Nitrogenase model systems composed of molybdate and sulfur containing ligands such as l(+> cysteine catalyze the reduction of CN-, N3-, NsO, and N2 with NaBH4 as the reductant. All reactions are stimulated by ATP and other nucleoside phosphates. The reduction of CNyields CH4, NH3, smaller amounts of C2H6, C2H4, and traces of CH3NH2; N3-and N20 are reduced to N2 and NH3 and N2 and H20, respectively, as with nitrogenase. The molybdate-cysteine catalysts also reduce ethylene oxide to C2H4 and H20 and ethyl diazoacetate to ethyl acetate plus N2; these reductions are also stimulated by ATP. In all reactions, added iron salts act as cocatalysts. Experiments with l5N2 indicate that the fixation of N2 is molybdenum-specific. A comparison of the relative rates of reduction of all substrates indicates parallels with the relative rates of reduction observed with nitrogenase. The Km values for reactions of substrates in the model system are as a rule larger than those

Nitrogenase model systems composed of molybdate and thiol ligands such as L(+)-cysteine reduce molecular nitrogen to ammonia slowly in the presence of NaBH4 as the reducing agent. In the presence of substrate amounts of ATP, the reduction of nitrogen is significantly stimulated but leads to the accumulation of diimide in the reaction solutions. The diimide decomposes or disproportionates to nitrogen, hydrogen, and hydrazine and is not reduced as such. The ammonia produced in all reactions arises primarily from the reduction

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Nitrogenase model systems, composed of molybdate and thiol ligands such as, e.g., cysteine, catalyze the reduction of aliphatic nitriles to alkanes and NHa in the presence of NaBH4. The reaction is significantly stimulated by substrate amounts of ATP, is faster in DzO than in HzO, and is inhibited up to 90 and 41 %, respectively, by CO and Nz at 1 atm of partial pressure. Experiments with IeNz indicate that molecular nitrogen is reduced under these conditions, while CO apparently inhibits by forming a complex with the catalyst. The catalytically active species in the model systems are mononuclear molybdenum-thiol complexes. In the active reduced form molybdenum is probably in the 4f oxidation state. The ATP stimulates the reduction of nitriles and of other substrates by accelerating the conversion of oxidized (i.e., Mo(V) and Mo(V1)) forms of the catalyst complexes to the active reduced form, by providing a better leaving group for the relatively inert molybdenum-bound OH groups. The initial interaction of the substrate with the catalyst is assumed to involve side-on bonding of the CN group, giving rise to the formation of an organomolybdenum species, whose subsequent reactions occur without Mo-C bond cleavage, until a terminal alkylmolybdenum complex is formed, whose hydrolysis yields the saturated hydrocarbon product plus the oxidized form of the catalyst. The reduction of unsaturated nitriles affords mixtures of alkanes and olefins in analogy to the reactions catalyzed by Azotobacter vinelandii nitrogenase. The olefinic organomolybdenum intermediates in this case undergo double bond and cis-trans isomerization prior to the hydrolysis of the molybdenum-carbon bond. The types of reaction of unsaturated nitriles are exemplified in detail for cis-and trans-crotononitrile as the substrate, also with catalyst systems containing 2-aminoethanethiol, glutathione, and bovine serum albumin as the thiol component. Since all previously observed reactions of nitriles with nitrogenase are essentially duplicated by the model systems, nitrile binding and reduction by nitrogenase must occur at the molybdenum active site of the Fe-Mo-protein of mol wt 270,000. Iron, which cannot replace molybdenum in the model system, exerts a modest cocatalyst effect. Its primary function in nitrogenase appears to be the activation of electron transport to the molybdenum active site.zotobacter vinelandii nitrogenase (Nz-ase) reduces A saturated, unbranched nitriles up to, and including, n-butyronitrile, to alkanes plus ammonia (eq 1). 2-5 6H +, .1TP. Mu'+, nitrogenase R-CN + 6e > R-CHa + NH3 (1) ResultsThe N2-ase model systems employed were identical with those used prev i o~s l y ,~J consisting either of molar 1:l mixtures of L(+)-cysteine (Cys) and or the binuclear Mo(V) complex NazMo204(Cys)2. 5 H z 0 (complex I). The catalytic activity of freshly prepared MOO? 2--Cys mixtures is greater than that of equivalent solutions of complex I, which as such is not active. Complex I is known to dissociate in solution into monomeric species, however, ...