Complex formation of Sn(II) with glycine: an IR, DTA/TGA and DFT investigation.

The novel Sn(Gly)2⋅H2O complex compound has been synthesized and characterized by TGA, IR and Raman spectroscopy. Molecular spectroscopy and ab initio simulation have given the evidence of glycine molecule being coordinated to Sn(II) as bidentate chelating ligand by oxygen atom of carboxyl group and nitrogen atom of amino group. Water molecule is bonded with amino and carboxylic groups by hydrogen bonds in the out sphere. The M06, TPSS, TPSSm, TPSSh and revTPSS density functionals have been tested for calculation of structural and vibrational data. The vibrational assignment of experimental IR and Raman and simulated spectra has been carried out. The TPSS and TPSSm density functionals and Def2-TZVP basis set have provided the most accurate results.

Complex formation of Sn(II) with L-cysteine: an IR, DTA/TGA and DFT investigation.

The novel complex of Sn(II) with L-cysteine (L-H2Cys) has been synthesized and characterized by elemental analysis, TGA and IR spectroscopy. Vibrational assignment and DFT/PBE0/def2-TZVP ab initio simulation give evidence of cysteine molecule being coordinated to Sn(II) as three-dentate chelating N,O,S-donor ligand. The four Perdew density functionals TPSS, PBE0, PBE, TPSSh have been tested to provide consistency of simulated and experimental IR spectra, the best result is provided by unweighted Hartree-Fock density functionals (PBE, TPSS). On the contrary, the Hartree-Fock weighted functionals (PBE0, TPPSh) provide the most accurate geometry optimization. Unharmonic frequencies are obtained via ab initio vibrational self-consistent field (PT2-VSCF) calculations at DFT/TPSS/Def2-TZVP level, the vibrational assignment of IR spectra has been carried out.

Exploring by pulsed EPR the electronic structure of ubisemiquinone bound at the QH site of cytochrome bo3 from Escherichia coli with in vivo 13C-labeled methyl and methoxy substituents.

The cytochrome bo(3) ubiquinol oxidase from Escherichia coli resides in the bacterial cytoplasmic membrane and catalyzes the two-electron oxidation of ubiquinol-8 and four-electron reduction of O(2) to water. The one-electron reduced semiquinone forms transiently during the reaction, and the enzyme has been demonstrated to stabilize the semiquinone. The semiquinone is also formed in the D75E mutant, where the mutation has little influence on the catalytic activity, and in the D75H mutant, which is virtually inactive. In this work, wild-type cytochrome bo(3) as well as the D75E and D75H mutant proteins were prepared with ubiquinone-8 (13)C-labeled selectively at the methyl and two methoxy groups. This was accomplished by expressing the proteins in a methionine auxotroph in the presence of l-methionine with the side chain methyl group (13)C-labeled. The (13)C-labeled quinone isolated from cytochrome bo(3) was also used for the generation of model anion radicals in alcohol. Two-dimensional pulsed EPR and ENDOR were used for the study of the (13)C methyl and methoxy hyperfine couplings in the semiquinone generated in the three proteins indicated above and in the model system. The data were used to characterize the transferred unpaired spin densities on the methyl and methoxy substituents and the conformations of the methoxy groups. In the wild type and D75E mutant, the constraints on the configurations of the methoxy side chains are similar, but the D75H mutant appears to have altered methoxy configurations, which could be related to the perturbed electron distribution in the semiquinone and the loss of enzymatic activity.

Proton environment of reduced Rieske iron-sulfur cluster probed by two-dimensional ESEEM spectroscopy.

The proton environment of the reduced [2Fe-2S] cluster in the water-soluble head domain of the Rieske iron-sulfur protein (ISF) from the cytochrome bc(1) complex of Rhodobacter sphaeroides has been studied by orientation-selected X-band 2D ESEEM. The 2D spectra show multiple cross-peaks from protons, with considerable overlap. Samples in which (1)H(2)O water was replaced by (2)H(2)O were used to determine which of the observed peaks belong to exchangeable protons, likely involved in hydrogen bonds in the neighborhood of the cluster. By correlating the cross-peaks from 2D spectra recorded at different parts of the EPR spectrum, lines from nine distinct proton signals were identified. Assignment of the proton signals was based on a point-dipole model for interaction with electrons of Fe(III) and Fe(II) ions, using the high-resolution structure of ISF from Rb. sphaeroides. Analysis of experimental and calculated tensors has led us to conclude that even 2D spectra do not completely resolve all contributions from nearby protons. Particularly, the seven resolved signals from nonexchangeable protons could be produced by at least 13 protons. The contributions from exchangeable protons were resolved by difference spectra ((1)H(2)O minus (2)H(2)O), and assigned to two groups of protons with distinct anisotropic hyperfine values. The largest measured coupling exceeded any calculated value. This discrepancy could result from limitations of the point dipole approximation in dealing with the distribution of spin density over the sulfur atoms of the cluster and the cysteine ligands, or from differences between the structure in solution and the crystallographic structure. The approach demonstrated here provides a paradigm for a wide range of studies in which hydrogen-bonding interactions with metallic centers has a crucial role in understanding the function.

Water concentration profiles in membranes measured by ESEEM of spin-labeled lipids.

Electron spin-echo envelope modulation (ESEEM) spectroscopy of phospholipids spin-labeled systematically down the sn-2 chain was used to detect the penetration of water (D2O) into bilayer membranes of dipalmitoyl phosphatidylcholine with and without 50 mol % cholesterol. Three-pulse stimulated echoes allow the resolution of two superimposed 2H-ESEEM spectral components of different widths, for spin labels located in the upper part of the lipid chains. Quantum chemical calculations (DFT) and ESEEM simulations assign the broad spectral component to one or two D2O molecules that are directly hydrogen bonded to the N-O group of the spin label. Classical ESEEM simulations establish that the narrow spectral component arises from nonbonded water (D2O) molecules that are free in the hydrocarbon chain region of the bilayer membrane. The amplitudes of the broad 2H-ESEEM spectral component correlate directly with those of the narrow component for spin labels at different positions down the lipid chain, reflecting the local H-bonding equilibria. The D2O-ESEEM amplitudes decrease with position down the chain toward the bilayer center, displaying a sigmoidal dependence on position that is characteristic of transmembrane polarity profiles established by other less direct spin-labeling methods. The midpoint of the sigmoidal profile is shifted toward the membrane center for membranes without cholesterol, relative to those with cholesterol, and the D2O-ESEEM amplitude in the outer regions of the chain is greater in the presence of cholesterol than in its absence. For both membrane types, the D2O amplitude is almost vanishingly small at the bilayer center. The water-penetration profiles reverse correlate with the lipid-chain packing density, as reflected by 1H-ESEEM intensities from protons of the membrane matrix. An analysis of the H-bonding equilibria provides essential information on the binding of water molecules to H-bond acceptors within the hydrophobic interior of membranes. For membranes containing cholesterol, approximately 40% of the nitroxides in the region adjacent to the lipid headgroups are H bonded to water, of which ca. 15% are doubly H bonded. Corresponding H-bonded populations in membranes without cholesterol are ca. 20%, of which ca. 6% are doubly bonded.

Determination of hyperfine tensor components from nuclear frequencies at canonical orientations of the g-tensor.

The analytical procedure for the determination of all components of the symmetric hyperfine tensor of the I=1/2 nucleus in the g-tensor coordinate system is described, assuming that nuclear frequencies corresponding to the principal directions of the g-tensor and exact values of the external magnetic field (or nuclear Zeeman frequencies) are experimentally available.