Monday, April 13, 2009

J Phys Chem B and C, Vol. 113, Issue 15

Structural Studies of the Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrafluoroborate in Dichloromethane Using a Combined DFT-NMR Spectroscopic Approach

Sergey A. Katsyuba*, Tatiana P. Griaznova, Ana Vidi and Paul J. Dyson
A.E.Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov Str. 8, 420088 Kazan, Russia, and Institut des Sciences et Ingnierie Chimiques, Ecole Polytechnique Fdrale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
J. Phys. Chem. B, 2009, 113 (15), pp 5046–5051

Abstract: DFT methods in combination with NMR spectroscopy are used to investigate possible variants of the molecular structure of the ion pairs of the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF4], in dichloromethane. According to the computations of the chemical shifts, experimental NMR spectra can be rationalized by an equilibrium between ca. 70−80% of structures with the anion positioned near to the C2 atom of the imidazolium ring and ca. 20−30% of structures with the anion close to the C5 and/or C4 atoms. The content of the latter structures, according to the computed Gibbs free energies, does not exceed 10%. Both the computations and the experimental NMR data suggest that the ratio of the two above-mentioned types of structures of the imidazolium-based ILs is influenced by the concentration/polarity of their dichloromethane solutions.

Connectivity and Proximity between Quadrupolar Nuclides in Oxide Glasses: Insights from through-Bond and through-Space Correlations in Solid-State NMR

Sung Keun Lee*, Michael Deschamps§, Julien Hiet§, Dominique Massiot§ and Sun Young Park
J. Phys. Chem. B, 2009, 113 (15), pp 5162–5167

Abstract:The connectivity and proximity among framework cations and anions in covalent oxide glasses yields unique information whereby their various transport and thermodynamic properties can be predicted. Recent developments and advances in the reconstruction of anisotropic spin interactions among quadrupolar nuclides (spin > 1/2) in solid-state NMR shed light on a new opportunity to explore local connectivity and proximity in amorphous solids. Here, we report the 2D through-bond (J-coupling) and through-space (dipolar coupling) correlation NMR spectra for oxide glasses where previously unknown structural details about the connectivity and proximity among quadrupolar nuclides (27Al, 17O) are determined. Nonbridging oxygen peaks in Ca−aluminosilicate glasses with distinct connectivity, such as Ca−O−Al and Al−O−(Al, Si) are well distinguished in {17O}27Al solid HMQC NMR spectra. Both peaks shift to a lower frequency in direct and indirect dimensions upon the addition of Si to the Ca−aluminate glasses. The 2D 27Al double quantum magic angle spinning NMR spectra for Mg-aluminoborate glasses indicate the preferential proximity between [4]Al and [5]Al leading to the formation of correlations peaks such as [4]Al−[4]Al, [4]Al−[5]Al, and [5]Al−O−[5]Al. A fraction of the [6]Al−[6]Al correlation peak is also noticeable while that of [4,5]Al−[6]Al is missing. These results suggest that [6]Al is likely to be isolated from the [4]Al and [5]Al species, forming [6]Al clusters. The experimental realization of through-bond and through-space correlations among quadrupolar nuclides in amorphous materials suggests a significant deviation from the random distribution among framework cations and a spatial heterogeneity due to possible clustering of framework cations in the model oxide glasses.

Spectroscopic and Computational Characterization of the Base-off Forms of Cob(II)alamin

Matthew D. Liptak, Angela S. Fleischhacker, Rowena G. Matthews§, Joshua Telser and Thomas C. Brunold*
Department of Chemistry, University of Wisconsin-Madison, Madison Wisconsin 53706, Department of Chemistry, and Life Sciences Institute, Department of Biological Chemistry, and Biophysics Research Division, University of Michigan, Ann Arbor, Michigan 48109, and Chemistry Program, Roosevelt University, Chicago, Illinois 60605
J. Phys. Chem. B, 2009, 113 (15), pp 5245–5254

Abstract: The one-electron-reduced form of vitamin B12, cob(II)alamin (Co2+Cbl), is found in several essential human enzymes, including the cobalamin-dependent methionine synthase (MetH). In this work, experimentally validated electronic structure descriptions for two “base-off” Co2+Cbl species have been generated using a combined spectroscopic and computational approach, so as to obtain definitive clues as to how these and related enzymes catalyze the thermodynamically challenging reduction of Co2+Cbl to cob(I)alamin (Co1+Cbl). Specifically, electron paramagnetic resonance (EPR), electronic absorption (Abs), and magnetic circular dichroism (MCD) spectroscopic techniques have been employed as complementary tools to characterize the two distinct forms of base-off Co2+Cbl that can be trapped in the H759G variant of MetH, one containing a five-coordinate and the other containing a four-coordinate, square-planar Co2+ center. Accurate spin Hamiltonian parameters for these low-spin Co2+ centers have been determined by collecting EPR data using both X- and Q-band microwave frequencies, and Abs and MCD spectroscopic techniques have been employed to probe the corrin-centered π → π* and Co-based d → d excitations, respectively. By using these spectroscopic data to evaluate electronic structure calculations, we found that density functional theory provides a reasonable electronic structure description for the five-coordinate form of base-off Co2+Cbl. However, it was necessary to resort to a multireference ab initio treatment to generate a more realistic description of the electronic structure of the four-coordinate form. Consistent with this finding, our computational data indicate that, in the five-coordinate Co2+Cbl species, the unpaired spin density is primarily localized in the Co 3dz2-based molecular orbital, as expected, whereas in the four-coordinate form, extensive Co 3d orbital mixing, configuration interaction, and spin−orbit coupling cause the unpaired electron to delocalize over several Co 3d orbitals. These results provide important clues to the mechanism of enzymatic Co2+Cbl → Co1+Cbl reduction.

Theoretical Study of the Effective Chemical Shielding Anisotropy (CSA) in Peptide Backbone, Rating the Impact of CSAs on the Cross-Correlated Relaxations in l-Alanyl-l-alanine

Ladislav Benda*, Petr Bou, Norbert Mller* and Vladimr Sychrovsk*
Institute of Organic Chemistry and Biochemistry v.v.i., Academy of Sciences of the Czech Republic, Flemingovo nm. 2, 166 10 Praha 6, Czech Republic, and Institute of Organic Chemistry, Johannes Kepler University, Altenbergerstrasse 69, 4040 Linz, Austria
J. Phys. Chem. B, 2009, 113 (15), pp 5273–5281

Abstract:The dependence of the effective chemical shielding anisotropy (effective CSA, Δσeff) on the and ψ peptide backbone torsion angles was calculated in the l-alanyl-l-alanine (LALA) peptide using the DFT method. The effects of backbone conformation, molecular charge including the cation, zwitterion, and anion forms of the LALA peptide, and the scaling taking into account the length of the dipolar vector were calculated for the effective CSAs in order to assess their structural behaviors and to predict their magnitudes which can be probed for the β-sheet and α-helix backbone conformations via measurement of the cross-correlated relaxation rates (CCR rates). Twenty different CSA−DD cross-correlation mechanisms involving the amide nitrogen and carbonyl carbon chemical shielding tensors and the CαHα (α-carbon group), NHN (amide group), CαHN, NHα, C′Hα, and C′HN (α = α1, α2) dipolar vectors were investigated. The X−CαHα (X = N, C′; α = α1, α2) cross-correlations, which had already been studied experimentally, exhibited overall best performance of the calculated effective CSAs in the LALA molecule; they spanned the largest range of values upon variation of the ψ and torsions and depended dominantly on only one of the two backbone torsion angles. The X−NHN (X = N, C′) cross-correlations, which had been also probed experimentally, depended on both backbone torsions, which makes their structural assignment more difficult. The N−NHα2 and N−C′Hα1 cross-correlations were found to be promising for the determination of various backbone conformations due to the large calculated range of the scaled effective CSA values and due to their predominant dependence on the ψ and torsions, respectively. The 20 calculated dependencies of effective CSAs on the two backbone torsion angles can facilitate the structural interpretation of CCR rates.

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