Single crystal EPR studies of the reduced active site of [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F

In the catalytic cycle of [NiFe] hydrogenase the paramagnetic Ni-C intermediate is of key importance, since it is believed to carry the substrate hydrogen, albeit in a yet unknown geometry. Upon illumination at low temperatures, Ni-C is converted to the so-called Ni-L state with markedly different spectroscopic parameters. It is suspected that Ni-L has lost the "substrate hydrogen". In this work, both paramagnetic states have been generated in single crystals obtained from the [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F. Evaluation of the orientation dependent spectra yielded the magnitudes of the g tensors and their orientations in the crystal axes system for both Ni-C and Ni-L. The g tensors could further be related to the atomic structure by comparison with the X-ray crystallographic structure of the reduced enzyme. Although the g tensor magnitudes of Ni-C and Ni-L are quite different, the orientations of the resulting g tensors are very similar but differ from those obtained earlier for Ni-A and Ni-B (Trofanchuk et al. J. Biol. Inorg. Chem. 2000, 5, 36-44). The g tensors were also calculated by density functional theory (DFT) methods using various structural models of the active site. The calculated g tensor of Ni-C is, concerning magnitudes and orientation, in good agreement with the experimental one for a formal Ni(III) oxidation state with a hydride (H(-)) bridge between the Ni and the Fe atom. Satisfying agreement is obtained for the Ni-L state when a formal Ni(I) oxidation state is assumed for this species with a proton (H(+)) removed from the bridge between the nickel and the iron atom.     

Synthese von [(η-C5H5)2Ti(Cl)(CCSiMe3)]Ni(CO) und dessen Reaktionsverhalten gegenüber Phosphiten: die Festkörperstruktur von (CO)2Ni[P(OC6H4CH3-2)3]2

Treatment of [Ti](Cl)(CCSiMe₃) (1) {[Ti]=(η⁵-C₅H₅)₂Ti} with Ni(CO)₄ (2) in a 1:1 molar ratio produces the heterobimetallic early-late transition metal complex {[Ti](Cl)(CCSiMe₃)}Ni(CO) (3), which features a low-valent Ni(CO) entity stabilized by a datively bonded Cl and a η²-coordinated Me₃SiCC ligand. As side-products [Ti]Cl₂ (8) and {[Ti](CCSiMe₃)₂}Ni(CO) (5) are formed. The latter complex can also be synthesized by the reaction of [Ti](CCSiMe₃)₂ (4) with equimolar amounts of 2. If 3 is reacted with stoichiometric amounts of P(OR)₃ (6a, R=C₆H₅; 6b, R=C₆H₄CH₃-2; 6c, R=C₆H₄^tBu-2) the bis(alkynyl) titanocene 4, (CO)₂Ni[P(OR)₃]₂ (7a, R=C₆H₅; 7b, R=C₆H₄CH₃-2; 7c, R=C₆H₄^tBu-2), complex 8, {[Ti](μ,η¹:η²-CCSiMe₃)}₂ (9) along with Me₃SiCCCCSiMe₃ (10) is produced. A possible mechanism for the formation of these species is presented. The solid-state structure of 7b is reported. Complex 7b crystallizes in the tetragonalic space group P-42₁c with the following parameters: a=14.852(2), b=14.852(2), c=19.410(4) Å, V=4281.5(12) ų, Z=4 and ρ=1.271 g cm⁻³. Mononuclear 7b features a Ni(0) centre in a pseudo-tetrahedral environment, caused by the CO and P(OC₆H₄CH₃-2)₃ ligands.     

Estimating probabilities of correct identification from results of mass spectral library searches

This work presents a method for using mass spectral match factors reported by library search systems to obtain certain probabilistic indicators of correct identification. The overall probability that a retrieval is correct is formally separated into two independent terms. One of these is the probability that a retrieval is correct assuming that the correct match is contained in the library. This can be computed directly from test results. The other term represents the probability that the spectrum of the unknown compound is actually in the library. While the absolute value of this term cannot be computed, a relative value based solely on search results can be derived. This value may, if desired, be used to refine an initial estimate of the overall probability. Parameters used in this calculation are based on changes in test results caused by the logical removal of the test compounds from the library. These methods were parameterized from results of searching the MST/EPA/NIH Mass Spectral Database with 12,592 good quality replicate spectra and a simple mass spectral comparison function. The methodology should be equal1 applicable to other libraries and search systems.     

A spin-dependent Popov-Fedotov trick and a new loop expansion for the strong coupling negativeU Hubbard model

We calculate fluctuation effects on Bose condensation type superconductivity in the strong coupling negative U Hubbard model by means of a new loop expansion. Our method is based on a spin-dependent modification of the Popov-Fedotov trick. We replace the Popov-Fedotov chemical potential by a fictitious imaginary magnetic field. This field is absorbed in spindependent semionic Matsubara frequencies, which allows for a mixed statistics representation of the anisotropic quantum spin 1/2 Heisenberg model. We report results at one loop order for the superconducting order parameter , for the critical temperature, for the chemical potential, and for the excitation spectra both above and belowT _c. We identify mean field results in zeroth loop order and we find both dimensional and filling (v-)depending singularities in interaction fluctuations at one loop order. Renormalization of dimensional singularities is carried out in 4 dimensions. Divergencies withv (1–v)0 in the dilute limits indicate the breakdown of mean field solutions, but superconductivity persists for arbitrarily smallv(1–v) if our loop expansion is interpreted by exponential behaviour as it is suggested by the abelian nonlinear sigma model.     

Linear dichroism and fluorescence polarization of diphenyl polyenes in stretched polyethylene films

A study of the linear diehroism and fluorescence polarization of diphenylpolyenes (C₆H₅—(CH=CH)_n—C₆H₅) with n = 1,2,3,4,6,8 in stretched polyethylene films shows that the polyenes orient in the anisotropic matrix with their transition dipole (emission and absorption, π → π^*) aligned with the direction of stretch. The maximum dichroic ratio is observed for 1,6-diphenyl-1,3,5-hexatriene (DPH), with n = 3, whereas the values are substantially lower for n < 3 or n > 3. The high orientation of DPH and poor alignment of the higher polyenes may be accounted for in terms of the growing flexibility of the polyene chains with increasing chain length. The results justify the use of DPH as a convenient fluorescence probe for biological and synthetic membranes.     

Surface modification of mesoporous, macroporous, and amorphous silica with catalytically active polyoxometalate clusters

Transition-metal-substituted polyoxometalates (TMSP) of the type [MII(H2O)PW11O39]5- (M = Co, Zn) and [SiW9O37(CoII(H2O))3]10- have been chemically anchored to modified macroporous (400 nm pores), mesoporous (2.8 nm pores), and amorphous silica surfaces. The materials were characterized by solid-state 31P MAS NMR, UV-vis, FT-IR spectroscopy, and N2 adsorption experiments to verify cluster attachment and the structure of the TMSP on the support. On the basis of the spectroscopic evidence, clusters were attached datively to the surface, and the integrity of the [CoPW11] cluster was maintained for nonaqueous impregnation with TBA5[CoPW11]; partial degradation of the cluster occurred when it was impregnated from aqueous solution using the K5[CoPW11] salt. Both the amine surface groups and the cobalt centers in the clusters were found to be necessary to prevent cluster loss during washing or reaction processes. The catalytic activities of the supported TMSP clusters were tested by the epoxidation of cyclohexene to cyclohexene oxide in the presence of isobutyraldehyde. The percent conversion of the substrate and the amount of product formed per unit time were similar for [CoPW11] clusters on each of the three silica supports, but slightly lower than for purely homogeneous reactions. [SiW9Co3] clusters with three available cobalt centers exhibited higher catalytic activity with nearly identical performance on a silica support or in homogeneous solution.     

Poly-N-hydroxyethylacrylamide as a novel, adsorbed coating for protein separation by capillary electrophoresis.

We present the polymer poly-N-hydroxyethylacrylamide (PHEA) (polyDuramide) as a novel, hydrophilic, adsorbed capillary coating for electrophoretic protein analysis. Preparation of the PHEA coating requires a simple and fast (30 min) protocol that can be easily automated in capillary electrophoresis instruments. Over the pH range of 3-8.4, the PHEA coating is shown to reduce electroosmotic flow (EOF) by about 2 orders of magnitude compared to the bare silica capillary. In a systematic comparative study, the adsorbed PHEA coating exhibited minimal interactions with both acidic and basic proteins, providing efficient protein separations with excellent reproducibility on par with a covalent polyacrylamide coating. Hydrophobic interactions between proteins and a relatively hydrophobic poly-N,N-dimethylacrylamide (PDMA) adsorbed coating, on the other hand, adversely affected separation reproducibility and efficiency. Under both acidic and basic buffer conditions, the adsorbed PHEA coating produced an EOF suppression performance comparable to that of covalent polyacrylamide coating and superior to that of adsorbed PDMA coating. The protein separation performance in PHEA-coated capillaries was retained for 275 consecutive protein separation runs at pH 8.4, and for more than 800 runs at pH 4.4. The unique and novel combination of hydrophilicity and adsorptive coating ability of PHEA makes it a suitable wall coating for automated microscale analysis of proteins by capillary array systems.     

On the thermal behavior of 4-(o-carboranyl)-1-butylmethyl siloxane–dimethyl siloxane copolymers

Polysiloxanes containing carboranylbutyl and methyl pendant groups were investigated, to determine their resistance to thermal decomposition when heated in air. Results indicate that the copolymers degrade through Si? C as well as Si? O bond rupture. At 300°C. thermooxidative scission of pendant groups was the predominant reaction. whereas Si? O and Si? C bond rupture occurred at 500°C. Variations in the behavior of the copolymers studied suggest that the carborane nucleus provides some inhibiting influence on their thermal decomposition. This is mainly attributed to steric hindrance effects rather than polarity factors. Chain extension with tin octoate caused a reduction of thermal stability, probably because of the addition of Sn? O bonds to the molecules.