Chemical evolution of a nitrogenase model. V. Reduction of nitriles

Abstract

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, ...

Reduction of Acetylene and Hydrazine with a Molybdenum-Glutathione Complex

“…Our attempts to demonstrate a capability of the molybdenumglutathione complex to catalyze the reduction of N2 to ammonia failed. In our opinion, however, the recently reported reduction of N2 with a ratio of 0.004 mol of N2 reduced per mol of Mo-cysteine complex during a period of 48 hr (10) should not be regarded as a significant catalytic reduction of N2. In contrast, we emphasize that all the model chemical systems studied thus far lack the essential quality of nitrogenase, which is the efficient catalysis of the binding and reduction of N2 to NH3.…”

“…4. The highest specific activity was observed with 10 complex was added under comparable conditions. The specific activity of the complex decreased rapidly at concentrations of complex above 50 Mug in a reaction mixture of 3 ml.…”

“…Some of the models involve components that are completely different from those that participate in biological dinitrogen fixation (6). Special interest has been shown in chemical model systems that involve essential nitrogenase-system components (8)(9)(10)(11), such as thiol compounds, molybdate, ATP, and iron. With the acetylene-reduction test, the highest activity reported for a chemical model in the presence of ATP is only 0.3% of that of nitrogenase when they are compared on a molar basis (9).…”

Fixation of Nitrogen in Solution in the Presence of Transition Metal Complexes

“…The authors regard this system as a close model of nitrogenase, although under the optimal conditions the yield of ammonia is extremely small, reaching 0.06-0.1%relative to the initial molybdenum compound after five days at a nitrogen pressure of 140 atm. In later communications 143 ' 144 the authors reported that the results were confirmed with the aid of labelled nitrogen 15 N 2 . Acetylene inhibits the reduction of nitrogen, while carbon monoxide does not affect the yield of ammonia appreciably.…”

“…For example, in the reduction of isonitriles molybdate can be replaced by compounds of other transition metals (WO4", VOi", Pd 2 *, Rh , Ru 3+ , Co 3+ , and Fe 2+ ), some of which are more effective than molybdenum compounds 143 .…”