Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

The role of the π-bonding network for trigonal level splittings of tris-bidentate Cr(acac)3 and Cr(ox) 3 3−

The varying -bonding contributions in the title compounds caused by the different electronic and molecular structure of the chelate rings are used for explaining the large band splittings in the absorption spectra by trigonal symmetry. It is shown that usual ligand field theory and the angular overlap model are not able to account for the trigonal level splitting of Cr(acac)₃ for which the coordination sphere of oxygen atoms is nearly octahedrally arranged. The experimental finding can, however, be rationalized by an extended angular overlap model which considers the phase coupling of -orbitals in the ligands leading to non-additive contributions to the metal-ligand bond energy.On leave of absence from the Bulgarian Academy of Sciences, Sofia, Bulgaria     

Spin-orbit electronic states of octahedral Pt-group d 6 complexes as derived from reflectance spectra and ligand field calculations

The 90 K diffuse reflectance spectra of some octahedral Rh(III), Ir(III) and Pt(IV) complex compounds with chloride, thiocyanate and amine ligands are reported. The spectra show in the visible and near ultraviolet distinct bands due to spin-orbit components of singlet-triplet d-d transitions which are explained by ligand field calculations including spin-orbit coupling considering all possible d-electron configurations. Model parameter sets are obtained by fitting the measured band peaks to possible transitions between calculated energy levels. For some of the bands the vibrational structure could be resolved which is assigned to metal-ligand stretching vibrations of the electronically excited complex octahedra with vibrational fundamentals lower than those of the ground state.     

An investigation of the two electron chemical bond calculating energy expectation values from density matrix partitionings. II. The heteronuclear case HeH+

Bond energy contributions calculated from first and second order density matrix terms as partitioned by Ruedenberg's procedure have been obtained for HeH⁺ in the ground state and in the first excited 1Σ⁺. For the chemically bonded ground state the full partitioning is investigated for all internuclear distances R. The wavefunctions used for calculating the density matrices are obtained from an SCF calculation at near Hartree-Fock quality, using Slater orbitals with exponents which for each R are optimized simultaneously with the coefficients. For the excited state a limited CI has been performed. The results for promotional, charge transfer, and interference terms for kinetic and potential bond contributions are presented in the form of energy plots E(R). Starting from the promoted atoms and subsequently allowing for charge transfer the importance of the electron interaction is demonstrated by the unusually low quasiclassical electron repulsion curve due to electronic charge transfer, which makes an essential contribution to the decrease in energy during bond formation.     

On the use of two-dimensional potential surface cross sections in the discussion of vibronic spectra of transition metal complexes

For purposes of interpreting broad band electronic spectra of complexes, the 3N-5 dimensional potential energy hypersurface may be reduced substantially. Consideration of the geometry of the orbital transition involved allows selection of an appropriate cross section, depending on which features should be represented. In addition, the orbital transition geometry predicts which vibrational modes will contribute significantly to the width of the spectrum. In the particular case of the first and second spin-allowed bands in d³ and d⁶ complexes, α_1g and ?_g modes are responsible for the band width.     

The virial theorem for non-coulombic interactions and scaled approximate wave functions

The virial theorem for various not necessarily homogeneous potential functions V(r) is derived. It is proved in a transparent way that approximate wave functions at optimal scale satisfy the general virial theorem for any radial potential V(r). The proposed method for improving a variational wave function which by the scaling procedure obeys the virial theorem is tested in the case of some simple screened coulomb potentials.

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Zusammenfassung Das Virialtheorem für verschiedene, nicht unbedingt homogene Potentialfunktionen V(r) wird abgeleitet. Es wird gezeigt, da\ eine NÄherungsfunktion mit optimalem Skalenparameter auch das allgemeine Virialtheorem für beliebige Radialpotentiale V(r) erfüllt. Die Methode der Streckung des Grundgebietes (scaling) dient zur Verbesserung von NÄherungsfunktionen, was am Beispiel einiger abgeschirmter Coulomb-Potentiale demonstriert wird.

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Résumé Le théorème du viriel pour quelques fonctions de potentiel V(r) non nécessairement homogènes est dérivé du théorème général. Il est prouvé d'une manière intuitive que des fonctions d'onde approchées, à l'échelle optimum, satisfont le théorème du viriel général pour tous les potentiels radiaux V(r). La méthode proposée pour améliorer une fonction d'onde variationnelle, qui obéit au théorème du viriel, est essayée dans le cas de quelques simples potentiels coulombiens écrantés.

     

Darstellung und Charakterisierung einige Thiocyanat- und Selenocyanat-Komplexe von Übergangsmetallen

Some 6:1 thiocyanate complexes of Fe^III, Mo^III, Ru^III, Os^III, Ir^III and some 6:1 or 4:1 selenocyanate complexes of Fe^II, Fe^III, Mo^III, Rh^III, Pd^II, Pt^II, and Au^III have been prepared as salts of large organic cations. The compounds are characterized by their infra-red spectra and by conductance measurements. In particular the linkage property of the thiocyanate and selenocyanate ligand is discussed together with the consequences it has on the AHRLAND -CHATT -DAVIES -PEARSON classification of the central ions.     

Investigation of the reaction between a CdSe-crystal and a Pb and S containing reactive gas phase

The mechanism of the reaction of a CdSe-crystal with a lead and sulfur containing gas phase is investigated by means of an electron microprobe and by optical microscopy in the temperature region from 600 to 900°C. The morphology of the reaction layer depends strongly on temperature. In the low temperature region the reaction zone exists of two product layers. Directly at the crystal surface we find a Cd(S, Se) interdiffusion layer but only at some distance from the crystal surface a Pb-chalcogenide layer has been formed.In the high temperature region needles of Pb-chalcogenide are formed directly at the crystal surface. These differences in the morphological feature can be explained by local thermodynamic equilibrium at the phase boundaries.     

Following the solvent directly during ultrafast excited state proton transfer

Excited state intramolecular proton transfer in 1-chloroacetylaminoanthraquinone is investigated from the perspective of the solvent. Using a new two-dimensional nonlinear optical spectroscopy the solvent response is probed directly as the proton transfer takes place. The measurements indicate that solvent reorganization controls the proton transfer in acetonitrile by dynamically shifting the position of equilibrium in the excited state, even on subpicosecond time scales.