2009. "Influence of the Charge State on the Structures and Interactions of Vancomycin Antibiotics with Cell-Wall Analogue Peptides: Experimental and Theoretical Studies." Chemistry - a European Journal 15(9):2081-2090. doi:10.1002/chem.200802010 Abstract In this study we examined the effect of the charge state on the energetics and dynamics of dissociation of the non-covalent complex between the vancomycin and the cell wall peptide analogue Nα,Nε-diacetyl-L-Lys-D-Ala-D-Ala (V-Ac2KDADA). The binding energies between the vancomycin and the peptide were obtained from the RRKM modeling of the time- and energy resolved surface-induced dissociation (SID) experiments. Our results demonstrate that the stability of the complex toward fragmentation increases in the order: [V+Ac2KDADA+H]+2 < [V+Ac2KDADA+H]+ < [V+Ac2KDADA-H]-. Dissociation of the singly protonated and singly deprotonated complex is characterized by very large entropy effects indicating substantial increase in the conformational flexibility of the resulting products. The experimental threshold energies of 1.75 eV and 1.34 eV obtained for the [V+Ac2KDADA-H]- and [V+Ac2KDADA+H]+ , respectively, are in excellent agreement with the results of density functional theory (DFT) calculations. The increased stability of the deprotonated complex observed experimentally is attributed to the presence of three charged sites in the deprotonated complex as compared to only one charged site in the singly protonated complex. The low binding energy of 0.93 eV obtained for the doubly protonated complex suggests that this ion is destabilized by Coulomb repulsion between the singly protonated vancomycin and the singly protonated peptide comprising the complex.
2009. "Fragmentation Mechanisms of Oxidized Peptides Elucidated by SID, RRKM Modeling and Molecular Dynamics." Journal of the American Chemical Society 20(9):1579-1592. doi:10.1016/j.jasms.2009.04.012 Abstract The gas phase fragmentation reactions of singly charged angiotensin II (AngII, DRVYIHPF) and the ozonolysis products AngII+O (DRVY*IHPF), AngII+3O (DRVYIH*PF), and AngII+4O (DRVY*IH*PF) were studied using SID FT-ICR mass spectrometry and molecular dynamics. Oxidation of Tyr (AngII+O) leads to an additional low-energy charge-remote selective fragmentation channel resulting in the b4 fragment ion. Modification of His leads to a series of new selective dissociation channels. For AngII+3O and AngII+4O, the formation of [MH+3O]+-45 and [MH+3O]+-71 are driven by charge-remote processes while it is suggested that the b5 and [MH+3O]+-88 fragments are a result of charge-directed reactions. Energy-resolved SID experiments and RRKM modeling were able to provide insight into the energetics of the lowest energy fragmentation channel for each of the parent ions. Destabilization of the ozonolysis products was found to be strictly due to entropic effects. Mechanistic details for each of the new dissociation pathways were determined by relating the SID FT-ICR MS results to parent ion conformations samples using molecular dynamics.
2009. "Experimental and Computational Studies of the Macrocyclic Effect of an Auxiliary Ligand on Electron and Proton Transfers Within Ternary Copper(II)–Histidine Complexes ." Journal of the American Society for Mass Spectrometry 20(6):972-984. Abstract The dissociation of [CuII(L)His]•2+ complexes [L = diethylenetriamine (dien) or 1,4,7-triazacyclononane (9-aneN3)] bears a strong resemblance to the previously reported behavior of [CuII(L)GGH]•2+ complexes. We have used low energy collision-induced dissociation experiments and density functional theory (DFT) calculations at the B3LYP/6-31+G(d) level to study the macrocyclic effect of the auxiliary ligands on the formation of His•+ from prototypical [CuII(L)His]•2+ systems. DFT revealed that the relative energy barriers of the same electron transfer (ET) dissociation pathways of [CuII(9-aneN3)His]•2+ and [CuII(dien)His]•2+ are very similar, with the ET reactions of [CuII(9-aneN3)His]•2+ leading to the generation of two distinct His•+ species; in contrast, the proton transfer (PT) dissociation pathways of [CuII(9-aneN3)His]•2+ and [CuII(dien)His]•2+ differ considerably. The PT reactions of [CuII(9-aneN3)His]•2+ are associated with substantially higher barriers (>13 kcal/mol) than those of [CuII(dien)His]•2+. Thus, the sterically encumbered auxiliary 9-aneN3 ligand facilitates ET reactions while moderating PT reactions, allowing the formation of hitherto non-observable histidine radical cations.
2009. "Molecular Characterization of Biomass Burning Aerosols Using High Resolution Mass Spectrometry." Analytical Chemistry 81(4):1512-1521. doi:10.1021/ac8020664 Abstract Chemical characterizations of atmospheric aerosols is a serious analytical challenge because of the complexity of particulate matter analyte composed of a large number of compounds with a wide range of molecular structures, physico-chemical properties, and reactivity. In this study chemical composition of biomass burning organic aerosol (BBOA) samples is characterized by high resolution electrospray ionization mass spectrometry (ESI/MS). Accurate mass measurement combined with Kendrick analysis allowed us to assign elemental composition for hundreds of compounds in the range of m/z values of 50-1000. ESI/MS spectra of different BBOA samples contain a variety of distinct, sample specific, characteristic peaks that can be used as unique markers for different types of biofuels. Our results indicate that a significant number of high-MW organic compounds in BBOA samples are highly oxidized polar species that can be efficiently detected using ESI/MS but are difficult to observe using the conventional GCMS analysis of aerosol samples. The average O:C ratios obtained for each of the BBOA samples studied in this work are in a strikingly good agreement with the previously reported values obtained using STXM/NEXAFS. The degree of unsaturation of detected organic compounds shows a clear decrease with increase in the molecular weight of the anyalyte molecules. The decrease is particularly pronounced for the samples containing a large number of CH2-based homologous series.
2009. "Kinetics for Tautomerizations and Dissociations of Triglycine Radical Cations." Journal of the American Society for Mass Spectrometry 20(6):996-1005. Abstract Fragmentations of tautomers of the α-centered radical triglycine radical cation, [GGG*]+, [GG*G]+, and [G*GG]+, are charge-driven, giving b-type ions; these are processes that are facilitated by a mobile proton, as in the fragmentation of protonated triglycine (Rodriquez, C.F. et al. J. Am. Chem. Soc. 2001, 123, 3006 - 3012). By contrast, radical centers are less mobile. Two mechanisms have been examined theoretically utilizing density functional theory and Rice-Ramsperger-Kassel-Marcus modeling: (1) a direct hydrogen-atom migration between two α-carbons, and (2) a two-step proton migration involving a canonical [GGG]*+ as an intermediate. Predictions employing the latter mechanism are in good agreement with results of recent CID experiments (Chu, I.K. et al. J. Am. Chem. Soc. 2008, 130, 7862 - 7872).
2009. "Molecular Characterization of Nitrogen Containing Organic Compounds in Biomass Burning Aerosols Using High Resolution Mass Spectrometry." Environmental Science & Technology 43(10):3764-3771. doi:10.1021/es803456n Abstract Although nitrogen-containing organic compounds (NOC) are important components of atmospheric aerosols, little is known about their chemical compositions. Here we present detailed characterization of the NOC constituents of biomass burning aerosol (BBA) samples using high resolution electrospray ionization mass spectrometry (ESI/MS). Accurate mass measurements combined with MS/MS fragmentation experiments of selected ions were used to assign molecular structures to individual NOC species. Our results indicate that N-heterocyclic alkaloid compounds - species naturally produced by plants and living organisms - comprise a substantial fraction of NOC in BBA samples collected from test burns of five biomass fuels. High abundance of alkaloids in test burns of ponderosa pine - a widespread tree in the western U.S. areas frequently affected by large scale fires - suggests that N-heterocyclic alkaloids in BBA can play a significant role in dry and wet deposition of fixed nitrogen in this region.
2009. "In situ Studies of Soft- and Reactive Landing of Mass-Selected Ions Using Infrared Reflection Absorption Spectroscopy." Analytical Chemistry 81(17):7302-7308. doi:10.1021/ac901149s Abstract Grazing incidence infrared reflection absorption spectroscopy (IRRAS) for in situ and in real time characterization of substrates modified by soft- and reactive landing (SL and RL) of complex ions was implemented on a mass-selected ion deposition instrument. Ions produced by electrospray ionization were mass-selected using a quadrupole mass filter and deposited onto inert and reactive self-assembled monolayer (SAM) surfaces. Surface composition during and after ion deposition was monitored using IRRAS. Physisorption of a cyclic peptide, Garmicidin S (GS), was studied for 8 hrs during deposition and additional 12 hrs after the end of deposition. The integrated signal of the characteristic amide bands followed a linear increase during the deposition and stayed unchanged after the deposition was finished. Similar linear increase in IRRAS signal was obtained following reactive deposition of the protonated dodecanediamine onto SAMs of dithiobis (succinimidyl undecanoate) (NHS-SAM) and 16-mercaptohexadecanoic acid fluoride (COF-SAM) on gold. IRRAS allowed us to monitor for the first time the formation of the amide bond between reactive SAM surfaces and the projectile molecule.
2009. "Effect of the Surface on Charge Reduction and Desorption Kinetics of Soft Landed Peptide Ions." Journal of the American Society for Mass Spectrometry 20(6):901-906. Abstract Charge reduction and desorption kinetics of ions and neutral molecules produced by soft-landing of mass-selected singly and doubly protonated Gramicidin S (GS) on different surfaces was studied using time dependant in situ secondary ion mass spectrometry (SIMS) integrated in a specially designed Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) research instrument. Soft-landing targets utilized in this study included inert self-assembled monolayers (SAMs) of 1-dodecane thiol (HSAM) and its fluorinated analog (FSAM) on gold and hydrophilic carboxyl-terminated (COOH-SAM) and amine-terminated (NH2-SAM) SAM surfaces. We observed efficient neutralization of soft-landed ions on the COOH-SAM surface, partial retention of only one proton on the HSAM surface and efficient retention of two protons on the FSAM surface. Slow desorption rates measured experimentally indicate fairly strong binding between peptide molecules and SAM surfaces with the binding energy of 20-25 kcal/mol.
2009. "Time-Resolved Molecular Characterization of Limonene/Ozone Aerosol using High-Resolution Electrospray Ionization Mass Spectrometry." Physical Chemistry Chemical Physics. PCCP (11):7931-7942. doi:10.1039/b905288g Abstract Molecular composition of limonene/O3 secondary organic aerosol (SOA) was investigated using high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) as a function of reaction time. SOA was generated by ozonation of D-limonene in a reaction chamber and sampled at different time intervals using a cascade impactor. The SOA samples were extracted into acetonitrile and analyzed using a HR-ESI-MS instrument with a resolving power of 100,000 (m/Δm). The resulting mass spectra provided detailed information about the extent of oxidation inferred from the O:C ratios, double bond equivalency (DBE) factors, and aromaticity indexes (AI) in hundreds of identified individual SOA species.
2008. "The Effect of the Secondary Structure on Dissociation of Peptide Radical Cations: Fragmentation of Angiotensin III and Its Analogues." Journal of Physical Chemistry B 112(39):12468-12478. doi:10.1021/jp805226x Abstract Fragmentation of protonated RVYIHPF and RVYIHPF-OMe and the corresponding radical cations was studied using time- and collision energy-resolved surface-induced dissociation (SID) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Peptide radical cations were produced by gas-phase fragmentation of CoIII(salen)-peptide complexes. Both the energetics and mechanisms of dissociation of even-electron and odd-electron angiotensin III ions are quite different. Protonated molecules are much more stable towards fragmentation than the corresponding radical cations. RRKM modeling of the experimental data suggests that this stability is largely attributed to differences in threshold energies for dissociation while activation entropies are very similar. Detailed analysis of the experimental data obtained for radical cations demonstrated the presence of two distinct structures separated by a high free-energy barrier. The two families of structures were ascribed to the canonical and zwitterionic forms of the radical cations produced in our experiments.
2008. "Experimental and Theoretical Studies of the Structures and Interactions of Vancomycin Antibiotics with Cell Wall Analogues." Journal of the American Chemical Society 130(39):13013-13022. doi:10.1021/ja802643g Abstract Surface-induced dissociation (SID) of the singly protonated complex of vancomycin antibiotic with cell wall peptide analogue (Nα,Nε-diacetyl-L-Lys-D-Ala-D-Ala) was studied using a 6 T Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS) specially configured for SID experiments. The binding energy between the vancomycin and the peptide was obtained from the RRKM modeling of the time- and energy resolved fragmentation efficiency curves (TFECs) of the precursor ion and its fragments. Electronic structure calculations of the geometries, proton affinities and binding energies were performed for several model systems including vancomycin (V), vancomycin aglycon (VA), Nα,Nε-diacetyl-L-Lys-D-Ala-D-Ala, and non-covalent complexes of VA with N-acetyl-D-Ala-D-Ala and Nα,Nε-diacetyl-L-Lys-D-Ala-D-Ala at the B3LYP/6-31G(d) level of theory. Comparison between the experimental and computational results suggests that the most probable structure of the complex observed in our experiments corresponds to the neutral peptide bound to the vancomycin protonated at the secondary amino group of the N-methyl-leucine residue. The experimental binding energy of 30.9 ± 1.8 kcal/mol is in good agreement with the binding energy of 29.3 ± 2.5 kcal/mol calculated for the model system representing the preferred structure of the complex.
2008. "Reactive Landing of Peptide Ions on Self-Assembled Monolayer Surfaces: A Alternative Approach for Covalent Immobilization of Peptides on Surfaces." Physical Chemistry Chemical Physics. PCCP 10(11):1512-1522. doi:10.1039/b717617a Abstract Soft landing of mass-selected peptide ions onto reactive self-assembled monolayer surfaces (SAMs) was performed using a newly constructed ion deposition apparatus. SAM surfaces before and after soft-landing were characterized ex situ using time of flight-secondary ion mass spectrometry (TOF-SIMS) and infrared reflection absorption spectroscopy (IRRAS). We demonstrate that reactive landing (RL) results in efficient covalent linking of lysine-containing peptides onto the SAM of N-hydroxysuccinimidyl ester terminated alkylthiol on gold (NHS-SAM). Systematic studies of the factors that affect the efficiency of RL revealed that the reaction takes place upon collision and is promoted by the kinetic energy of the ion. The efficiency of RL maximizes at ca. 40 eV collision energy. At high collision energies the RL efficiency decreases because of the competition with scattering of ions off the surface. The reaction yield is independent of the charge state of the projectile ions suggesting that peptide ions undergo efficient neutralization upon collision. Chemical and physical properties of the SAM surface are also important factors that affect the outcome of RL. The presence of chemically reactive functional groups on the SAM surface significantly improves the reaction efficiency. RL of mass- and energy-selected peptide ions on surfaces provides a highly specific approach for covalent immobilization of biological molecules onto SAM surfaces.
2008. "Helical Peptide Arrays on Self-Assembled Monolayer Surfaces Through Soft and Reactive Landing of Mass-Selected Ions." Angewandte Chemie International Edition 47:6678-6680. doi:10.1002/anie.200801366 Abstract The α-helix – the common building block of the protein secondary structure - plays an important role in determining protein structure and function. The biological function of the α-helix is mainly attributed to its large macrodipole originating from the alignment of individual dipole moments of peptide bonds. Preparation of directionally aligned α-helical peptide layers on substrates has attracted significant attention because the resulting strong net dipole is useful for a variety of applications in photonics, , molecular electronics, and catalysis. - In addition, conformationally-selected α-helical peptide arrays can be used for detailed characterization of molecular recognition steps critical for protein folding, enzyme function and DNA binding by proteins. Existing technologies for the production of α-helical peptide surfaces are based on a variety of solution phase synthetic strategies - that usually require relatively large quantities of purified materials.
2008. "High-Resolution Mass Spectrometric Analysis of Secondary Organic Aerosol Produced by Ozonation of Limonene." Physical Chemistry Chemical Physics. PCCP 10(7):1009-1022. doi:10.1039/b712620d Abstract Secondary organic aerosol (SOA) particles formed from the ozone-initiated oxidation of limonene are characterized by high-resolution electrospray ionization mass spectrometry in both the positive and negative ion modes. The mass spectra reveal a large number of both monomeric (m/z < 300) and oligomeric (m/z > 300) products of oxidation. A combination of high resolving power (m/Δm ~60,000) and Kendrick mass defect analysis makes it possible to unambiguously determine the composition for hundreds of individual compounds in SOA samples. Van Krevelen analysis shows that the SOA compounds are heavily oxidized, with average O:C ratios of 0.43 and 0.50 determined from the positive and negative ion mode spectra, respectively. An extended reaction mechanism for the formation of the first generation SOA molecular components is proposed. The mechanism includes known isomerization and addition reactions of the carbonyl oxide intermediates generated during the ozonation of limonene, and numerous isomerization pathways for alkoxy radicals resulting from the decomposition of unstable carbonyl oxides. The isomerization reactions yield numerous products with a progressively increasing number of alcohol and carbonyl groups, whereas C-C bond scission reactions in alkoxy radicals shorten the carbon chain. Together these reactions yield a large number of isomeric products with broadly distributed masses. A qualitative agreement is found between the number and degree of oxidation of the predicted and measured reaction products in the monomer range.
2008. "Soft-Landing of Peptide IOns Onto Self-Assembled Monolayer Surfaces: an Overview." Physical Chemistry Chemical Physics. PCCP 10(8):1079-1090. doi:10.1039/b712710c Abstract This review is focused on what has been learned in recent research studies concerned with fundamental aspects of soft-landing and reactive landing of peptide ions on self-assembled monolayer surfaces (SAMs). Peptide ions are particularly attractive model systems that provide important insights on the behavior of soft landed proteins, while SAMs provide a convenient and flexible platform for tailoring the interfacial properties of metals and semiconductor surfaces. Deposition of mass-selected ions on surfaces is accompanied by a number of processes including charge reduction, neutralization, covalent and non-covalent binding, and thermal desorption of ions and molecules from the substrate. Factors that affect the competition between these processes are discussed.
2008. "Energetics and Dynamics of Electron Transfer and Proton Transfer in Dissociation of Metal III (salen)-Peptide Complexes in the Gas Phase." Journal of the American Chemical Society 130(10):3218-3230. doi:10.1021/ja077690s Abstract Time- and collision energy-resolved surface-induced dissociation (SID) of ternary complexes of CoIII(salen)+, FeIII(salen)+, and MnIII(salen)+ with several angiotensin peptide analogs was studied using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved fragmentation efficiency curves (TFECs) were modeled using an RRKM-based approach developed in our laboratory. The approach utilizes a very flexible analytical expression for the internal energy deposition function that is capable of reproducing both single-collision and multiple-collision activation in the gas phase and excitation by collisions with a surface. The energetics and dynamics of competing dissociation pathways obtained from the modeling provides important insight on the competition between proton transfer, electron transfer, loss of neutral peptide ligand, and other processes that determine gas-phase fragmentation of these model systems. Similar fragmentation behavior was obtained for various CoIII(salen)-peptide systems of different angiotensin analogs. In contrast, dissociation pathways and relative stabilities of the complexes changed dramatically when cobalt was replaced with trivalent iron or manganese. We demonstrate that the electron transfer efficiency is correlated with redox properties of the metalIII(salen) complexes (Co > Fe > Mn), while differences in the types of fragments formed from the complexes reflect differences in the modes of binding between the metal-salen complex and the peptide ligand. RRKM modeling of time- and collision energy-resolved SID data suggests that the competition between proton transfer and electron transfer during dissociation of CoIII(salen)-peptide complexes is mainly determined by differences in entropy effects while the energetics of these two pathways are very similar.
2008. "The Effect of Solvent on the Analysis of Secondary Organic Aerosol Using Electrospray Ionization Mass Spectrometry." Environmental Science & Technology 42(19):7341-7346. doi:10.1021/es801226w Abstract Solvent-analyte reactions in organic aerosol (OA) extracts prepared for analysis by electrospray ionization mass spectrometry (ESI-MS) were examined. Secondary organic aerosol (SOA) produced by ozonation of d-limonene as well as several test organic chemicals with functional groups typical for OA constituents were dissolved and stored in methanol, d3-methanol, acetonitrile, and d3-acetonitrile to investigate the extent and relative rates of reactions between analyte and solvent. High resolution ESI-MS showed that reactions of carbonyls with methanol produce significant amounts of hemiacetals and acetals on time scales ranging from several minutes to several days, with the reaction rates increasing in acidified solutions. Carboxylic acid groups were observed to react with methanol resulting in the formation of esters. In contrast, acetonitrile extracts showed no evidence of reactions with analyte molecules, suggesting that acetonitrile is the preferred solvent for SOA extraction. The use of solvent-analyte reactivity as an analytical chemistry tool for the improved characterization of functional groups in complex organic mixtures was also demonstrated. Direct comparison between ESI mass spectra of the same SOA samples extracted in reactive (methanol) versus non-reactive (acetonitrile) solvents was used to estimate the relative fractions of ketones (38%), aldehydes (6%), and carboxylic acids (55%) in d-limonene SOA.
2007. "Effect of the Surface Morphology on the Energy Transfer in Ion-Surface Collisions." International Journal of Mass Spectrometry 265(1):124-129. doi:10.1016/j.ijms.2007.01.018 Abstract Time- and energy-resolved surface-induced dissociation (SID) of singly protonated des-Arg1-bradykinin (PPGFSPFR) combined with RRKM modeling was used to explore the effect of surface morphology on the energy transfer in collisions of large ions with surfaces. Experiments were performed in a Fourier Transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for SID studies. Massselected and vibrationally relaxed ions were collided with three diamond surfaces of varying degree of roughness. The results demonstrate that internal energy distributions resulting from collisions of large ions with surfaces are rather independent of the surface morphology: the translational to vibrational (T→V) energy transfer efficiency is 19.5±0.5% for all three diamond surfaces. However, the scattered ion signal increases with decrease in the degree of roughness of the SID target suggesting that smooth diamond surfaces are better targets for analytical applications.
2007. "Covalent Immobilization of Peptides on Self-Assembled Monolayer Surfaces Using Soft-Landing of Mass-Selected Ions." Journal of the American Chemical Society 129:8682-8683. Abstract Covalent immobilization of peptides on solid supports plays an important role in biochemistry with applications ranging from characterization of molecular recognition events at the amino acid level and identification of biologically active motifs in proteins to development of novel biosensors and substrates for improved cell adhesion. Self-assembled monolayer surfaces (SAMs) provide a simple and convenient platform for tailoring chemical properties of a variety of substrates. Existing techniques for linking peptides to SAMs are based on solution-phase synthetic strategies and require relatively large quantities of purified material. Here, we report a novel approach for highly selective covalent binding of peptides to SAMs using soft-landing (SL) of mass-selected ions. SL is defined as intact deposition of ions onto suitable substrates at hyperthermal (<100 eV) energies.Recent studies have demonstrated that SAMs are excellent deposition targets for SL due to their ability to dissipate kinetic energies of the projectiles and their efficiency in trapping captured species. It has been proposed that SL could be utilized for controlled preparation of protein arrays.
2007. "Energetics and Dynamics of the Fragmentation Reactions of Protonated Peptides Containing Methionine Sulfoxide or Aspartic Acid via Energy- and Time-Resolved Surface Induced Dissociation." Journal of Physical Chemistry A 111(42):10580-10588. doi:10.1021/jp073040z Abstract The surface-induced dissociation (SID) of six model peptides containing either methionine sulfoxide or aspartic acid (GAILM(O)GAILR, GAILM(O)GAILK, GAILM(O)GAILA, GAILDGAILR, GAILDGAILK, and GAILDGAILA) have been studied using a specially configured Fourier transform ion-cyclotron resonance mass spectrometer (FT-ICR MS). In particular, we have investigated the energetics and dynamics associated with (i) preferential cleavage of the methionine sulfoxide side chain via the loss of CH3SOH (64Da), and (ii) preferential cleavage of the amide bond C-terminal to aspartic acid. The role of proton mobility on these selective bond cleavage reactions was examined by changing the C-terminal residue of the peptide from arginine (non-mobile proton conditions) to lysine (partially-mobile proton conditions) to alanine (mobile proton conditions). Time- and energy-resolved fragmentation efficiency curves (TFEC) reveals that selective cleavages due to the methionine sulfoxide and aspartic acid residues are characterized by slow fragmentation kinetics. RRKM modeling of the experimental data suggests that the slow kinetics is associated with large negative entropy effects and these may be due to the presence of rearrangements prior to fragmentation. It was found that the Arrhenius pre-exponential factor (A) for peptide fragmentations occurring via selective bond cleavages are 1–2 orders of magnitude lower than non-selective peptide fragmentation reactions, while the dissociation threshold (E0) is relatively invariant. This means that selective bond cleavage is kinetically disfavored compared to non-selective amide bond cleavage. It was also found that the energetics and dynamics for the preferential loss of CH3SOH from peptide ions containing methionine sulfoxide are very similar to selective C-terminal amide bond cleavage at the aspartic acid residue. These results suggest that while preferential cleavage can compete with amide bond cleavage energetically, dynamically, these processes are much slower compared to amide bond cleavage, explaining why these selective bond cleavages are not observed if fragmentation is performed under mobile proton conditions. This study further affirms that fragmentation of peptide ions in the gas phase are predominantly governed by entropic effects.
2007. "Is Dissociation of Peptide Radical Cations an Ergodic Process?" Journal of the American Chemical Society 129(31):9598-9599. Abstract Achieving a fundamental understanding of the mechanism of unimolecular dissociation of internally excited complex molecules is one of the most important challenges in modern mass spectrometry. One of the central questions is whether the dissociation of large molecules is properly described by statistical theories—RRKM/QET or Phase Space Theories —that have proved to be remarkably successful both for small molecules and a number of small and medium size peptides. The concept question is whether the ergodic assumption that the internal excitation of the ion is randomly redistributed among the vibrational degrees of freedom prior to fragmentation is satisfied for large molecules. The validity of the ergodic hypothesis for dissociation of gas-phase biomolecules has been recently reviewed and will be only briefly discussed here.
2007. "Charge-Remote Fragmentation of Odd-Electron Peptide Ions." Analytical Chemistry 79(17):6607-6614. doi:10.1021/ac070777b Abstract Comparison between gas-phase fragmentation of odd-electron M+• and [M-2H]-• ions of model peptides reported in this study demonstrates that charge-remote radical-driven fragmentation pathways play an important role in dissociation of odd-electron peptide ions. We found that charge-remote processes are responsible for a variety of side chain losses from the precursor ion and some backbone fragmentation. These fragmentation pathways most likely involve hydrogen abstraction from the β-carbon of the side chain by the radical site that initiates subsequent cleavages. The results reported in this work are generally relevant for understanding of fragmentation patterns of odd-electron peptide ions produced by different approaches including capture of low-energy electrons, electron detachment and electron transfer.
2007. "Charge Retention by Peptide Ions Soft-Landed onto Self-Assembled Monolayer Surfaces." International Journal of Mass Spectrometry 265(1):237-243. Abstract Soft-landing of singly and doubly protonated peptide ions onto three self-assembled monolayer surfaces (SAMs) was performed using a novel ion deposition instrument constructed in our laboratory and a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially designed for studying collisions of large ions with surfaces.. Modified surfaces were analyzed using in situ 2 keV Cs+ secondary ion mass spectrometry or ex situ 15 keV Ga+ time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results demonstrate that a fraction of multiply protonated peptide ions retain more than one proton following soft-landing on the FSAM surface. [M+2H]2+ ions observed in FT-ICR SIMS spectra are produced by desorption of multiply charged ions from the surface, while re-ionization of singly protonated ions or neutral peptides is a source of [M+2H]2+ ions in Tof-SIMS spectra. Differences in neutralization efficiency of soft-landed ions following exposure of surfaces to laboratory air has a measurable effect on the results of ex situ ToF-SIMS analysis of soft-landed ions on SAM surfaces.
2007. "First Observation of Charge Reduction and Desorption Kinetics of Multiply Protonated Peptides Soft Landed onto Self-Assembled Monolayer Surfaces." Journal of Physical Chemistry C 111(49):18220-18225. doi:10.1021/jp075293y Abstract The kinetics of charge reduction and desorption of different species produced by soft-landing of mass-selected ions was studied using in situ secondary ion mass spectrometry (SIMS) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). The improved SIMS capability described in this work utilizes an in-line 8 keV Cs+ ion gun and allows us to interrogate the surface both during the ion deposition and after the deposition is terminated. As a model system doubly protonated ions of Gramicidin S were deposited onto a fluorinated self-assembled monolayer (FSAM) surface. Our results demonstrate for the first time that various peptide-related peaks in FT-ICR SIMS spectra follow very different kinetics. We obtained unique kinetics signatures for doubly protonated, singly protonated and neutral peptides retained on the surface and followed their evolution as a function of time. The experimental results are in excellent agreement with a kinetic model that takes into account charge reduction and thermal desorption of different species from the surface.
2007. "Design and Performance of a Novel Instrument for Soft-Landing of Biomolecular Ions on Surfaces." Analytical Chemistry 79(17):6566-6574. doi:10.1021/ac070600h Abstract A new ion deposition apparatus was designed and constructed in our laboratory. Our research objectives were to investigate interactions of biomolecules with hydrophilic and hydrophobic surfaces and to carry out exploratory experiments aimed at highly-selective deposition of spatially defined and uniquely selected biological molecules on surfaces. The apparatus includes a high-transmission electrospray ion source, quadrupole mass filter, bending quadrupole that deflects the ion beam and prevents neutral molecules originating in the ion source from impacting the surface, an ultrahigh vacuum (UHV) chamber for ion deposition by soft landing, and a vacuum-lock system for introducing surfaces into the UHV chamber without breaking vacuum. Ex situ analysis of surfaces following soft-landing of mass-selected peptide ions was performed using 15 keV Ga+ time-of-flight secondary ion mass spectrometry (TOF-SIMS) and grazing incidence infrared reflection-absorption spectroscopy (IRRAS). It will be shown that these two techniques are highly complementary methods for characterization of surfaces prepared with a range of doses of mass-selected biomolecular ions.
2007. "Electronic Structure and Fragmentation Properties of [Fe4S4(SEt)4-X(SSEt)x]2-." International Journal of Mass Spectrometry 263(2-3):260-266. Abstract A limited exposure of (n-Bu4N)2[Fe4S4(SEt)4] solutions in acetonitrile to air was found to produce a new series of [4Fe-4S] cluster complexes, [Fe4S4(SEt)4-x(SSEt)x]2- (x = 1-4), with the original –SEt ligands substituted by –SSEt di-sulfide ligands, which were formed due to partial decomposition of the [4Fe-4S] core in parent [Fe4S4(SEt)4]2-. The products were first observed in the experiments with an ESI-Ion Trap-TOF mass spectrometer and were further identified using high resolution FTICR (Fourier Transform Ion Cyclotron Resonance) mass spectrometer. Photoelectron spectra of the [Fe4S4(SEt)4-x(SSEt)x]2- dianions revealed that the –SSEt coordination induced little change in the electronic structure of the [4Fe-4S] cluster, but the electron binding energies of [Fe4S4(SEt)4-x(SSEt)x]2- increased from 0.52 to 0.73 eV with increase in x from 0 to 4, suggesting a greater electron withdrawing ability of –SSEt than -SEt. In high resolution MS/MS experiments on [Fe4S4(SEt)3(SSEt)]2-/1-, clusters with both charge states yielded fragment [Fe4S4(SEt)3]-, suggesting that –SSEt could be lost either as a negatively charged ion SSEt- from the doubly charged precursor, or as a radical •SSEt from the singly charged species. The biological implication of the interaction between [Fe4S4(SEt)4]2- and O2 is discussed in comparison to the air exposure of [4Fe-4S] proteins to the air.
2007. "Evaluation of the Influence of Amino Acid Composition on the Propensity for Collision-Induced Dissociation of Model Peptides Using Molecular Dynamics Simulations." Journal of the American Society for Mass Spectrometry 18(9):1625-1637. doi:10.1016/j.jasms.2007.06.005 Abstract The dynamical behavior of model peptides was evaluated with respect to their ability to form internal proton donor-acceptor pairs using molecular dynamics simulations. The proton donor-acceptor pairs are postulated to be prerequisites for peptide bond cleavage resulting in formation of b and y ions during low energy collision-induced dissociation in tandem mass spectrometry (MS/MS). The simulations for the polyalanine pentamer Ala5H+ were compared to experimental data from collision energy-resolved surface induced dissociation (SID) studies. The results of the simulation are insightful into the events that likely lead up to the fragmentation of peptides. 9-mer polyalanine-based model peptides were used to examine the dynamical effect of each of the 20 common amino acids on the probability to form donor-acceptor pairs at labile peptide bonds. A continuous range of probabilities was observed as a function of the substituted amino acid. However, the location of the peptide bond involved in the donor-acceptor pair plays a critical role in the dynamical behavior. This influence of position on the probability of forming a donor-acceptor pair would be hard to predict from statistical analyses on experimental spectra of aggregate, diverse peptides. In addition, the inclusion of basic side chains in the model peptides alters the probability of forming donor-acceptor pairs across the entire backbone. In this case there are still more ionizing protons than basic residues, but the side chains of the basic amino acids form stable hydrogen bond networks with the peptide carbonyl oxygens and thus act to prevent free access of “mobile protons” to labile peptide bonds. It is clear from the work that the identification of peptides from low-energy CID using automated computational methods should consider the location of the fragmenting bond as well as the amino acid composition.
2006. "Energy and Entropy Effects in the Gas-Phase Dissociation of Peptides and Proteins." In Principles of Mass Spectrometry Applied to Biomolecules: Wiley-Interscience Series on Mass Spectrometry, ed. J. Laskin and C. Lifshitz, pp. 619-666. John Wiley & Sons, Hoboken, NJ. Abstract In the last decade characterization of complex molecules, particularly biomolecules, became a focus of both fundamental and applied research in mass spectrometry. Most of these studies utilize tandem mass spectrometry (MS/MS) for obtaining structural information for complex molecules. MS/MS typically involves the mass selection of a primary ion, its activation by collision or photon excitation, unimolecular decay into fragment ions characteristic of the ion structure and its internal excitation, and mass analysis of the fragment ions. A wide variety of mass filters and ion excitation methods can be employed in these experiments, making tandem mass spectrometry an extremely flexible analytical technique that can be implemented on almost any type of mass spectrometer.
2006. "Peptide Ozonolysis: Product Structures and Relative Reactivities for Oxidation of Tyrosine and Histidine Residues ." Journal of the American Society for Mass Spectrometry 17(9):1289-1298. doi:10.1016/j.jasms.2006.05.009 Abstract Angiotensin II (DRVYIHPF) and two analogs (DRVYIAPA, and DRVAIHPA) were used as model systems to study the ozonolysis of peptides containing tyrosine and histidine residues. The ESI mass spectrum of angiotensin II following exposure to ozone showed the formation of adducts containing one, three and four oxygen atoms. CID and SID spectra of these adducts were consistent with formation of Tyr + O and His + 3O as expected from previous work with amino acids. Additional ions in the CID and SID spectra suggested formation of Tyr + 3O and a small amount of Phe + O. Two analogs were also studied, one in which His and Phe were replaced by Ala (DRVYIAPA) and the other in which Tyr and Phe were replaced by Ala (DRVAIHPA). Exposure of DRVYIAPA to ozone resulted in the addition of one and three oxygen atoms, while DRVAIHPA showed only the addition of three oxygen atoms. Tandem mass spectra of these adducts confirmed the formation of Tyr + 3O in addition to Tyr + O and His + 3O. Other noteworthy minor oxidation products were observed from these analogs including Tyr + 34 u, His + 34 u, and His + 82 u. Modified reaction schemes for peptide ozonolysis are proposed which account for each of these newly observed adducts.
2006. "Mechanisms of Peptide Fragmentation from Time-and Energy-Resolved Surface-Induced Dissociation Studies: Dissociation of Angiotensin Analogs." International Journal of Mass Spectrometry 249-250:462-472. Abstract Energetics and mechanism of dissociation of singly protonated angiotensin III (RVYIHPF) and its analogs RVYIFPF, RVYIYPF, RVYIHAF, and RVYIHDF was studied using surface-induced dissociation (SID) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for studying ion activation by collisions with surfaces. The energetics and dynamics of peptide fragmentation were deduced by modeling the time- and energy-resolved survival curves for each precursor ion using an RRKM based approach developed in our laboratory. Fragmentation mechanisms were inferred from comparison of time- and energy-resolved fragmentation efficiency curves (TFECs) of different fragment ions followed by RRKM modeling of dissociation of angiotensin III into six major families of fragment ions. Detailed modeling demonstrated that dissociation of these peptides is dominated by loss of ammonia from the precursor ion and characterized by a high energy barrier of 1.6 eV. Loss of NH3 and subsequent rearrangement of the MH-NH3 ion results in proton mobilization and release of ca. 30 kcal/mol into internal excitation of the MH-NH3 ion. The resulting highly excited ion accesses a variety of non-specific dissociation pathways with very high rate constants. Fast fragmentation of excited MH-NH3 ion forms a variety of abundant bn-NH3 and an-NH3 fragment ions. Abundant XH and HX internal fragments are also formed, reflecting the stability of histidine-containing diketopiperazine structures.
2006. "Energetics and Dynamics of Fragmentation of Protonated Leucine Enkephalin from Time-and Energy-Resolved Surface-Induced Dissociation Studies." Journal of Physical Chemistry A 110(27):8554-8562. Abstract Dissociation of singly protonated leucine enkephalin (YGGFL) was studied using surface-induced dissociation (SID) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for studying ion activation by collisions with surfaces. The energetics and dynamics of seven primary dissociation channels were deduced from modeling the time- and energy-resolved fragmentation efficiency curves for different fragment ions using an RRKM based approach developed in our laboratory. The following threshold energies and activation entropies were determined in this study: E0=1.20 eV and ∆S‡=-20 e.u.1 (MH+→b5); E0=1.14 eV and ∆S‡=-14.7 e.u. (MH+→b4); E0=1.42 eV and ∆S‡=-2.5 e.u. (MH+→b3); E0=1.30 eV and ∆S‡=-4.1 e.u. (MH+→a4); E0=1.37 eV and ∆S‡=-5.2 e.u. (MH+→y ions); E0=1.50 eV and ∆S‡=1.6 e.u. (MH+→internal fragments); E0=1.62 eV and ∆S‡=5.2 e.u. (MH+→F). Comparison with Arrhenius activation energies reported in the literature demonstrated for the first time the reversal of the order of activation energies as compared to threshold energies for dissociation.
2006. "Collision Induced Dissociation of [4Fe-4S] Cubane Cluster Complexes: [Fe4S4C14-x(SC2H5)x]2-/1- (x=0-4)." International Journal of Mass Spectrometry 255-256:102-110. doi:10.1016/j.ijms.2005.12.009 Abstract Collision-induced dissociation (CID) experiments on a series of [4Fe-4S] cluster ions, [Fe4S4Cl4-x(SC2H5)x]2-/1- (x = 0 - 4), revealed that their fragmentation channels change with the coordination environment. Among the three Coulomb repulsion related channels for the doubly charged species, the collision induced electron detachment channel was found to become more significant from x = 0 to 4 due to the decreasing electron binding energies and the magnitude of the repulsion Coulomb barrier, while both the ligand detachment of Cl- and the fission of the [Fe4S4]2+ core became more and more significant with the increase of the Cl- coordination, and eventually became the dominant channel at x = 0. From the parents containing the -SC2H5 ligand, neutral losses of HSC2H5 (62) and/or HSCH=CH2 (60) were observed. It was proposed that interand intra-ligand proton transfer could happen during the CID process, resulting in hydrogen coordination to the [4Fe-4S] cluster. In the presence of O2, [Fe4S4Cl3(SC2H5)]2- and [Fe4S4Cl4]2- can form the O2-substituted products [Fe4S4Cl2(SC2H5)O2]- and [Fe4S4Cl3O2]-, respectively. It was shown that the O2 complexation occurs by coordination to the empty iron site of the [4Fe-4S] cubane core after dissociation of one Cl- ligand.
2006. "Protein Identification Via Surface-Induced Dissociation in an FT-ICR Mass Spectrometer and a Patchwork Sequencing Approach." Journal of the American Society for Mass Spectrometry 17(5):700-709. Abstract Surface-induced dissociation (SID) and collision-induced dissociation (CID) are ion activation techniques based on energetic collisions with a surface or gas molecules, respectively. One noticeable difference between CID and SID is that SID does not require a collision gas for ion activation and is therefore directly compatible with the high vacuum requirement of Fourier Transform Ion Cyclotron Resonance mass spectrometers (FT-ICR MS). Eliminating the introduction of collision gas into the ICR cell for collisional activation dramatically shortens the acquisition time for MS/MS experiments, suggesting that SID could be utilized for high-throughput MS/MS studies in FT-ICR MS. We demonstrate for the first time the utility of SID combined with FT-ICR MS for protein identification. Tryptic digests of standard proteins were analyzed using a hybrid 6-Tesla FT-ICR MS with SID and CID capabilities. SID spectra of mass-selected singly and doubly charged peptides were obtained using a diamond-coated target mounted at the rear trapping plate of the ICR cell. The broad internal energy distribution deposited into the precursor ion following collision with the diamond surface allowed a variety of fragmentation channels to be accessed by SID. Composition and sequence qualifiers produced by SID of tryptic peptides were used to improve the statistical significance of database searches. Protein identification MASCOT scores obtained using SID were comparable or better than scores obtained using sustained off-resonance irradiation collision-induced dissociation (SORI-CID) –the conventional ion activation technique in FT-ICR MS.
2006. "Soft-Landing of Peptides onto Self-Assembled Monolayer Surfaces." Journal of Physical Chemistry A 110(4):1678-1687. doi:10.1021/jp0555044 Abstract Mass-selected peptide ions produced by electrospray ionization were deposited as ions by soft-landing onto fluorinated and hydrogenated self-assembled monolayer surfaces (FSAM and HSAM) surfaces using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially designed for studying interactions of large ions with surfaces. Analysis of the modified surface was performed in situ by combining 2 keV Cs+ secondary ion mass spectrometry with FT-ICR detection of the sputtered ions (FT-ICR-SIMS). Collision energy dependent data indicated that peptide fragmentation observed in Cs+ SIMS analysis of surfaces modified by soft landing occurred in the analysis step (SIMS) rather than during ion deposition. The peptide ion deposition efficiency showed a gradual decrease with increase in collision energy which parallels the decrease in the Langevin cross section with ion velocity at impact. Peptide ion soft-landing on FSAM surfaces gave twice the sputtered ion signal seen for hydrocarbon self-assembled monolayer (HSAM) surfaces. This indicates stronger interaction of peptide ions with the FSAM surface, in good agreement with larger polarizability of fluorinated hydrocarbons. The efficiency of soft landing of different peptides on the FSAM surface increases with the charge state of the ion, consistent with an ion-polarizable molecule model for the initial stage of soft landing on SAM surfaces.
2005. "Activation of Large Ions in FT-ICR Mass Spectrometry." Mass Spectrometry Reviews 24(2):135-167. Abstract The advent of soft ionization techniques, notably electrospray and laser desorption ionization methods, has enabled the extension of mass spectrometric methods to large molecules and molecular complexes. This both greatly extends the applications of mass spectrometry and makes the activation and dissociation of complex ions an integral part of these applications. This review emphasizes the most promising methods for activation and dissociation of complex ions and presents this discussion in the context of general knowledge of reaction kinetics and dynamics largely established for small ions. We then introduce the characteristic differences associated with the higher number of internal degrees of freedom and high density of states associated with molecular complexity. This is reflected primarily in the kinetics of unimolecular dissociation of complex ions, particularly their slow decay and the higher energy content required to induce decomposition-the kinetic shift (KS). The longer trapping time for Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) significantly reduces the KS, which presents several advantages over other methods for the investigation of dissociation of complex molecules.
2005. "Ion/Surface Reactions and Ion Soft-Landing." Physical Chemistry Chemical Physics. PCCP 7:1490-1500. Abstract Ion/surface collision phenomena in the hyperthermal collision energy regime (1-100 eV) are reviewed, with emphasis on chemical processes associated with the impact of small organic and biological ions at functionalized self-assembled monolayer surfaces. Inelastic collisions can lead to excitation the projectile ion and can result in fragmentation, a process known as surface-induced dissociation which is useful in chemical analysis using tandem mass spectrometry.. Changes in charge can accompany ion/surface collisions and those associated with a change in polarity (positive to negative ions or vice-versa) are an attractive method for ion structural characterization and isomer differentiation. The energetics, thermochemistry and dynamics of surface-induced charge inversion of nitrobenzene and other substituted aromatics is discussed. Reactive collisions also occur between gaseous ions and surfaces and the reactions depend on the chemical nature of the collision partners. These reactions can be used for selected chemical modifications of surfaces as well as for surface analysis. Particular emphasis is given here to one ion/surface interaction, ion soft-landing, a process in which the projectile ion is landed intact at the surface, either as the corresponding neutral molecule or, interestingly but less commonly, in the form of the ion itself. The ion soft-landing experiment allows preparative mass spectrometry, for example the preparation of pure biological compounds by using the mass spectrometer as a separation device. After separation, the mass-selected ions are collected by soft-landing, at different spatial points in an array. If the experiment is done using a suitable liquid medium, at least some proteins retain their biological activity.
2005. "Preparation and in situ Characterization of Surfaces Using Soft-Landing in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer." Analytical Chemistry 77(11):3452-3460. Abstract Mass-selected peptide ions produced by electrospray ionization were deposited onto fluorinated self-assembled monolayer surfaces (FSAM) surfaces by soft-landing using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially designed for studying interactions of large ions with surfaces. Analysis of the modified surface was performed in situ by combining 2 keV Cs+ secondary ion mass spectrometry with FT-ICR detection of the sputtered ions (FT-ICR-SIMS). Regardless of the initial charge state of the precursor ion, the SIMS mass spectra included singly-protonated peptide fragment ions and peaks characteristic of the surfaces in all cases. In some experiments multiply-protonated peptide ions and [M+Au]+ ions were also observed upon SIMS analysis of modified surfaces. For comparison with the in situ analysis of the modified surfaces, ex situ analysis of some of the modified surfaces was performed by 25 kV Ga+ time of flight – secondary ion mass spectrometry (ToF-SIMS). The ex situ analysis demonstrated that a significant number of soft-landed peptide ions remain charged on the surface even when exposed to air for several hours after deposition. Charge retention of soft-landed ions dramatically increases the ion yields obtained during SIMS analysis very sensitive detection of deposited material at less than 1% of monolayer coverage. Accumulation of charged species on the surface undergoes saturation due to Coulomb repulsion between charges at close to 30% coverage. We estimated that close to 1 ng of peptide could be deposited on the spot area of 4 mm2 of the FSAM surface without reaching saturation.
2004. "Isolation, Characterization of an Intermediate in an Oxygen Atom-Transfer Reaction, and the Determination of the Bond Dissociation Energy." Journal of the American Chemical Society 126(28):8604-8605. Abstract Redox reactions coupled with the formal loss or gain of an oxygen atom are ubiquitous in chemical processes. Such reactions proceed through the reduction of th edonor center (XO) and the oxidation of the acceptor (Y) molecule. Among many examples of the metal centered oxygen atom transfer (OAT) reactivity, those involving molybdenum complexes have been widely investigated due to their involvement in mononuclear molydenum enzymes. The heat of reaction of the overall atom transfer process can be expressed as a difference between the bond dissociation energies (BDEs) of the oxygen-donor(X) and oxygen-acceptor(Y) bond, i.e., H=DX=o-DY=O.
2004. "Surface-Induced Dissociation of Ions Produced by Matrix-Assisted Laser Desorption Ionization in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer." Analytical Chemistry 76(2):351-356. Abstract Intermediate pressure matrix assisted laser ionizaton (MALDI) source was constructed and interfaced with a 6T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for surface-induced dissociation (SID) studies.
2004. "Fragmentation Energetics for Angiotensin II and its Analogs from Time-and Energy-Resolved Surface-Induced Dissociation Studies." International Journal of Mass Spectrometry 234(1-3):89-99. Abstract Surface-induced dissociation (SID) of four model peptides: DRVYIHPF, RVYIHPF, RVYIHAF, and RVYIHDF was studied using a novel Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for SID experiments.
2004. "Energetics and Dynamics of Peptide Fragmentation from Multiple-Collision Activation and Surface-Induced Dissociation Studies." European Journal of Mass Spectrometry 10(2):259-267. doi:10.1255/ejms.641 Abstract This account summarizes the energetics and dynamics of peptide fragmentation obtained using a new approach recently developed in our laboratory. The approach involves RRKM modeling of time- and energy-resolved MS/MS data obtained using collisional activation. We demonstrate that surface-induced dissociation (SID) on a long timescale of Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) is perfectly suited for studying the energetics and dynamics of peptide fragmentation.
2004. "Energetics and Dynamics of Peptide Fragmentation from Multiple-Collision Activation and Surface-Induced Dissociation Studies." European Journal of Mass Spectrometry 10(2):259-267. Abstract This account summarizes the energetics and dynamics of peptide fragmentation obtained using a new approach recently developed in our laboratory. The approach involves RRKM modeling of time- and energy-resolved MS/MS data obtained using collisional activation. We demonstrate that surface-induced dissociation (SID) on a long timescale of Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) is perfectly suited for studying the energetics and dynamics of peptide fragmentation. The advantages provided by SID include very fast ion activation, which eliminates possible discrimination against higher-energy dissociation pathways, and efficient “amplification†of small changes in dissociation parameters. We present a summary of results obtained for small alanine-containing peptides as well as larger peptides including angiotensin analogs and a series of peptides containing the LDIFSDF motif.
2004. "Relative Proton Affinities from Kinetic Energy Release Distributions for Dissociation of Proton-Bound Dimers: 2. Diamines as a Test Case." International Journal of Mass Spectrometry 233:223-231. Abstract Dissociation of proton-bound dimers containing diamines is often characterized by a substantial entropy difference between the two competing reaction channels. Kinetic energy release distributions (KERDs) upon dissociation of diamine-containing dimers are utilized along with Finite Heat Bath theory analysis to obtain relative proton affinities of monomeric species composing the dimer. When dissociation of a proton-bound dimer is not associated with reverse activation barrier our method provides reliable relative energetics and dynamics.
2003. "Surface-Induced Dissociation of Peptide Ions: Kinetics and Dynamics." Journal of the American Society for Mass Spectrometry 14(12):1340-1347. doi:10.1016/j.jasms.2003.08.004 Abstract Kinetics and dynamics studies have been carried out for the surface-induced dissociation (SID) of a set of model peptides utilizing a specially designed electro spray ionization Fourier Transform Ion cyclotron resonance mass spectrometer in which mass-selected and vibrationally relaxed ions are collided on an orthogonally-mounted fluorinated self-assembled monolayer on Au {111} crystal. The sampling time in this apparatus can be varied from hundreds of microseconds to tens of seconds, enabling the investigation of kinetics of ion decomposition over an extended range of decomposition rates. RRKM-based modeling of these reactions for a set of polyalanines demonstrates that kinetics of these simple peptides is very similar to slow, multiple-collision activation and that the distribution of internal energies following collisional activation is indistinguishable from a thermal distribution. For more complex peptides comprised of several amino acids and with internal degrees of freedom (DOF) of the order of 350 there is a dramatic change in kinetics in which RRKM kinetics is no longer capable of describing the decomposition of these complex ions. A combination of RRKM kinetics and the “sudden death” approximation, according to which decomposition occurs instantaneously, is a satisfactory description. This implies that a population of ions – which is dependant on the nature of the peptide, kinetic energy and sampling time – decomposes on or very near the surface. The shattering transition is described quantitatively for the limited set of molecules examined to date.
2003. "Shattering of Peptide Ions on Self-Assembled Monolayer Surfaces. ." Journal of the American Chemical Society 125(6):1625-1632. Abstract ABSTRACT Time- and energy-resolved surface-induced dissociation (SID) of des-Arg1- and des-Arg9-bradykinin on a fluorinated self-assembled monolayer (SAM) surface was studied using a novel Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved data was modeled using an RRKM-based approach developed in our laboratory. Both experimental observations and modeling establish a very sharp transition in the dynamics of ion-surface interaction: the shattering transition. Shattering opens a variety of dissociation pathways that are not accessible to slow collisional and thermal ion activation. This results in a much better sequence coverage for the singly protonated peptides than dissociation patterns obtained with any of the slow activation methods. Modeling demonstrated that for short reaction delays dissociation of these peptides is solely determined by shattering. Internal energies required for shattering transition are approximately the same for des-Arg1 and des-Arg9-bradykinin, resulting in the overlap of fragmentation efficiency curves obtained at short reaction delays. At longer delay times parent ions depletion is mainly determined by a slow decay rate and fragmentation efficiency curves for des-Arg1 and des-Arg9-bradykinin diverge. Dissociation thresholds of 1.17 and 1.09 eV and activation entropies of -22.2 and -23.3 cal/mol K were obtained for des-Arg1 and des-Arg9-bradykinin from RRKM modeling of time-resolved data. Dissociation parameters for des-Arg1-bradykinin are in good agreement with parameters derived from thermal experiments. However, there is a significant discrepancy between the thermal data and dissociation parameters for des-Arg9-bradykinin obtained in this study. The difference is attributed to the differences in conformations that undergo thermal activation and activation by ion-surface collisions prior to dissociation.
2003. "Shattering of Peptide Ions on Self-Assembled Monolayer Surfaces." Journal of the American Chemical Society 125(6):1625-1632. Abstract Time- and energy-resolved surface-induced dissociation (SID) of des-Arg1- and des-Arg9-bradykinin on a fluorinated self-assembled monolayer (SAM) surface was studied using a novel Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved data was modeled using an RRKM-based approach developed in our laboratory. Both experimental observations and modeling establish a very sharp transition in the dynamics of ion-surface interaction: the shattering transition. Shattering opens a variety of dissociation pathways that are not accessible to slow collisional and thermal ion activation. This results in a much better sequence coverage for the singly protonated peptides than dissociation patterns obtained with any of the slow activation methods. Modeling demonstrated that for short reaction delays dissociation of these peptides is solely determined by shattering. Internal energies required for shattering transition are approximately the same for des-Arg1 and des-Arg9-bradykinin, resulting in the overlap of fragmentation efficiency curves obtained at short reaction delays. At longer delay times parent ions depletion is mainly determined by a slow decay rate and fragmentation efficiency curves for des-Arg1 and des-Arg9-bradykinin diverge. Dissociation thresholds of 1.17 and 1.09 eV and activation entropies of ?22.2 and -23.3 cal/mol K were obtained for des-Arg1 and des-Arg9-bradykinin from RRKM modeling of time-resolved data. Dissociation parameters for des-Arg1-bradykinin are in good agreement with parameters derived from thermal experiments. However, there is a significant discrepancy between the thermal data and dissociation parameters for des-Arg9-bradykinin obtained in this study. The difference is attributed to the differences in conformations that undergo thermal activation and activation by ion-surface collisions prior to dissociation.
2003. "Entropy is the Major Driving Force for Fragmentation of Proteins and Protein-Ligand Complexes in the Gas-Phase." Journal of Physical Chemistry A 107:5836-5839. Abstract This paper presents a critical analysis of Arrhenius parameters for gas-phase fragmentation of proteins and protein-ligand complexes reported in the literature. We demonstrate that there is a surprisingly strong correlation between the Arrhenius activation energy (Ea) and the pre-exponential factor (A). This correlation becomes extremely important for reactions characterized by very high or very low values of A. This correlation is a direct consequence of the relative change in the spacing between vibrational levels of the reactant and the transition state for reaction. Converting the Arrhenius activation energy into the threshold energy for the reaction using Tolman’s theorem reveals the true magnitude of the correlation between molecular complexity and stability. Tolman’s correction factor (Ecorr) increases linearly with Log(A) from 3 kcal/mol for Log(A) of 16.2 to 36.4 kcal/mol for Log(A) of 39.2. Threshold energies extracted from the Arrhenius activation parameters for 56 different reactions are the same within the experimental error bars, while the pre-exponential factors differ by many orders of magnitude. This indicates that activation entropy is the major driving force for dissociation of proteins and protein-ligand complexes in the gas phase.
2003. "Energy Transfer in Collisions of Peptide Ions with Surfaces." Journal of Chemical Physics 119(6):3413-3420. Abstract Time- and energy-resolved surface induced (SID) dissociation of a singly protonated octapeptide des-Arg1-bradykinin (PPGFSPFR) was used to study the effect of physical properties of the SID target on the efficiency of translational to vibrational energy transfer (T -> V) in collisions of peptide ions with surfaces. Four SID targets of varying chemical composition and stiffness were examined in this work: self-assembled monolayers of 1-dodecane thiol (HSAM) and its fluorinated analog (CF3(CF2)9C2H4SH - FSAM) on gold, a 300 nm thick layer of lithium fluoride (LiF) on a polished titanium surface, and a 2 µm carbon vapor deposited diamond layer on a titanium surface. An RRKM-based modeling approach was utilized to extract internal energy distributions deposited into the precursor ion upon collisions with different surfaces. We found that the percent of T -> V transfer increases in the order: HSAM (10.1%), LiF (12.0 %), diamond (19.2 %), FSAM (20.5 %). Furthermore, the width of the energy deposition function (EDF) is affected by the properties of the SID target. Collisions of peptide ions with the HSAM surface results in deposition of relatively narrow internal energy distributions with the width of the EDF increasing in the order: HSAM < FSAM < LiF < Diamond. The results demonstrate that surface stiffness has a major effect on the width of the EDF, while the average energy deposited into the ion is mainly affected by the mass of the chemical moiety representing an immediate collision partner for the ion impacting the surface.
2003. "Collisional Activation of Peptide Ions in FT-ICR Mass Spectrometry." Mass Spectrometry Review 22:158-181. Abstract In the last decade characterization of complex molecules, particularly biomolecules became a focus of both fundamental and applied research in mass spectrometry. Most of these studies utilize tandem mass spectrometry (MS/MS) for obtaining structural information for complex molecules. . Tandem mass spectrometry (MS/MS) typically involves the mass selection of a primary ion, its activation by collision or photon excitation, unimolecular decay into fragment ions characteristic of the ion structure and its internal excitation, and mass analysis of the fragment ions. Although the fundamental principles of tandem mass spectrometry of relatively small molecules are fairly well understood, our understanding of the activation and fragmentation of large molecules is much more primitive. For small ions a single energetic collision is sufficient to dissociate the ion but this is not the case for complex molecules. For large ions two fundamental limits severely constrain fragmentation in tandem mass spectrometry. First the center-of-mass collision energy?the absolute upper limit of energy transfer in a collision process?decreases with increasing mass of the projectile ion for fixed ion kinetic energy and neutral mass. Secondly, the dramatic increase in density of states with increasing internal degrees of freedom of the ion decreases the rate of dissociation by many orders of magnitude at a given internal energy. Consequently most practical MS/MS experiments with complex ions involve multiple collision activation (MCA-CID), multi-photon activation or surface-induced dissociation (SID). This review is focused on what has been learned in recent research studies concerned with fundamental aspects of MCA-CID and SID of model peptides with emphasis on experiments carried out using Fourier transform ion cyclotron resonance mass spectrometers (FT-ICR MS). These studies provide the first quantitative comparison of gas-phase multiple-collision activation and SID of peptide ions. Combining collisional energy-resolved data with RRKM-based modeling revealed the effect of peptide size and identity on energy transfer in collisions ? very important characteristics of ion activation from both fundamental and the analytical perspectives. Finally, the combination of FT-ICR with SID was utilized to carry out the first time-resolved experiments, which examine the kinetics of peptide fragmentation. This has lead to the discovery that the time dependence of ion dissociation varies smoothly up to certain collision energy and then shifts dramatically to a time independent, extensive dissociation. This near instantaneous ?shattering? of the ion generates a large number of relatively small fragment ions. Shattering of ions on surfaces opens up a variety of dissociation pathways that are not accessible using multiple-collision and multiphoton excitation.
2003. "Energetics of Selective Cleavage at Acidic Residues Studied by Time-and Energy-Resolved Surface-Induced Dissociation in FT-ICR-MS." International Journal of Mass Spectrometry 222(1-3):313-327. Abstract Surface-induced dissociation (SID) of four model peptides: LDIFSDF, LDIFSDFR, RLDIFSDF, and LEIFSEFR, was studied using a novel Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. The energetics and dynamics of selective cleavages at acidic residues were deduced by modeling the time- and energy-resolved fragmentation efficiency curves (TFECs) using an RRKM based approach developed in our laboratory. RRKM modeling revealed that addition of a basic residue to the C-terminus of a peptide has a very small effect on the dissociation threshold. However, the dynamics of dissociation is dramatically affected by the presence of the arginine. The Arrhenius pre-exponential factorfor dissociation of LDIFSDF is two orders of magnitude higher than the pre-exponential factor for dissociation of arginine-containing peptides. The difference in the pre-exponential factors is indicative of a complex rearrangement process associated with selective fragmentation. Molecular mechanics modeling of the four parent ions gives some qualitative insight into the differences in fragmentation mechanisms.
2002. ""Dissociation of Noncovalent Protein Complexes by Triple Quadrupole Tandem Mass Spectrometry: Comparison of Monte Carlo Simulation and Experiment"." International Journal of Mass Spectrometry 221(3):245-262. Abstract We present Monte Carlo simulations of collisionally activated dissociation of noncovalent protein complexes in the collision cell of a triple quadrupole mass spectrometer. These simulations account for the influence of the precursor ion mass-to-charge ration, acceleration potential, nature of the collision gas and the number of collisions experienced by the ion on the efficiency of ion dissociation. Evolution of the translational and internal energies of activated precursor ions along the collision cell was simulated using the hard-sphere and diffuse scattering models. Dissociation rate constants and ion survival probability were calculated based on the estimated internal energy content of the excited ion. It was found that dissociation of the precursor ion could occur anywhere in the collision cell provided there is enough gain of relative internal energy accumulated during collisions. Simulated dissociation curves are in a good agreement with experimental results obtained for different collision gases at different pressures. Monte Carlo simulations of collision events modeled by the hard-sphere and diffuse scattering approximations resulted in different dynamics of precursor ion dissociation. Comparison of Monte Carlo simulations with results obtained using the previously proposed collision model showed a good correlation between the integral results obtained using these two approaches over a variety of experimental parameters. However, in contrast to the collision model, Monte Carlo simulations allow to obtain detained information on the dynamics of ion dissociation in the collision cell. Moreover, Monte Carlo simulations provided a first insight into collisionally activated dissociation of the precursor ion under unfolding conditions including realistic increase in collision cross section during ion passage through the collision cell.
2002. "Surface-induced dissociation in a Fourier transform ion cyclotron resonance mass spectrometer: Instrument design and evaluation." Analytical Chemistry 74:3255-3261. Abstract A new Fourier Transform Ion Cyclotron Resonance mass spectrometer (FT-ICR MS) has been constructed in our laboratory. The instrument employs surface-induced dissociation (SID) as an activation method for obtaining structural information on biomolecules in the gas phase. Tandem SID mass spectra can be acquired using either a continuous or a pulsed mode of operation. Collision energy of precursor ion is controlled by a dc offset of the ICR cell. This approach eliminates defocusing of the ion beam by the ion transfer optics as a function of ion kinetic energy and constitutes a significant improvement over our previous experimental setup. Furthermore, it can be easily implemented on any FT-ICR mass spectrometer. Very high signal-to-noise ratios of 200-500 were obtained in single-scan SID mass spectra of model peptides with acquisition time less than 1.1 s. Reasonable SID signal was detected in single-scan spectra with total acquisition time of only 0.3 s. The high signal-to-noise ratio and the fast acquisition time point on a potential application of SID for high-throughput studies in FT-ICR MS.
2002. "On the Relative Stability of Singly Protonated des-Arg1 and des-Arg9 Bradykinins. ." Journal of Physical Chemistry A (106):9832-9836. Abstract Normal-incidence surface-induced dissociation of singly protonated des-Arg1-bradykinin (PPGFSPFR) and des-Arg9-bradykinin (RPPGFSPF) has been studied using a specially designed Fourier Transform Ion Cyclotron Resonance mass spectrometer (FT-ICR MS). We found that with a reaction time of 1 s the collision energy-resolved fragmentation efficiency curve (FEC) for des-Arg9-bradykinin is shifted to lower energies by about 4 eV relative to the FEC for des-Arg1-bradykinin. Because the Arrhenius activation energies found by Williams and co-workers in BIRD experiments are 0.82 eV and 1.2 eV for des-Arg1 and des-Arg9 bradykinin, respectively, we expected to find the reverse order in our long reaction time FTICR-SID experiments. We rationalize the difference between our data and the thermal kinetics using Tolman?s theorem to calculate threshold energies from the Arrhenius activation parameters. The threshold energies are 1.15 eV and 1.24 eV for des-Arg1 and des-Arg9 bradykinin, respectively. However, des-Arg1-bradykinin dissociates via a very tight transition state. Consequently, microcanonical rate-energy dependencies for these two peptides cross at low internal energies. Rate constants of 1 s-1 are reached at internal energies of 7.9 eV for des-Arg1-bradykinin and 6.9 eV for des-Arg9 bradykinin, in excellent agreement with the energy shift in our FECs.
2002. "On the Efficiency of Energy Transfer in Collisional Activation of Small Peptides." Journal of Chemical Physics 116(10):4302-4310. Abstract We present a study of the efficiency of internal excitation of small peptide ions upon multiple-collision activation with Ar and ion-surface interaction with self-assembled monolayers of fluorinated alkylthiol on gold. Internal energy distributions are extracted from RRKM modeling of collision energy-resolved fragmentation efficiency curves for protonated tri-, tetra-, penta-alanine, and prolyltetra-alanine. The efficiency of T?V transfer in surface collisional activation decreases for larger peptides. This is readily rationalized by the corresponding decrease in the center-of-mass collision energy. For all peptides except protonated dialanine, energy transfer upon multiple-collision activation is rather insensitive to the peptide size and composition. The average energy deposited into protonated dialanine is substantially lower than the excitation level achieved for other peptides. Master equation modeling revealed that energy-transfer efficiency in peptide collisions with Ar is the same for all peptides excluding (AA)H+. The results suggest that protonated dialanine has a more extended structure than larger peptides studied in this work.
2002. "Fragmentation Energetics of Small Peptides from Multiple-Collision Activation and Surface-Induced Dissociation in FT-ICR MS." International Journal of Mass Spectrometry 219:189-201. Abstract Collision-induced and surface-induced dissociation of protonated tri- and tetraalanine ((AAA)H+ and (AAAA)H+) were studied using a 7 T Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS). Energy-resolved fragmentation efficiency curves obtained using both techniques were modeled using RRKM/QET formalism. The stability of small polyalanines decreases with increasing peptide size. The major reaction channels of protonated di- tri-and tetraalanine have dissociation thresholds of 2.11 eV, 1.46 eV and 1.23 eV, respectively. The rates of formation of C-terminal and N-terminal fragments from protonated tetraalanine are the same, while C-terminal fragmentation of trialanine is energetically more demanding than N-terminal fragmentation.
2002. "Relative Proton Affinities from Kinetic Energy Release Distributions for Dissociation of Proton-Bound Dimers." Journal of Physical Chemistry A 106(50):12051-12057. Abstract Kinetic energy release distributions (KERDs) upon dissociation of proton-bound dimers are utilized along with Finite Heat Bath theory analysis to obtain relative proton affinities of monomeric species composing the dimer. The proposed approach allows accurate measurement of relative proton affinities based on KERD measurements for the compound with unknown thermochemical properties versus a single reference base. It also allows distinguishing the cases when dissociation of proton-bound dimers is associated with reverse activation barrier, for which both our approach and the kinetic method become inapplicable. Results are reported for the n-butanol-n-propanol dimer, for which there is no significant difference in entropy effects for two reactions and for the pyrrolidine-1,2-ethylenediamine dimer, which is characterized by a significant difference in entropy effects for the two competing reactions. Relative protonation affinities of -1.0?0.3 kcal/mol for the n-butanol-n-propanol pair and 0.27?0.10 kcal/mol for the pyrrolidine-1,2-ethylenediamine pair are in good agreement with literature values. Relative reaction entropies were extracted from the branching ratio and KERD measurements. Good correspondence was found between the relative reaction entropies for the n-butanol-n-propanol dimer (D(DS?) =-0.3?1.5 cal/mol K) and the relative protonation entropy for the two monomers (D(DSp)=0). However, the relative reaction entropy for the pyrrolidine-1,2-ethylenediamine dimer is higher than the difference in protonation entropies (D(DS?) =8.2?0.5 cal/mol K vs. D(DSp)=5 cal/mol K).
2001. "A Comparative Study of Collision-Induced and Surface-Induced Dissociation. II. Fragmentation of Small Alanine-Containing Peptides in FT-ICR MS. ." Journal of Physical Chemistry B 105(9):1895-1900. Abstract Multiple collision activation (MCA-CID) and surface-induced dissociation (SID) of protonated trialanine, (AAA)H+; tetraalanine, (AAAA)H+; pentaalanine, (AAAAA)H+; and prolyltetraalanine, (PAAAA)H+ were studied using a 7 T Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS). Fragmentation efficiency curves obtained using both techniques were compared by converting the SID collision energy into an "effective" center-of-mass frame using an arbitrary neutral encounter of mass, MN. The best overlap between the SID and MCA-CID fragmentation efficiency curves was obtained using very similar values of MN for all the peptides, indicating that all the protonated precursor ions were undergoing similar interaction with terminal chemical groups on the fluorinated self-assembled monolayer (SAM) surface. Collision energy-resolved fragmentation efficiency curves obtained using both methods are very similar and both methods of collisional activation result in a quasi-thermal population of ion internal energies. We suggest that ion-surface collisions involve multiple interactions of the projectile ion with chemical groups on the surface with efficient transfer of impact energy into the surface and into the internal energy of the ion. This mechanism, except for time frame for the activation process, is analogous to the sequential gas phase collisional activation of these model peptides.
2000. "The Theoretical Basis of the Kinetic Method from the point of View of Finite Heat Bath Theory." Journal of Physical Chemistry A 104(38):8829-8837. Abstract We present a rigorous theoretical basis of the kinetic method based on Finite Heat Bath Theory (FHBT) developed by Klots. A simple analytical expression for the branching ratio is derived from FHBT formalism. This expression simplifies to the expression given by the absolute reaction rate theory 1) for very large clusters or 2) for reactions having a negligible kinetic shift. The reacting population is described by two different temperatures rather than by the "effective" temperature as suggested previously. Simulations performed using both RRKM and FHBT revealed that the kinetic plots are slightly non-linear. The observed curvature is related to the changes in the transition state temperature as a function of the critical energy for fragmentation. The curvature of the plots decreases for larger clusters. We show that the "effective" temperature closely resembles the average value of the transition state temperature. This allows us to assign a new definition of the effective temperature and predict its properties. The results of simulations confirm that the extended version of the kinetic method introduced by Fenselau and co-workers provides accurate relative energetics for competitive reactions for both small and large ions. However, accurate thermochemical information can be obtained from the kinetic method only if reactions under investigation have negligible reverse activation energies. A new approach for extracting relative fragmentation energetics and entropy differences for two competing reactions is proposed. This approach requires a measurement of kinetic energy release distributions (KERDs) for the two fragmentation channels; the relative energetics and dynamics can be extracted from a single measurement.
2000. "Internal Energy Distributions Resulting from Sustained Off-Resonance Excitation in Fourier Transform Ion Syclotron Resonance Mass Spectrometry.II. Fragmentation of the 1-Bromonaphthalene Radical Cation ." Journal of Physical Chemistry A 104(23):5484-5494. Abstract The collision energy dependence of the fragmentaiton of C10H7Br+? was studied using sustained off-rsonance excitation (SORI) in a 7 T Fourier Trnasform Mass Spectrometer (FTMS). Fragmentation efficiency curves were obtained as a function of collision energy at four different pressures of Ar bath gas corresponding to collision numbers of 3, 5, 15 and 20. The results were modeled using RRKM/QET formalism. A refined analytical form for the collisional energy deposition function is prposed. The ability to obtain accurate framentation energetics of a complex system using the present approach is demonstrated. The "effective temperatures" deduced from the average internal energies for C6H5Br+? and C10H7Br+? were found to be the same for both ions provided the bath gas pressure and the maximum value of center-of-mass collision energy were the same.
2000. "A Comparative Study of Collision-Induced and Surface-Induced Dissociation. I Fragmentation of Protonated Dialanine." Journal of the American Chemical Society 122(40):9703-9714. Abstract Collision-induced and surface-induced dissociation of protonated dialanine (Ala-AlaH+) was studied using a 7 T Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS). Energy-resolved fragmentation efficiency curves were obtained using both techniques. The results were modeled using RRKM/QET formalism. The energetics and dynamics of the four primary and some secondary fragmentation pathways were determined from the RRKM modeling. Both multiple-collision CID and SID results could be reproduced within the same model. The strong correspondence of fragmentation efficiency curves obtained in the high-pressure CID and SID experiments indicates that the internal energy distributions of Ala-AlaH+ activated by multiple collisions and by surface impact are remarkably similar.