Moderating influence of proteins on nonplanar tetrapyrrole deformations: coenzyme F430 in methyl-coenzyme-M reductase

Normal-coordinate structural decomposition, cluster analysis, and molecular mechanics calculations were undertaken to examine the effect of methyl-coenzyme-M reductase (MCR) on the nonplanar deformations of coenzyme F430. Although free 12,13-diepi-F430 has a lower energy conformation than free F430, the protein restraints exerted by MCR are responsible for F430 having a lower energy conformation than the 12,13-diepimer in MCR. According to the NSD analysis, the crystal structure of free diepimerized F430M is highly distorted. In MCR the protein prevents 12,13-diepi-F430 from undergoing nonplanar deformations; therefore, MCR favors F430 over the 12,13-diepimeric form. The strain imposed on 12,13-diepi-F430 in the protein is so large that although 88% of free F430 is found in the diepimeric form, none of the diepimeric form is found in MCR. This is of significance since the two forms have different chemistries. MCR also moderates the nonplanar deformations of coenzyme F430, which are known to affect redox potentials and axial ligand affinities in tetrapyrroles, suggesting that the protein environment (MCR) is responsible for tuning the chemistry of the active site nickel ion. F430 is bound to MCR by hydrogen bonds between the protein and the F430 carboxylate groups. Conformational searches have shown that F430 has very little rotational and translational freedom within MCR.     

Ligand K-edge X-ray absorption spectroscopy of [Fe4S4]1+,2+,3+ clusters: changes in bonding and electronic relaxation upon redox

Sulfur K-edge X-ray absorption spectroscopy (XAS) is reported for [Fe(4)S(4)](1+,2+,3+) clusters. The results are quantitatively and qualitatively compared with DFT calculations. The change in covalency upon redox in both the [Fe(4)S(4)](1+/2+) (ferredoxin) and the [Fe(4)S(4)](2+/3+) (HiPIP) couple are much larger than that expected from just the change in number of 3d holes. Moreover, the change in the HiPIP couple is higher than that of the ferredoxin couple. These changes in electronic structure are analyzed using DFT calculations in terms of contributions from the nature of the redox active molecular orbital (RAMO) and electronic relaxation. The results indicate that the RAMO of HiPIP has 50% ligand character, and hence, the HiPIP redox couple involves limited electronic relaxation. Alternatively, the RAMO of the ferredoxin couple is metal-based, and the ferredoxin redox couple involves extensive electronic relaxation. The contributions of these RAMO differences to ET processes in the different proteins are discussed.     

Methylation of 3-methyl- and 3-phenyl-4-arylhydrazonoisoxazol-5-ones with diazomethane

3-Methyl(or phenyl)-4-arylhydrazonoisoxazol-5-ones on methylation with diazomethane afford 3-methyl(or phenyl)-4-(N-methylarylhydrazono)isoxazol-5-ones and 3-methyl(or phenyl)-5-methoxy-4-arylazoisoxazoles. The latter compounds readily rearrange to 4-methoxycarbonyl-5-methyl(or phenyl)-2-aryl-2H-1,2,3-triazoles.     

Synthesis of tellurobispyridines

All six isomers of tellurobispyridine, namely 2,2′-tellurobispyridine, 2,3′-tellurobispyridine, 2,4′-tellurobis-pyridine, 3,3′-tellurobispyridine, 3,4′-tellurobispyridine and 4,4′-tellurobispyridine have been synthesised.     

Electronic decay constant of carotenoid polyenes from single-molecule measurements

The conductance of carotenoid polyenes chemically bound at each end to gold contacts has been measured for single molecules containing 5, 7, 9, and 11 carbon-carbon double bonds in conjugation. The electronic decay constant, beta, is determined to be 0.22 +/- 0.04 A-1, in close agreement with the value obtained from first principles simulations (0.22 +/- 0.01 A-1). The absolute values of the molecular conductance are within a factor of 3 of those calculated from first principles. The small value of beta demonstrates that conductivity drops off only slowly with chain length, confirming that carotenoid conjugated chains are relatively good molecular "wires".     

The effect of benzonitrile on the hydrogenation of phenylacetylene and 1-phenyl-1-propyne over a palladium/carbon catalyst

Benzonitrile reduces the rate of alkyne hydrogenation. However, it enhances the rate of styrene hydrogenation while reducing the rate of -methylstyrene hydrogenation.     

Intramolecularly competitive Ireland-Claisen rearrangements: scope and potential applications to natural product synthesis

A variety of bis-allylic esters were prepared by vinylmetal addition to cycloalkenones followed by esterification either in situ or in a separate operation. For chiral cyclohexenones, the vinyl additions generally occurred with >10:1 diastereoselectivity. Although in some cases the bis-allylic esters proved to be sensitive to silica gel or other adsorbents, all of the esters examined could be isolated in acceptable purity. The Ireland-Claisen rearrangement of the bis-allylic esters occurred with complete regioselectivity via the exocyclic alkene. The alkene stereochemistry and the stereochemistry at C-2 and C-3 of the pentenoic acid products were consistent with a chairlike transition state in the rearrangement. Substituents at the carbons adjacent to the allylic carbinol carbon (i.e., C-2 or C-6 in cyclohexenone-derived substrates) directed the stereochemical course of the rearrangement. The rearrangements generally proceeded so as to place the larger of the C-2 or C-6 substituents in the pseudoequatorial position with respect to the chairlike transition state. For a bis-allylic ester bearing both a C-2-CH(3) and a C-6-OMEM substituent, the rearrangement product resulted from the nominally smaller OMEM substituent occupying a pseudoequatorial position with respect to the chairlike transition state.     

Synthesis and characterisation of new triple halide-bridged mixed valence binuclear complexes of ruthenium and osmium

Several isomers of the type [M₂Cl₅L₄] (M = Ru, L = AsPh₃, As(p-tol)₃, As(p-PhCl)₃, PEt₂Ph, PMe₂Ph; L₂ = Ph₂As(CH₂)₂AsPh₂; M = Os, L = PPh₃, AsPh₃) have been synthesised by various routes and characterised by magnetic, ESR and electrochemical measurements, and for [(PEt₂Ph)Cl₂RuCl₃Ru(PEt₂Ph)₃] by X-ray structural analysis.     

Grid-type metal ion architectures: functional metallosupramolecular arrays

Recent advances in supramolecular coordination chemistry allow access to transition-metal complexes of grid-type architecture comprising two-dimensional arrays of metal ions connecting a set of organic ligands in a perpendicular arrangement to generate a multiple wiring network. General design principles for these structures involve the thermodynamically driven synthesis of complex discrete objects from numerous molecular components in a single overall operation. Such supramolecular metal ion arrays combine the properties of their constituent metal ions and ligands, showing unique optical, electrochemical, and magnetic behavior. These features present potential relevance for nanotechnology, particularly in the area of supramolecular devices for information storage and processing. Thus, a dense organization of addressable units is represented by an extended "grid-of-grids" arrangement, formed by interaction of grid-type arrays with solid surfaces.     

Tight binding prediction of the α-Gd2S3 magnetic structure

Spin-dependent extended Hückel tight binding (EHTB) calculations were carried out for the magnetic solid Gd₂S₃ by considering 20 different variations in the ordering of the 4f⁷ moments. The tight-binding calculations are used to interpolate the band structure of a nonmagnetic congener (Y₂S₃) and the 4f/5d,6s exchange interactions are introduced as perturbations via the introduction of spin-dependent H_dd and H_ss parameters. The calculations predict that Gd₂S₃ adopts an antiferromagnetic ordering of the 4f⁷ moments that is consistent with published neutron diffraction results. Our attempt to account for the calculated energies of the spin patterns using an Ising model was unsuccessful.