Scientific Publications 2001
2001. "Comment on "Atomic Many-Body Effects for the p-Shell Photoelectron Spectra of Transition Metals" - Reply ." Physical Review Letters 86(16):3693. Abstract The comment by Taguchi et al. on our Letter about the importance of atomic contributions to the 2p and 3p x-ray photoelectron spectra (XPS) of ionic transition metal materials claims that we have "made serious misconceptions on" their calculations. They also criticize our work because we have not used empirical parameters to improve our agreement with experiment. We disagree with their criticisms.
2001. "Theoretical Studies of Diiron(II) Complexes that Model Features of the Dioxygen-Activating Centers in Non-Heme Diiron Enzymes." Israel Journal of Chemistry 41(3):173-186. Abstract We have applied high-level Density Functional Theory to investigate the properties of recently characterized carboxylate-bridged diiron(II) complexes supported by 2,6-di(p-tolyl)benzoate (ArTolCO2-) ligands. These compounds, prepared as synthetic models for the reduced non-heme diiron centers in the enzymes MMO, RNR-R2, and D9D, reproduce the composition of the first coordination sphere ligands as well as the core geometry. The experimentally observed flexibility of the diiron cores in the model compounds, a main design target, was confirmed computationally. Details of a possible interconversion mechanism that transforms quadruply and doubly carboxylate-bridged isomers of [Fe2(ArTolCO2)4L2], L = pyridine or related ligand, were examined. The orientation of the pyridine ligands plays a major role and promotes an initial carboxylate shift of the bridging carboxylate ligand that is orthogonal to the pyridine ring plane. Alternative mechanisms were explored and evaluated. Structural features of the strongly coupled diiron centers could only be reproduced reliably by using the experimentally determined antiferromagnetic spin-coupling properties of the high-spin d6 iron(II) centers. Use of the ferromagnetic-coupling scheme gave rise to a poor correlation of the computed structure with the experiment. The broken-symmetry orbitals required to describe the antiferromagnetic coupling are compared to the MOs as classical symmetry-adapted linear combinations of atomic orbitals that form the basis for the magnetic coupling scheme. The molecular orbitals responsible for the dependence of the structural results on spin coupling were identified and used to evolve an intuitive explanation for the structural differences observed.
2001. "The Effect of Doping with TI(IV) and Sn(IV) on Oxygen Reduction at Hematite Electrodes ." Journal of the Electrochemical Society 148(2):E85-E91. Abstract We examined the reduction of oxygen at doped hematite (a-Fe2O3) electrodes to determine how the identity of the dopant affected the reaction and how changes in reactivity compared with changes in dopant concentration. Our results indicated that Sn(IV) and Ti(IV) dopants have a similar effect on the open-circuit potential, cathodic transfer coefficient, and exchange current density, suggesting that any bandgap states associated with the introduction of dopants and directly involved in the reduction of oxygen have similar energies. The greatest difference between the electrodes doped with Sn(IV) and Ti(IV) was in the apparent cathodic transfer coefficient. The cathodic transfer coefficients for the Sn-doped electrodes were slightly smaller than those for the Ti-doped, suggesting that the density of interface states is greater in the Sn-doped electrodes. In comparing electrodes with two different dopant concentrations, we found that the relative increase in reactivity was significantly less than the increase in dopant concentration. This may be due to the electrochemical creation of surface Fe(II) sites that catalyze the reduction of O2. Other factors that may also contribute include bandgap states associated with the dopants and the fact that not all the dopants lead to creation of Fe(II) sites.
2001. "A Systematic Study of the Reactions of OH- with Chlorinated Methanes: 1. Benchmark Studies of the Gas-Phase Reactions." Journal of Physical Chemistry A 105(32):7724-7736. Abstract Nothing available at this timeNothing available at this timeNothing available at this timeNothing available at this timeNothing available at this timeNothing available at this time
2001. "Crystal Structures and Vibrational and Solid-State (CPMAS) NMR Spectroscopy of Some Bis(triphenylphosphine)silver(I) Sulfate, Selenate and Phosphate Systems." Journal of the Chemical Society. Dalton Transactions (2001):20-28. Abstract The complexes [Ag2(PPh3)4EO4].2H2O(E=S, Se) (1,2), [Ag(PPh3)2HEO4].H2O (E=S, Se)(3,4) and [Ag9PPh3)2H2PO4].2EtOH (5) have been prepared and studied by X-ray crystallography and by infrared and solid-state 13C and 31 P cross-polarization, magic-angle-spinning (CPMAS) NMR spectroscopy.
2001. "Structural Analysis of BAG1 Cochaperone and Its Interactions With Hsc70 Heat Shock Protein." Nature Structural Biology 8(4):349-352. Abstract BAG-family proteins share an evolutionary conserved protein interaction domain, called the "BAG domain"1, which binds and regulates the ATPase domain of Hsp70/Hsc70 molecular chaperones2-5. This family of co-chaperones functionally regulate several signal transducing proteins and transcription factors important for cell stress responses, apoptosis, proliferation, cell migration and hormone regulation.
2001. "Structural Characterization of MAO and Related Aluminum Complexes. 1. Solid-State 27 Al NMR with Comparison to EFG Tensors from ab Initio Molecular Orbital Calculations." Journal of the American Chemical Society 123:12009-12017. Abstract Aminato and propanolato aluminum clusters with 3-, 4-, and 6-coordinate aluminum sites are studied with three 27Al NMR techniques optimized for large 27Al Quadrupole coupling constants: field-swept, frequency-stepped, and high-field MAS NMR. The 27Al quadrupole coupling constants and asymmetry parameters of molecular species, both experimental and derived from ab initio molecular orbital calculations, are correlated with structure.
2001. "Characterization of Tricoordinate Boron Chemical Shift Tensors: Definitive High-FieldSolid-State NMR Evidence for Anisotropic Boron Shielding." Journal of Physical Chemistry A 105(14):3633-3640. Abstract Despite the large known chemical shift (CS) range for boron and the large number of 11B NMR studies of glasses, no boron CS tensors have been characterized to date. We report the application of solid-state NMR techniques at moderate (9.4 T) and high (17.63 T) applied magnetic field strengths to the characterization of the boron CS tensors in trimesitylborane (BMes3) and triphenyl borate (B(OPh)3). The boron CS tensor of the former compound exhibits a remarkably large span, � ) 121 ( 1 ppm, which encompasses the known range of isotropic chemical shifts for tricoordinate boron compounds. Conversely, the effect of the boron CS tensor on the 11B NMR spectra of B(OPh)3 is difficult to observe and quantify even at field strengths as high as 17.63 T; we find � e 10 ppm. This marked difference in the boron nuclear magnetic shielding tensors is reproduced accurately by a series of ab initio and DFT calculations with a range of basis sets. The difference is rationalized in the context of Ramsey’s theory of nuclear magnetic shielding by considering contributions to the paramagnetic shielding in the tricoordinate boron plane. Differences in the in-plane shielding tensor components for the molecules considered are a result of variations in the effectiveness of the mixing of occupied ۠orbitals with virtual � orbitals under the influence of an applied magnetic field. A similar explanation has been invoked to rationalize 13C isotropic chemical shifts in classical and nonclassical carbocations. We also report experimental and calculated boron nuclear quadrupolar coupling constants and asymmetry parameters for BMes3 and B(OPh)3. A combination of experimental and theoretical results provides the orientation of the CS and electric field gradient tensors in the molecular framework.
2001. "High-Field Chlorine NMR Spectroscopy of Solid Organic Hydrochloride Salts: A Sensitive Probe of Hydrogen Bonding Environment." Journal of Physical Chemistry A 105(45):10413-10421. Abstract A series of organic hydrochloride salts has been investigated using solid-state 35Cl and 37Cl NMR spectroscopy at applied magnetic field strengths of 9.4 and 18.8 T. Magic-angle spinning, static Hahn-echo, and quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) echo experiments have been applied to investigate the chlorine electric field gradient (EFG) and chemical shift (CS) tensors for L-tyrosine hydrochloride, L-cysteine methyl ester hydrochloride, L-cysteine ethyl ester hydrochloride, quinuclidine hydrochloride, and tris sarcosine calcium chloride. Chlorine-35 nuclear quadrupolar coupling constants for these compounds range from 2.23 to 5.25 MHz, and isotropic chemical shifts range from approximately 9 to 53 ppm relative to the chloride ion in aqueous solution. The results demonstrate the feasibility and benefits of high-field 35/37Cl NMR studies of organic chloride salts. A discussion of the data in the context of the known X-ray or neutron diffraction structures for these compounds suggests that the chlorine EFG tensor is a valuable probe of hydrogen bonding to the chloride ion. Because the anisotropies of the CS tensors are rather small, precise determination of the chlorine CS tensors proved to be challenging and was only feasible for L-cysteine ethyl ester hydrochloride, where the span, �, was found to be 47 ( 4 ppm. This represents the first determination of �(Cl) from a powder sample. Results of ab initio calculations of the chlorine EFG and CS tensors in L-tyrosine hydrochloride are presented and compared with the experimental data.
2001. "Practical Aspects of Modern Routine Solid-State Multinuclear Magnetic Resonance Spectroscopy: One Dimensional Experiments." Canadian Journal of Analytical Sciences and Spectroscopy 46:46-82. Abstract Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is becoming increasingly important and accessible to the non-NMR specialist. At the same time, NMR spectroscopists are actively developing new techniques to address a large variety of problems relating to diverse areas of chemistry, physics, biology, biochemistry, materials science, and beyond.
2001. "The First Chromium-53 Solid-State Nuclear Magnetic Resonance Spectra of Diamagnetic Chromium(0) and Chromium(VI) Compounds." Physical Chemistry Chemical Physics. PCCP 3(23):5154-5157. Abstract Chromium-53 is a spin-3/2 nucleus with a relatively small magnetic moment, low natural abundance, and large quadrupole moment. These properties have severely hampered the development of 53Cr NMR, especially in the solid state. In this Communication, the first 53Cr solid-state NMR spectra of prototypal diamagnetic chromium(0) and chromium(VI) compounds are presented. Specifically, analyses of 53Cr NMR spectra of solid hexacarbonylchromium(0), caesium chromate(VI), and potassium chromate(VI) have allowed for the determination of 53Cr quadrupolar coupling parameters and the first chromium chemical shift (CS) tensors. This work demonstrates the potential of 53Cr solid-state NMR, in particular the extreme sensitivity of the 53Cr quadrupolar coupling constant to the local chromium environment. Comparisons are made to known 53Cr NMR parameters available from solution studies, and to the 95Mo solid-state NMR parameters of analogous molybdenum compounds. The influence of crystal symmetry present in isomorphic Cr(CO)6 and Mo(CO)6 is strongly reflected in the magnitudes of the metal nuclei CS tensors and in their orientation with respect to their corresponding electric field gradient tensors.
2001. "Structure of a BRCA1/BARD1 Complex: a Heterodimeric RING-RING Interaction." Nature Structural Biology 8(10):833-837. Abstract The N-terminal RING domain of the breast and ovarian cancer tumor suppressor BRCA1 interacts with multiple cognate proteins, including the RING protein, BARD1. Proper function of the BRCA1 RING domain is critical, as evidenced by the many cancer-predisposing mutations found within this domain. We present the solution structure of the N-terminal RING domain heterodimer of BRCA1 and BARD1. Comparison with the RAG1 RING homodimer reveals the structural diversity of complexes formed by interactions between different RING domains. The BRCA1/BARD1 structure provides a model for its ubiquitin ligase activity, illustrates how the BRCA1 RING domain can be involved in associations with multiple protein partners, and provdes a framework for understanding cancer-causing mutations at the molecular level.
2001. "Human Nucleotide Excision Repair Protein XPA: NMR Spectroscopic Studies of an XPA Fragment Containing the ERCC1-Binding Region and the Minimal DNA-Binding Domain (M59-F219)." Mutation Research 486(1):1-10. Abstract XPA is a central protein component of nucleotide excision repair (NER), a ubiquitous, multi-component cellular pathway responsible for the removal and repair of many structurally distinct DNA lesions from the eukaryotic genome.