J. T. Spence

Reactions of molybdenum compounds in solution — model reactions for biological systems☆

Abstract Kinetic investigations of the reduction of flavins by Mo(V) complexes in aqueous solution as model systems for Mo—flavin interaction in xanthine oxidase and other molybdenum enzymes are discussed. Data are presented indicating that Mo(V)—dimers may react by either one- or two-electron mechanisms, depending on conditions and ligand environment. The reduction of flavin mononucleotide by the model complex, μ-oxo-bis[oxodihydroxo(L-cysteinato)molybdate(V)], is reported in detail, and mechanisms for its reaction under a variety of conditions are presented. The reduction of this complex with NaBH4 to produce a Mo(III) species is also discussed, and investigations of the oxidation of aldehydes by Mo(VI)—thiol complexes in non-aqueous solvents are reported.

Modeling the molybdenum sites of the molybdenum hydroxylases

Abstract Molybdenum hydroxylases are multicomponent enzymes which catalyze two electron oxidations of purines, aldehydes, formate and sulfite in animals and microorganisms [1]. In addition, eukaryotic nitrate reductase [2] and several as yet poorly characterized molybdenum containing enzymes have properties similar to those of the hydroxylases. Recent EPR and EXAFS investigation indicate the presence of a terminal oxo and a terminal sulfido group on Mo in oxidized (Mo(VI)) xanthine oxidase and xanthine dehydrogenase and two oxo groups in oxidized sulfite oxidase [1, 3]. In the reduced state (Mo(V), (IV)) the sulfido group appears to be converted to SH (xanthine oxidase, xanthine dehydrogenase) or one oxo to OH (sulfite oxidase) [3]. In addition, 2–3 Mo thiolate sulfur ligands are present for both oxidized and reduced enzymes [3]. One or more of the thiolate ligands may be located on a side chain of a reduced pterin proposed to be the cofactor common to all Mo hydroxylases [4]. The reduction potentials of the Mo centers of the hyroxylases have been determined and are found to differ considerably between enzymes (−0.355 V for the Mo(VI)/(V) couple in xanthine oxidase [5], 0.038 V in sulfite oxidase [6], e.g.). Recent model studies have concentrated on synthesis and structural characterization of dioxo-Mo(VI) complexes with N, S donor sets which mimic the EXAFS results [7] (Fig. 1a), on monomeric oxo-Mo(V) complexes having EPR parameters similar to those of the enzymes [8, 9] (Fig. 1b), and on oxo-Mo(VI), (V) and (IV) complexes which mimic the redox behavior of the enzymes [9, 10] (Fig. 1b). These results are briefly reviewed. Current work in this laboratory is directed towards the synthesis and characterization of dioxo-Mo(VI) complexes with sterically bulky bi-, tri- and tetradentate ligands which may be electrochemically or chemically reduced to monomeric Mo(V)(O)(OH) complexes: Normally, such reductions give oxo bridged Mo(V) dimers; the presence of bulky groups on the ligands, however, inhibits the dimerization. Representative ligands: Synthetic methods for the preparation of the complexes and their properties (IR, electronic and EPR spectra; electrochemistry) are reported. The relationships between EPR and electrochemical parameters and structures of the complexes are explored and the implications for the molybdenum hydroxylases are discussed. Current problems in modeling the molybdenum centers of the hydorxylases and possible directions for research toward the solutions of these problems are presented.

The reduction of nitrate by molybdenum(V) complexes in dimethylformamide

Abstract As a model system for the molybdenum enzyme nitrate reductase, the reduction of NO−3 by monomeric Mo(V) complexes of the formula MoOCl3L (L = o-phenanthroline, cis- andtrans-bipyridyl) in anhydrous DMF has been investigated. The complexes reduce NO−3 in a one-electron reduction, producing NO2 and the corresponding Mo(VI) complex MoO2Cl2L. The reaction is first order in MoOCl2L and is inhibited by Cl−. The order with respect to NO−3 is complex and the kinetics are best represented by the rate expression — d[MoOCl 3 L] d t = a[ NO − 3 ] (b[ Cl − 3 ] + c[ NO − 3 ]) . A mechanism involving the preliminary dissociation of Cl− from the complex, followed by the formation of an intermediate NO−3 complex which subsequently undergoes electron transfer to produce products, has been developed.

Model Reactions of Molybdenum Complexes

Publisher Summary This chapter discusses the model reactions of molybdenum complexes. Model studies have proven particularly useful for interpreting the properties and reactions of many metal-containing enzymes and proteins. Model studies for molybdenum enzymes have suffered, by comparison, because of a lack of biochemical data. The application of the new extended X-ray absorption edge fine-structure (EXAFS) technique and the isolation of low molecular weight molybdenum cofactors have improved the situation markedly. Results from studies of model reactions of molybdenum complexes have led to a number of interesting and biochemically relevant conclusions. The failure of model systems to oxidize purines, aldehydes, or SO3 successfully is more likely because of kinetic than thermodynamic reasons, although this conclusion is far from certain. While recent EXAFS results indicate that oxo groups are not present on Mo in nitrogenase, the demonstration by Schrauzer (1975) that the reduction of N2 in aqueous solution is effected by oxo Mo(IV), or possibly Mo(III) complexes, supports the hypothesis that the binding and reduction of N2 occur at, or near, the Mo center of this enzyme. This postulate is further strengthened by the fact that acid hydrolysis of formally zero valent Mo–(N2) complexes produces NH3. Ligand exchange reactions indicate that no oxidation state of Mo from +3 to +6 may be eliminated because of inertness to substitution, while S ligands appear to enhance the rates of such processes.