The crystal structure of the dimeric gene V protein of bacteriophage fl was determined using multiwavelength anomalous diffraction on the selenomethioninecontaining wild-type and isoleucine-47 methionine mutant proteins with x-ray diffraction data phased to 2.5 A resolution.The structure of the wild-type protein has been refined to an R factor of 19.2% using native data to 1.8 A resolution. The structure of the gene V protein was used to obtain a model for the protein portion of the gene V protein-single-stranded DNA complex.Gene V protein of bacteriophage fltt is a member of a class of proteins involved in DNA replication that bind to singlestranded nucleic acids with high affinity and cooperativity but little sequence specificity (1-4). Gene V protein coats the single-stranded DNA (ssDNA) intermediate in bacteriophage fl DNA replication, forming an ordered superhelical protein-DNA complex (1,(5)(6)(7)(8). This protein-DNA complex facilitates packaging of the ssDNA into new phage particles. Gene V protein also binds with some specificity to a translational operator sequence on phage fl gene II mRNA (9-12).The gene V protein provides a general model for proteinssDNA interactions that are strong, yet not sequencespecific. Gene V protein binding to single-stranded nucleic acids and to oligonucleotides has been studied using chemical modification, spectroscopic techniques, and mutagenesis (13-24). The structure of the protein-ssDNA complex has been studied using electron microscopy and solution scattering methods and is found to consist of a regular left-handed superhelix in which the gene V protein dimers are arrayed on the outside of the superhelix and the ssDNA strands are inside (8,25). The structure of the gene V protein is also of interest because its small size and the large number of mutants available have made it a useful model for determining effects ofamino acid substitutions on protein stability and function (23,26,27).A model for the crystal structure of the wild-type (WT) gene V protein has been reported (28), but recent NMR studies have demonstrated that the positions of amino acids involved in the segments of antiparallel (-structure are not compatible with those in the model (21), and a new determination of the structure was necessary. The multiwavelength anomalous diffraction (MAD) technique (29) was ideally suited for this purpose, as the WT gene V protein contains two methionine residues (Met-1 and Met-77) that might be substituted in vivo in Escherichia coli by selenomethionine. Here we report the determination of the gene V protein structure using the MAD technique, the refinement of the structure using x-ray diffraction data on the WT gene V proteinAt and a model for protein-protein contacts in the gene V protein-ssDNA superhelical complex. MATERIALS AND METHODSModeling of the Protein Portion of Gene V Protein-ssDNA Complex. The gene V protein dimer was placed with its internal two-fold axis of symmetry perpendicular to the axis of the superhelix to be generated and Phe-73 pointing ei...

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The complex [Mo(C,F,) (CO)2(q-C7H,)] can be reduced either chemically or electrochemically by one electron. Electrochemical studies suggest that the reduction is followed by a rapid but reversible structural change. The orange intermediate reduction product has not yet been isolated in pure form. Chemical reduction with sodium amalgam in tetrahydrofuran in the presence of trimethyl phosphite initially forms this orange product, but precipitation with hexane yields the pale yellow zerovalent o-aryl molybdenum complex Na[cis-mer-Mo(C,F,) (CO),{P(OMe),},],in which the cycloheptatrienyl ligand has been displaced by three trimethyl phosphite groups. This complex has been fully characterized by IR, 'H and 31 P N M R spectroscopy and X-ray crystallography.No stable molybdenum(0) a-alkyl or -aryl complexes have been reported. Whereas the other Group 6 transition metals form a variety of anionic [MR(CO),L]-complexes [M = Cr or W; L = CO, PPh,, PMe,, or P(OMe),; R = Me, Et, Ph, CH,Ph, etc.], lP3 the corresponding molybdenum complexes are apparently not stable. The only reported molybdenum(o) complexes containing organic o-carbon-bound ligands are the anionic heptafluoro-1 -methylpropenyl complex, [Mo(CO),-(C,F,)] -,, and the alkynyl complexes [Mo(C=CR)(CO),L] -(L = C O or PPh,, R = Me or Ph).' The only reported X-ray crystal structure of a zerovalent Group 6 o-alkyl or -aryl complex is that of [N(PPh3)2][cis-WMe(CO),(PMe3)].3 We report here the synthesis, characterization and X-ray crystal structure determination of the zerovalent molybdenum 0-perfluoroaryl complex, Na[c~s-rner-Mo(C,F,)(~o),-{P(OMe),),] 2. This compound was synthesised as part of an investigation of the redox chemistry of the cycloheptatrienylmolybdenum complex [ M o ( C , F , ) ( C ~) ~( ~-C ~H 7)] 1. Isoelectronic cylopentadienyliron complexes are known to undergo both oxidatively-and reductively-induced migratory C O inser-tions.6-s An investigation of the oxidative electrochemistry of compound 1 revealed a reversible one-electron oxidation.' However, chemical oxidation in methanol produced an approximately 50% yield of methylpentafluorobenzoate, indicating that some C O insertion did occur after oxidation. This paper presents the results of the reductive electrochemistry of compound 1 and the characterization and X-ray structural analysis of the product resulting from the reaction of the reduced species 1 -with trimethyl phosphite. Results and DiscussionElectrochemical Reduction of [ Mo( C,F,)( CO),( -C7H,)] 1.-In a previous investigation it was found that complex 1 displayed a chemically and electrochemically reversible oneelectron oxidation at + 0.49 V us. ferrocenium-ferrocene in CH,CI,.' The reductive electrochemistry of this compound is substantially more complicated. In tetrahydrofuran (thf), cyclic voltammetry of compound 1 at a platinum electrode at a scan t Sodium cis-dicarbonyl-mu-(pentafluorophenyl)tris( trimethyl phosphi te)molybdate(o).

The need for accurate absorption data on small single crystals has led to the development of a new optical beam condenser design for UV-Vis spectroscopy. This design is based on the style of beam condensers used in IR spectrophotometers as opposed to the standard light pipes and lens focusing devices. The six-mirrored unit focuses the slit image on samples as small as 300 µm. The design and standardization of the beam condenser unit are presented.