Molybdenum complexes nitrogenase

Nitrogen fixation is the conversion of N2 gas into ammonia, a process carried out by some soil bacteria, cyanobacteria and the symbiotic bacteria Rhizobium that invade the root nodules of leguminous plants. This process is carried out by the nitrogenase complex, which consists of a reductase and an iron-molybdenum-containing nitrogenase. At least 16 ATP molecules are hydrolyzed to form two molecules of ammonia. Leghemoglobin is used to protect the nitrogenase in the Rhizobium from inactivation by 02. 

The yields were found also to increase in the presence of phosphines, particularly trimethyl or tributyl phosphine. After all the improvements of the catalyst and reaction conditions the system became by far the most active of known non-biological catalytic systems for the reduction of dinitrogen at ambient temperature and pressure. The specific activity (the rate of N2 reduction per mole of the complex) reached and even exceeded that of nitrogenase. Up to 1000 turnovers relative to the molybdenum complex can be observed at atmospheric pressure and more than 10 000 turnovers at elevated N2 pressures. 

A nitrogenase model reaction has been accomplished in which significant amounts of NH3 (0.013-0.044 mol NH3/M0) are produced when trans-[Mo(N2)2(Ph2PCH2CH2PPh2)2] is added to[FeS(SCHPhCHPhS)]4 reduced to the 4 — or lower level. The molybdenum complex is also reducible by an excess of Na (naphthalene) to afford NH3 (0.100—0.300 mol NH3/M0). No N2H4 was observed in either reduction. The iron-dithiolen system alone is capable of fixing N2 at the 4— level under an N2 atmosphere NH3 up to... 

Coupling the results from the Mo model complex [57] with the explanation for the presence and possible action of homocitrate, these results provide the best model to date for N2 reduction by conventional molybdenum nitrogenases. Vanadium-dependent nitrogenases probably undergo similar reactions, but iron-only nitrogenases [66] and the unique molybdenum-dependent nitrogenase from Streptomyces thermoautrophicus [67] present further mysteries. 

Molybdenum complexes with sulfur-donor atoms play an important role in the nitrogenase enzyme system, and the study of their chemistry is of continuing interest. The present synthesis concerns cis-dinitrosylbis(lV,Af-diethyldithiocarbamato)molybdenum, originally described by Johnson et al. Their method of synthesis involves the conversion of molybdenum hexa-carbonyl to the unstable MoBr2(NO)2 using NOBr, followed by reaction... 

Ni(CN)3(NO)] - + iOa + HaO—[Ni(CN)3(NO)]2- + 20H-The importance of the redox nature of molybdenum centres in biological systems has been recognized in several studies. Model systems for nitrogenase, involving molybdate, catalytic amounts of iron(ii), and cysteine, effect the reduction of aliphatic nitriles to alkanes and ammonia in the presence of sodium borohydride. The reaction is accelerated by substrate concentrations of ATP, the active species being considered to be mononuclear molybdenum-thiol complexes. The dinuclear molybdenum complex (7) formed by... 

Further work has been reported - with Fe-Mo models for nitro-genase, and a molecular mechanism has been proposed for the action of molybdenum in enzymes. In all reactions catalysed by Mo enzymes, the product and substrate differ by two electrons and two protons (or some multiple thereof). The co-ordination chemistry of Mo suggests that there is a distinct relationship between acid-base and redox properties of Mo complexes, and that a coupled electron-proton transfer (to or from substrate) may be mediated by Mo in enzymes. Each of the molybdenum enzymes (nitrogenase, nitrate reductase, xanthine oxidase, aldehyde oxidase, and sulphite oxidase) is discussed and it is shown that a simple molecular mechanism embodying coupled proton-electron transfer can explain many key experimental observations. 

Complexes of molybdenum and tungsten with bidentate sulfur ligands have been investigated extensively. In recent years, the work in this field has been escalated by the impetus of designing models of such bioinorganic enzymes as nitrogenase and xanthine oxidase (125). The early work reviewed by Coucouvanis (1) dealt exclusively with the isolation of oxomolybdenum(V) and -(VI) species. 

However, when the X-ray crystal structure of the MoFe protein was examined, it was clear that homocitrate could not directly hydrogen bond to the histidine, since the carboxylate group and imidazole are stacked parallel to each other in the crystal. Nevertheless, as noted in the previous section, studies on model complexes have suggested that homocitrate can become monodentate during nitrogenase turnover, with the molybdenum carboxylate bond breaking to open up a vacant site at molybdenum suitable for binding N2. 

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