Scientific Publications 2005
2005. "The Cbf5-Nop10 Complex is a Molecular Bracket that Organizes Box H/ACA RNPs." Nature Structural & Molecular Biology 12(12):1101-1107. doi:10.1038/nsmb1036 Abstract Box H/ACA ribonucleoprotein particles (RNPs) catalyze RNA pseudouridylation and direct processing of ribosomal RNA, and are essential architectural components of vertebrate telomerases. H/ACA RNPs comprise four proteins and a multihelical RNA. Two proteins, Cbf5 and Nop10, suffice for basal enzymatic activity in an archaeal in vitro system. We now report their cocrystal structure at 1.95-A resolution. We find that archaeal Cbf5 can assemble with yeast Nop10 and with human telomerase RNA, consistent with the high sequence identity of the RNP componenets between archaea and eukarya. Thus, the Cbf5-Nop10 architecture is phylogenetically conserved. The structure shows how Nop10 buttresses the active site of Cbf5, and it reveals two basic troughs that bidirectionally extend the active site cleft. Mutagenesis results implicate an adjacent basic patch in RNA binding. This tripartite RNA-binding surface may function as a molecular bracket that organizes the multihelical H/ACA and telomerase RNAs.
2005. "Optical, Physical and Chemical Properties of Tar Balls Observed During the Yosemite Aerosol Characterization Study." Journal of Geophysical Research. D. (Atmospheres) 110:Art. No. D21210. doi:10.1029/2004JD005728 Abstract The Yosemite Aerosol Characterization Study of summer 2002 (YACS) occurred during an active fire season in the western U. S., and provided an opportunity to investigate many unresolved issues related to the radiative effects of biomass burning aerosols. Single particle analysis was performed on field collected aerosol samples using an array of electron microscopy techniques. Amorphous carbon spheres, or “tar balls”, were present in samples collected during episodes of high particle light scattering coefficients that occurred during the peak of a smoke/haze event. The highest concentrations of light-absorbing carbon from a dual-wavelength aethalometer (λ = 370 and 880 nm) occurred during periods when the particles were predominantly tar balls, indicating they do absorb light in the UV and near-IR range of the solar spectrum. Closure experiments of mass concentrations and light scattering coefficients during periods dominated by tar balls did not require any distinct assumptions of organic carbon molecular weight correction factors, density, or refractive index compared to periods dominated by other types of organic carbon aerosols. Measurements of the hygroscopic behavior of tar balls using an environmental SEM indicate that tar balls do not exhibit deliquescence, but do uptake some water at high (~83 %) relative humidity. The ability of tar balls to efficiently scatter and absorb light, and to absorb water has important implications for their role in regional haze and climate fence.
2005. "Finding Adiabatically Bound Anions of Guanine through a Combinatorial Computational Approach." Angewandte Chemie International Edition 44(40):6585-6588. doi:10.1002/anie.200501671 Abstract In summary, guanine supports many adiabatically bound valence anions, which result from enamine-imine transformations of the most stable neutral tautomers. These stable anionic tautomers have been found using combinatorial-computational prescreening at the B3LYP level of theory followed by CCSD(T)/aug-cc-pVDZ calculations. The new anionic tautomers contradict a common opinion that guanine has the lowest electron affinity among nucleobases. The new anionic tautomers might be formed in the course of dissociative electron attachment followed by a hydrogen atom attachment to a carbon atom. They might affect the structure and properties of DNA and RNA exposed to low-energy electrons. Chemical transformations of DNA triggered by the new anionic tautomers will be explored in our future studies.
2005. "Efficient Simulation Method for Polarizable Protein Force Fields: Application to the Simulation of BPTI in Liquid Water." Journal of Chemical Theory and Computation 1(1):169-180. Abstract A methodology for large scale molecular dynamics simulation of a solvated polarizable protein, using a combination of permanent and inducible point dipoles with fluctuating and fixed charges, is discussed and applied to the simulation of water solvated bovine pancreatic trypsin inhibitor (BPTI). The electrostatic forces are evaluated using a generalized form of the P3M Ewald method which includes point dipoles in addition to point charge sites. The electrostatic configuration is propagated along with the nuclei during the course of the simulation using an extended Lagrangian formalism. For the system size studied, 20000 atoms, this method gives only a marginal computational overhead relative to nonpolarizable potential models (1.23-1.45) per time step of simulation. The models employ a newly developed polarizable dipole force field for the protein1 with two commonly used water models TIP4P-FQ and RPOL. Performed at constant energy and constant volume (NVE) using the velocity Verlet algorithm, the simulations show excellent energy conservation and run stably for their 2 ns duration. To characterize the accuracy of the solvation models the protein structure is analyzed. The simulated structures remain within 1 Å of the experimental crystal structure for the duration of the simulation in line with the nonpolarizable OPLS-AA model.
2005. "Search for Improved Host Architectures: Application of de Novo Structure-Based Design and High-Throughput Screening Methods to Identify Optimal Building Blocks for Multidentate Ethers." Journal of the American Chemical Society 127(48):17043-17053. doi:10.1021/ja055169x Abstract This paper presents a computational approach to the deliberate design of improved host architectures. The approach, which involves the use of computer-aided design software, is illustrated by application to cation hosts containing multiple aliphatic ether oxygen binding sites. De novo molecule building software, HostDesigner, is interfaced with molecular mechanics software, GMMX, providing a tool for generating and screening millions of potential bidentate building block structures. Enhanced cation binding affinity can be achieved when highly organized building blocks are used to construct macrocyclic hosts.
2005. "Structural Design Criteria for Anion Hosts: Strategies for Achieving Anion Shape Recognition through the Complementary Placement of Urea Donor Groups." Journal of the American Chemical Society 127(6):1810-1819. Abstract The arrangement of urea ligands about different shaped anions has been evaluated with electronic structure calculations. Geometries and binding energies are reported for urea complexes with Cl—, NO3—, and ClO4—. The results yield new insight into the nature of urea-anion interactions and provide structural criteria for the deliberate design of anion selective receptors containing two or more urea donor groups.
2005. "Relationship between the structural and magnetic properties of Co-doped SnO₂ nanoparticles." Physical Review. B, Condensed Matter and Materials Physics 72(7):518-524. Abstract In this paper, we present the results of a detailed investigation of the structural, optical and magnetic properties of chemically synthesized pure and Co doped SnO₂ powders using x-ray diffraction (XRD), diffuse reflectance spectroscopy, Raman spectroscopy, x-ray photoelectron spectroscopy and magnetometry. For low doping concentrations of Co (≤ 1%), the SnO₂ lattice contracts and a ferromagnetic behavior is developed. Increasing the Co doping concentration to > 1% leads to a rapid expansion of the lattice and significant structural disorder evidenced by changes in the XRD and Raman spectra presumably due to additional interstitial incorporation of Co. This higher Co doping completely destroys the ferromagnetism. The striking similarity between the changes in the lattice parameters and the magnetic properties of Sn₁-xCoxO₂ indicates a structure-magnetic property relationship.
2005. "Acetone and Water on TiO₂ (110): Competition for Sites." Langmuir 21(8):3443-3450 . Abstract The competitive interaction between acetone and water for surface sites on TiO₂ (110) was examined using temperature programmed desorption (TPD). Two surface pretreatment methods were employed, one involving vacuum reduction of the surface by annealing at 850 K in ultrahigh vacuum (UHV) and another involving surface oxidation with molecular oxygen. In the former case the surface possessed about 7% oxygen vacancy sites and in the latter reactive oxygen species (adatoms and molecules) were deposited on the surface as a result of oxidative filling of vacancy sites. On the reduced surface, excess water displaced all but about 20% of a saturated d6-acetone first layer to physisorbed desorption states, whereas about 40% of the first layer d6-acetone was stabilized on the oxidized surface against displacement by water through a reaction between oxygen and d6-acetone. The displacement of acetone on both surface is explained in terms of the relative desorption energies of each molecule on the clean surface and role of intermolecular repulsions in shifting their respective desorption features to lower temperatures with increasing coverage. Although first layer water desorbs from TiO₂ (110) at slightly lower temperature (275 K) than submonolayer coverages of d6-acetone (340 K), intermolecular repulsions between d6-acetone molecules shift its leading edge for desorption to 170 K as the first layer is saturated In contrast, the desorption leading edge for first layer water (with or without coadsorbed d6-acetone) was at 210 K. This small difference in the onsets for d6-acetone and water desorption resulted in the majority of d6-acetone being compressed into islands by water and eventually displaced from the first layer when excess water was adsorbed. On the oxidized surface the species resulting from reaction of d6-acetone and oxygen was not influence by increasing water coverages. This species was stable on the clean surface up to 375 K (well past the first layer water TPD feature) where it decomposed mostly back to d6-acetone and atomic oxygen. These results are discussed in terms of the influence of water in inhibiting acetone photo-oxidation on TiO₂ surfaces.
2005. "Acetone and Water on TiO₂(110): H/D Exchange." Langmuir 21(8):3451-3458. Abstract Isotopic H/D exchange between coadsorbed acetone and water on the TiO₂(110) surface was examined using temperature programmed desorption (TPD) as a function of coverage and two surface pretreatments (oxidation and reduction). Coadsorbed acetone and water interact repulsively on reduced TiO₂(110) based on results from the companion paper to this study, with water exerting a greater influence in destabilizing acetone and acetone having only a nominal influence on water. Despite the repulsive interaction between these coadsorbates, about 0.02 ML of a 1 ML d6-acetone on the reduced surface exhibits H/D exchange with coadsorbed water, with the exchange occurring exclusively in the high temperature region of the d₆-acetone TPD spectrum at ~340 K. The effect was confirmed with combinations of d₀-acetone and D₂O. The extent of exchange decreased on the reduced surface with water coverages above ~0.3 ML due to the ability of water to displace coadsorbed acetone from first layer sites to the multilayer. In contrast, the extent of exchange increased by a factor of 3 when the surface was pre-oxidized prior to coadsorption. In this case, there was no evidence for the negative influence of high water coverages on the extent of H/D exchange. Comparison of the TPD spectra from the exchange products (either d₁- or d₅-acetone depending on the coadsorption pairing) suggests that, in addition to the 340 K exchange process seen on the reduced surface, a second exchange process was observed on the oxidized surface at ~390 K. In both cases (oxidized and reduced), desorption of the H/D exchange products appeared to be reaction limited and to involve the influence of OH/OD groups (or water formed during recombinative desorption of OH/OD groups) instead of molecularly adsorbed water. The 340 K exchange process is assigned to reaction at step sites and the 390 K exchange process is attributed to the influence of oxygen adatoms deposited during surface oxidation. The H/D exchange mechanism likely involves an enolate or propenol surface intermediate formed transiently during the desorption of oxygen-stabilized acetone molecules.
2005. "Photooxidation of Acetone on TiO2(110): Conversion to Acetate via Methyl Radical Ejection." Journal of Physical Chemistry B 109(24):12062-12070. Abstract It is generally held that radicals form and participate in heterogeneous photocatalytic processes on oxide surfaces, although understanding the mechanistic origins and fates of such species is difficult. In this study, photodesorption and thermal desorption techniques show that acetone is converted into acetate on the surface of TiO₂(110) in a two step process that involves, first, a thermal reaction between acetone and coadsorbed oxygen to make a surface acetone-oxygen complex, followed second by a photochemical reaction that ejects a methyl radical from the surface and converts the acetone-oxygen complex into acetate. Designation of the photodesorption species to methyl radicals was confirmed using isotopically labeled acetone. The yield of photodesorbed methyl radicals correlates well with the amount depleted of acetone and with the yield of acetate left on the surface, both gauged using post-irradiation temperature programmed desorption (TPD). The thermal reaction between adsorbed acetone and oxygen to form the acetone-oxygen complex exhibits an approximate activation barrier of about 10 kJ/mol. A prerequisite to this reaction is the presence of surface Ti³⁺ sites that enable O₂ adsorption. Creation of these sites by vacuum reduction of the surface prior to acetone and oxygen co-adsorption results in an initial spike in the photodecomposition rate, but replenishment of these sites by photolytic means (i.e., by trapping excited electrons at the surface) appears to be a slow step a sustained reaction. Evidence in this study for the ejection of organic radicals from the surface during photo-oxidation catalysis on TiO₂ provides support for mechanistic pathways that involve both adsorbed and non-adsorbed species.
2005. "Parsing ERK Activation Reveals Quantitatively Equivalent Contributions From Epidermal Growth Factor Receptor and HER2 In Human Mammary Epithelial Cells." Journal of Biological Chemistry 280(7):6157-6169. Abstract HER2, a member of the EGFR tyrosine kinase family, functions as an accessory EGFR signaling component and alters EGFR trafficking by heterodimerization. HER2 overexpression leads to aberrant cell behavior including enhanced proliferation and motility. Here we apply a combination of computational modeling and quantitative experimental studies of the dynamic interactions between EGFR and HER2, and their downstream activation of extracellular signal-related kinase (ERK) to understand this complex signaling system. Using cells expressing different levels of HER2 relative to the EGFR, we can separate relative contributions of EGFR and HER2 to signaling amplitude and duration. Based on our model calculations, we demonstrate that, in contrast with previous suggestions in the literature, the intrinsic capabilities of EGFR and HER2 to activated ERK are quantitatively equivalent . We find that HER2-mediated effects on EGFR dimerization and trafficking are sufficient to explain the detected HER2-mediated amplification of EGF-induced ERK signaling. Our model suggests that transient amplification of ERK activity by HER2 arises predominantly from the 2-to-1 stoichiometry of receptor kinase to bound ligand in EGFR/HER2 heterodimers compared to the 1-to-1 stoichiometry of the EGFR homodimer, but alterations in receptor trafficking, with resultant EGFR sparing, cause the sustained HER2-mediated enhancement of ERK signaling.
2005. "Surface Electronic Properties and Site-Specific Laser Desorption Processes of Highly Structured Nanoporous MgO Thin Films." Surface Science 593(1-3):242-247. Abstract The surface electronic properties of metal oxides critically depends on low-coordinated sites, such as kinks, corners and steps . In order to characterize experimentally those surface states as well as their role for laser desorption processes, we prepare defect enriched surfaces by growing thin MgO films using reactive ballistic deposition  on crystalline dielectric substrates. With samples held at room temperature, the resulting MgO films are highly textured and consist of porous columns with column lengths ranging from tens of nanometers up to six micrometers. Measurements by x-ray photoelectron spectroscopy (XPS) are carried out in-situ for MgO films, vacuum-cleaved MgO crystals, and water vapor exposed samples. In the case of thin films, we observe O 1s spectra with a significant shoulder feature at 2.3 eV higher binding energy (HBE) than the corresponding peak at 530.0 eV representing regular lattice oxygen. We evaluate this feature in terms of non-stoichiometric oxygen and formation of an oxygen-rich layer at the topmost surface of the MgO columns. In contrast, no HBE-features are detectable from clean single crystal MgO surfaces, while the hydroxyl O 1s band peaks at 531.6 eV. Under excitation with 266-nm-laser-pulses, known to be resonant with low-coordinated surface anions , we observe preferential depletion of defective oxygen-states (HBE signal) and temporary restoration of ideal surface stoichiometry. Furthermore, auxiliary signals are observed on several micrometer thick films, acting like satellites to major photoelectron-peaks (O 1s, Mg 2s, and Mg 2p) but shifted by approximately 4 eV towards lower kinetic energy. These features are depleted by UV-light exposure, pointing to the occurrence of surface-charge imbalance, accompanied by photon stimulated charge-transfer reactions. These results are in line with desorption experiments of neutrals, stimulated by laser excitation at 266 nm. According to the low-coordination nature of nanoporous MgO films, we find that the laser fluence required for oxygen desorption is much lower in comparison to the single-crystal MgO (100) surface. The detected neutral oxygen desorption occurs with a single photon power dependence from nanoporous MgO thin films. This contrasts to the two-photon power dependence observed from MgO single crystals. In fact, the single photon energy of the laser lies within the charge-transfer transition in surface ion pairs with three-coordinated anions (~ 4.7 eV), while a two-photon absorption regime easily lies within the four-coordinated anion charge transfer transition near 5.4 eV . In summary, our XPS studies and laser desorption experiments indicate new surface-site-specific excitation regimes, which might eventually allow for site-specific manipulation of surface morphology.
2005. "Direct Fabrication of Enzyme-Carrying Polymer Nanofibers by Electrospinning." Journal of Materials Chemistry 15(31):3241-3245. Abstract Nanofibers of an enzyme-polymer composite were successfully electrospun with retained biocatalytic activity. The enzyme, -chymotrypsin, was solubilized in toluene via the aid of a surfactant and mixed with a polymer solution of polystyrene and poly(styrene-co-maleic anhydride). The enzyme-polymer solution could be directly electrospun to produce nanofibers with diameters of ~873 nm. The enzyme activity remained stable over the course of one week when nanofibers were treated with 0.1% glutaraldehyde solution. Untreated nanofibers rapidly lost the enzyme activity due to leaching of the enzyme from nanofibers. The nanofiber-based mats were durable and easily recovered from a solution.
2005. "Computational Investigation and Hydrogen/Deuterium Exchange of the Fixed Charge Derivative Tris(2,4,6-Trimethoxyphenyl)Phosphonium: Implications for the Aspartic Acid Cleavage Mechanism." Journal of the American Society for Mass Spectrometry 16(7):1067-1080. Abstract Aspartic acid (Asp)-containing peptides with the fixed charge derivative tris(2,4,6-trimethoxyphenyl) phosphonium (tTMP-P+) were explored computationally and experimentally by H/D exchange and fragmentation studies in order to probe the phenomenon of selective cleavage C-terminal to Asp in the absence of a "mobile" proton. Ab initio modeling of the tTMP-P+ electrostatic potential demonstrates the positive charge is distributed on the phosphonium group and therefore is not initiating or directing fragmentation as would a "mobile" proton. Geometry optimizations and vibrational analyses of different aspartic acid conformations show the aspartic acid structure with a hydrogen bond between the side chain hydroxy and backbone carbonyl lies 2.8 kcal/mol above the lowest energy conformer. In reactions with D2O, the phosphonium-derived doubly charged peptide (H+)P+LDIFSDF rapidly exchanges all 12 of its exchangeable hydrogens for deuterium and also displays a non-exchanging population. With no added proton, P+LDIFSDF exchanges a maximum of four of eleven exchangeable hydrogens for deuterium. No exchange is observed when all acidic groups are converted to the corresponding methyl esters. Together, these H/D exchange results indicate that the acidic hydrogens are "mobile locally" because they are able to participate in exchange even in the absence of an added proton. Fragmentation of two distinct (H+)P+LDIFSDF ion populations shows the non-exchanging population displays selective cleavage, while the exchanging population fragments more evenly across the peptide backbone. This result demonstrates that H/D exchange can sometimes distinguish between and provide a means of separation of different protonation motifs, and that these protonation motifs can have an effect on the fragmentation.
2005. "Laser Control of Desorption Through Selective Surface Excitation." Journal of Physical Chemistry B 109(42):19563-19578. doi:10.1021/jp0523672 Abstract We review recent developments in controlling photo-induced desorption processes of alkali halides and magnesium oxide. We focus primarily on hyperthermal desorption of halogen (and oxygen) atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. Laser desorption can serve as a solid-state source of halogen and oxygen atoms in well defined electronic states and with controlled velocity distributions. The latter can be used for studying chemical processes in the gas phase and at surfaces. We demonstrate that the exciton mechanism of desorption developed for alkali halides can be extended to cubic oxide surfaces.
2005. " Third- and Fourth-Order Perturbation Corrections to Excitation Energies from Configuration Interaction Singles." Journal of Chemical Physics 122:094105-094115. Abstract Abstract for this article not available at this time.
2005. "Fast Electron Correlation Methods for Molecular Clusters in the Ground and Excited States." Molecular Physics 103(15-16):2255-2265. Abstract An efficient and accurate electronic structure method for clusters of weakly interacting molecules has been proposed, on the basis of the pair-interaction method of Kitaura et al., and combined with density functional, many-body perturbation, coupled-cluster, equation-of-motion coupled-cluster, configuration-interaction singles, and time-dependent density functional theories. The method retains the one- and two-body Coulomb, exchange, and correlation energies exactly and higher-order Coulomb energies in the leading order of multipole expansion (hence the dipole polarisation effects). It typically recovers the total energies within 0.001 %, binding energies within a few kilocalories per mole, and excitation energies within a few hundredths of an electron volt of the conventional implementations. The size dependence of the computational cost of the method is asymptotically linear for total energies and constant for excitation energies. The method has been applied to the total energies of water clusters, to the total energies of zwitterionic and neutral glycine–water clusters, and to the excitation energies of formaldehyde–water clusters. The largest calculation was performed at an equation-of-motion coupled-cluster singles and doubles level for a formaldehyde–(H2O)81 cluster containing 247 atoms that predicted the solvatochromic shift of 1360 cm–1 in the lowest transition energy of formaldehyde in water.
2005. "Influence of Surface Morphology on D-2 Desorption Kinetics from Amorphous Solid Water ." Journal of Chemical Physics 122(12):124701-1 - 11. doi:10.1063/1.1874934 Abstract The influence of surface morphology/porosity on the desorption kinetics of weakly bound species was investigated by depositing D2 on amorphous solid water sASWd films grown by low temperature vapor deposition under various conditions and with differing thermal histories. A broad distribution of binding energies of the D2 monolayer on nonporous and porous ASW was measured experimentally and correlated by theoretical calculations to differences in the degree of coordination of the adsorbed H2 sD2d to H2O molecules in the ASW depending on the nature of the adsorption site, i.e., surface valleys vs surface peaks in a nanoscale rough film surface. For porous films, the effect of porosity on the desorption kinetics was observed to be a reduction in the desorption rate with film thickness and a change in peak shape. This can be partly explained by fast diffusion into the ASW pore structure via a simple one-dimensional diffusion model and by a change in binding energy statistics with increasing total effective surface area. Furthermore, the D2 desorption kinetics on thermally annealed ASW films were investigated. The main effect was seen to be a reduction in porosity and in the number of highly coordinated binding sites with anneal temperature due to ASW restructuring and pore collapse. These results contribute to the understanding of desorption from porous materials and to the development of correct models for desorption from and catalytic processes on dust grain surfaces in the interstellar medium.
2005. "Single-Molecule Triplet-State Photon Antibunching at Room Temperature." Journal of Physical Chemistry B 109(20):9861-9864. Abstract We have demonstrated probing single-molecule metal-to-ligand charge transfer (MLCT) dynamics at room temperature. Using photon antibunching measurements under CW laser excitation, non-classical photon statistics, and excitation power dependent measurements, we were able to selectively measure the single-molecule MLTC state lifetime. This work demonstrated the first single-molecule photon antibunching of triplet excited state and presents a step forward in studying single-molecule electron transfer in proteins using MLTC complex as an electron transfer donor or acceptor.
2005. "Conversion of Biomass Syngas to DME Using a Microchannel Reactor." Industrial and Engineering Chemistry Research 44(6):1722-1727. Abstract The capability of a microchannel reactor for direct synthesis of dimethylether (DME) from biomass syngas was explored. The reactor was operated in conjunction with a hybrid catalyst system consisting of methanol synthesis and dehydration catalysts, and the influence of reaction parameters on syngas conversion was investigated. The activities of different dehydration catalysts were compared under DME synthesis conditions. Reaction temperature and pressure exhibited similar positive effects on DME formation. A catalytic stability test of the hybrid catalyst system was performed for 880 hours, during which CO conversion only decreased from 88% to 81%. In the microchannel reactor, the catalyst deactivation rate appeared to be much slower than in a tubular fixed-bed reactor tested for comparison. Test results also indicated that the dehydration reaction rate and the water depletion rate via a water-gas-shift reaction should be compatible in order to achieve high selectivity to DME. Using the microchannel reactor, it was possible to achieve a space time yield almost three times higher than commercially demonstrated performance results. A side-by-side comparison indicated that the heat removal capability of the microchannel reactor was at least six times greater than that of a commercial slurry reactor under similar reaction conditions.
2005. "Line narrowing in H-1 MAS spectrum of mesoporous silica by removing adsorbed H2O using N-2 ." Solid State Nuclear Magnetic Resonance 27(3):200-205. Abstract The peaks for silanol protons in the high resolution 1H NMR spectrum obtained on mesoporous silica materials may be broadened and shifted downfield by hydrogen bonding with absorbed water molecules. The spectrum may be further complicated by overlapping of the resonance for hydrogen-bonded silanol with the corresponding broad peak due to hydrogen-bonded water. These complications hamper a quantitative analysis of the spectra for these and similar materials. It is demonstrated in this paper that absorbed water can be removed by exposing the sample to dry N2 during magic angle spinning. This results in significantly enhanced spectral resolution for the silanol protons in the 1H MAS spectrum. The enhanced spectral resolution makes it possible to quantify the various hydroxyl groups in a complex metal-oxide catalyst. Results obtained on tungsten oxide supported on SBA-15 mesoporous silica materials are reported.
2005. "Observation of d-Orbital Aromaticity." Angewandte Chemie International Edition 44(44):7251-7254. Abstract Aromatic molecules are usually planar species formed by main group elements, whose bonding is primarily from the s or p orbitals. Here we report experimental and theoretical evidence for aromaticity from d-orbitals. Two metal oxide clusters, W3O9- and Mo3O9-, are studied using photoelectron spectroscopy and theoretical calculations, which show that W3O9 and Mo3O9 both possess a D3h structure with a low-lying unoccupied molecular orbital formed from the metal d orbitals. Occupation of this orbital by one or two electrons gives rise to strong three-center one-electron and three-center two-electron metal-metal bonds, respectively, significantly reducing the M-M distances in M3O9- and M3O92-. The totally delocalized three-center bond renders aromaticity for both M3O9- and M3O92-, further evidenced by a large calculated resonance energy. The M3O9- and M3O92- species are highly stable and represent a new class of d-orbital aromatic molecules, which may be synthesized in the condensed phase.
2005. "Sites of Proteolytic Processing and Noncovalent Association of the Distal C-terminal Domain of CaV1.1 Channels in Skeletal Muscle." Proceedings of the National Academy of Sciences of the United States of America 102(14):5274-5279. doi:10.1073/pnas.0409885102 Abstract In skeletal muscle cells, voltage-dependent potentiation of Ca2+ channel activity requires phosphorylation by cyclic AMP-dependent protein kinase (PKA) anchored via an A-kinase anchoring protein (AKAP15), and the most rapid sites of phosphorylation are located in the C-terminal domain. Surprisingly, the site of interaction of the complex of PKA and AKAP15 with the 1 subunit of CaV1.1 channels lies in the distal C-terminus, which is cleaved from the remainder of the channel by in vivo proteolytic processing. Here we report that the distal C-terminus is noncovalently associated with the remainder of the channel via interaction with a site in the proximal C-terminal domain when expressed as a separate protein in mammalian non-muscle cells. Deletion mapping of the C-terminus of the 1 subunit using the yeast two-hybrid assay revealed that a distal C-terminal peptide containing amino acids 1802 to 1841 specifically interacts with a region in the proximal C-terminus containing amino acid residues 1556 to 1612. Analysis of the purified 1 subunit of CaV1.1 channels from skeletal muscle by saturation sequencing of the intracellular peptides by tandem mass spectrometry identified the site of proteolytic processing as alanine 1664. Our results support the conclusion that a noncovalently associated complex of the 1 subunit truncated at A1664 with the proteolytically cleaved distal C-terminal domain, AKAP15, and PKA is the primary physiological form of CaV1.1 channels in skeletal muscle cells.