James L. Corbin

NG, NG-Dimethylarginine in myosin during muscle development

Abstract NG, N G - Dimethylarginine ( unsym - DMA ) has now been found to be present in myosin prepared from developing leg muscle. Neither monomethylarginine nor sym - DMA (NG N ′ G - dimethylarginine ) could be detected in our preparations. Cultured muscle cell myosin contains up to four residues of this amino acid per 5 × 105 grams of protein. Myosins from leg muscle of chick embryos and neonatal rats contained less than two residues of unsym - DMA , while none was detected in adult chicken or rat myosins. Cardiac myosins from developing as well as mature animals lacked or contained very little unsym - DMA . Actin was devoid of this amino acid.

Preparations and properties of tripodal and linear tetradentate N,S-Donor ligands and their complexes containing the MoO22+core

Abstract New linear and tripodal tetradentate ligands, LH 2 , are reported and their syntheses are described. The new linear ligands L = HSCH 2 CH 2 SCH 2 CH 2 NRCH 2 CR 2 SH, R = H, CH 3 ) and the new tripodal ligands N(CH 2 CH 2 SH) 2 CH 2 Z, Z = CH 2 NH 2 , CH 2 N(CH 3 ) 2 , CH 2 N(C 2 H 5 ) 2 , CH 2 SCH 3 and CO 2 - were synthesized. The known linear ligands HSCH 2 CH 2 NCH 3 (CH 2 ) n NCH 3 CH 2 CH 2 SH (n = 2, 3) and HSCR 2 CH 2 NHCH 2 CH 2 NHCH 2 CR 2 SH (R = H, CH 3 ) were also utilized. These ligands react with MoO 2 (acac) 2 in CH 3 OH to yield MoO 2 L complexes in high yield. Infra-red and 1 H nmr spectra provide evidence to supplement X-ray crystallographic results reported elsewhere for selected numbers of the series. Octahedral structures with cis MoO 2 2+ groupings are assigned. Solution 1 H nmr studies are consistent with a trans placement of the two thiolate donors in agreement with the X-ray studies.

Mo(VI) Complexes of N,S-Donor Ligands: Relevance to Molybdenum Enzymes

The molybdenum enzymes other than nitrogenase have a common molybdenum cofactor1,2 and spectroscopic studies of their molybdenum sites reveal these to be similar although not identical.3 EPR* studies on xanthine oxidase in the Mo(V) oxidation state,4 together with results from model compounds,5 led to the suggestion of sulfur as a donor atom to molybdenum. However, in these original studies and in subsequent investigations, 3,6 the inorganic compounds used in the comparison were not structurally and, at times, not even stoichiometrically defined. In order to provide a comprehensive set of structurally defined oxo-molybdenum complexes containing sulfur-donor ligands for comparison with the enzymes by EPR*, EXAFS* and other spectroscopic probes, we have embarked on an exten sive synthetic and isolation program to obtain relevant compounds in the Mo(IV), Mo(V) and MoiVI) oxidation states. Recently, EXAFS studies on xanthine oxidase and sulfite oxidase8 have confirmed the presence of sulfur and terminal oxo ligands in the coordination sphere of molybdenum and have revealed distinct similarities to some of the model compounds discussed here.9

Cobalt(II) Complexes of N-Aryl (OR Cyclopropyl)- 4-Amino-3-Pentene-2-Thiones

Abstract Neutral cobalt(II) complexes of various N-aryl(or cyclo-propyl)-4-amino-3-pentene-2-thiones were prepared for the first time. The metal ion could be removed, thus providing a route to the free ligands which were also new compounds, and generally not obtainable in a pure state without the utilization of the cobalt complex.

Cyanide and methylisocyanide: Probes for nitrogenase reactivity

Abstract Nitrogenase (N 2 ase) is composed of two separately purified proteins, the molybdenum-iron (MoFe) protein and the iron (Fe) protein. Nitrogen fixation requires both proteins, a reductant, protons and MgATP. The Fe protein is generally accepted as a specific one-electron donor for the MoFe protein, which is believed to contain the substrate-reduction site. Besides, N 2 , N 2 ase catalyzes the reduction of protons and a number of alternative substrates [ e.g. 1], including the two six-electron substrates, cyanide and methylisocyanide, we have recently studied. The rate-limiting step for N 2 ase turnover occurs prior to substrate reduction. Thus, total electron flow through the enzyme should be essentially independent of the substrate being reduced. Although this appears true N 2 fixation, H 2 evolution and C 2 H 2 reduction [2], both CN − [3] and CH 3 NC dramatically inhibit the rate of total electron flow through N 2 ase. Inhibition by both substrates is completely reversed by CO. Not only do CN− and CH 3 NC inhibit nitrogenase turnover, they also reduce the enzymes efficiency by increasing the amount of MgATP hydrolyzed for each electron pair used to reduce substrate. These data are interpreted in terms of CN− or CH 3 NC binding to the MoFE protein in such a way as to prevent electron transfer to substrate. With nowhere to go, the electrons fall back to the Fe protein to complete a futile cycle. Are the substrates N 2 , HCN and CH 3 NC reduced in one six-electron step or via a series of lesser reduced intermediates? Previously, we proposed that N 2 is reduced to ammonia via the two-electron intermediates N 2 H 2 and N 2 H 4 [4]. For the six-electron reductions of HCN to CH 4 + NH 3 and CH 3 NC to CH 4 + CH 3 NH 2 , we have definitely identified the four-electron products, CH 3 NH 2 (for HCN) and CH 3 NHCH 3 (for CH 3 NC), and suggest them as intermediates. The formation of two-electron reduced intermediates for both HCN and CH 3 NC is suggested by the product ratio of NH 3 -to-CH 4 (for HCN) and CH 3 NHCH 2 -to-CH 4 (for CH 3 NC) being greater than one. The data support mechanisms whereby the six-electron reduction of N 2 , HCN and CH 3 NC occur via a series of analogous two- and four-electron reduced intermediates. Thus, a common phenomenon is likely as an intimate part of the mechanisms of N 2 , HCN and CH 3 NC reduction. Although H 2 evolution is suggested as an obligatory part of the N 2 -fixation mechanism, it is not required for either HCN or CH 3 NC reduction. This apparent anomaly might be explained if N 2 , HCN or CH 3 NC were either reduced at different sites or bound and reduced by different redox states of the MoFe protein. So, as increasing the ratio of Fe protein-to-MoFe protein increases electron flow, component protein ratio titration experiments in the presence of N 2 ,HCN and CH 3 NC were used. They indicate that HCN and CH 3 NC bind to and are reduced at a redox state of the MoFe protein more oxidized than that responsible for either N 2 fixation or H 2 evolution. Do all substrates and inhibitors of nitrogenase bind to the same site on nitrogenase? Experiments with various combinations of substrates (N 2 , HCN, CH 3 NC, C 2 H 2 , N 2 O, N − 3 and inhibitors (H 2 , CO, CN − , CH 3 NC) indicate that either C 2 H 2 or N 2 O stimulate HCN reduction and influence its product distribution, implying simultaneous binding and at least two interaction sites on N 2 ase. CH 3 NC appears to act as both substrate and inhibitor on binding to the same N 2 ase site, implying productive and non-productive modes of binding.

Synthesis and chemistry of some binuclear oxomolybdenum(V) xanthate (O-alkyl dithiocarbonate) complexes

A simple method for the preparation of pure µ-oxo-bis[bis(O-alkyl dithiocarbonato)oxomolybdenum(V)] complexes, [Mo2O3(RO·CS·S)4](I; R = Me, Et, Pri, Bun, or Bui), and difficulties encountered with previous preparative methods are reported. Assignments of molybdenum–oxygen stretching frequencies in the i.r. spectra have been made and visible spectra have been reinvestigated. Reaction with dialkylamines, alcohols, or hydrogen sulphide results in loss of xanthate ligand and produces, in the last two cases, a series of new di-µ-sulphido-bis[(O-alkyl dithiocarbonato)oxomolybdenum(V)] complexes, [Mo2O2S2(RO·CS·S)2](II). Spectroscopic studies of these products indicate that those containing unbranched O-alkyl dithiocarbonate groups are polymers in the solid state, which dissociate in solution. Complexes (I) and (II) react with excess of xanthate ligand to produce the ion [Mo2O2S2(O·CS·S)2]2–, (III).