Molybdenum(VI) and Molybdenum(V) Complexes with N,N'-Dimethyl-N,N'-bis(2-mercaptophenyl)ethylenediamine. Electrochemical and Electron Paramagnetic Resonance Models for the Molybdenum(VI/V) Centers of the Molybdenum Hydroxylases and Related Enzymes

Article abstract of DOI:10.1021/ja00253a016

As models for the molybdenum(VI/V) centers of the molybdenum hydroxylases and related enzymes, MoO2L and MoOClL (LH2=N,N'-dimethyl-N,N'-bis(2-mercaptophenyl)ethylenediamine) have been synthesized.The structure of MoO2L hasbeen determined by X-ray crystallography.The compound crystallizes in space group P2₁/n, with a=10.049(2) Angstroem, b=14.538(2) Angstroem, c=12.143(1) Angstroem, β=103.73(1) deg, and Z=4.MoO2L is six-coordinate with approximate C₂ symmetry, with the two thiolate S atoms trans to one another and the two tertiary amine N atoms approximately trans to the terminal oxo groups.MoO2L undergoes reversible one-electron reduction on both cyclic voltammetric and coulometric time scales to give various oxo-Mo(V) complexes.The EPR spectra of these complexes including 98Mo, 2H, and 17O substituted species, are consistent with formulation as ^-, cis-MoO(OH)L, ^-, and cis-MoO(SH)L. 1H coupling constants for cis-MoO(OH)L (1.65 mT) and cis-MoO(SH)L (1.05 mT) and the 17O coupling constants for Mo17O(17OH)L (0.25 and 0.82 mT), ^- (0.43 mT), and Mo17O(SH)L (0.22 mT) have been measured.MoOClL exhibit reversible one-electron electrochemical reduction in its cyclic voltammogram but decomposes upon coulometric reduction.EPR data indicate it is trans-MoOClL.Substitution of Cl^- by OH^-, F^-, and SH^- has been effected in solution.The implications of the results for the structures of Mo(VI/V) centers of the enzymes are discussed.In particular, Mo^VOS(OR) and Mo^VO(SH)(OR) (OR=bound product) are suggested as the centers responsible for the "very rapid" and "rapid" EPR signals of xanthine oxidase.An Mo^VO(OH) center appears to be involved in the "slow" EPR signal of xanthine oxidase and the low pH forms of sulfite axidase and nitrate reductase; the high pH forms of the latter enzymes, however, do not apear to involve a conjugate base Mo^VO2 center.