MOLYBDENUM(V) SITES IN XANTHINE-OXIDASE AND RELEVANT ANALOG COMPLEXES - COMPARISON OF MO-95 AND S-33 HYPERFINE COUPLING

Mo-95 (I = 5/2) hyperfine matrices have been defined from multifrequency (2-4 and 9 GHz) electron spin resonance spectra of the Rapid Type 1, Rapid Type 2, and Slow centers of milk xanthine oxidase. For comparison, Mo-95 hyperfine matrices were similarly extracted for the synthetic species [MoOXL]- and [MoO(XH)L] (X = O, S; LH2 = N,N'-dimethyl-N,N'-bis(2-mercaptophenyl)-1,2-diaminoethane). In addition, S-33 (I = 3/2) hyperfine matrices were derived for the thio ligand of [MoOSL]- and for the mercapto ligand SH of [MoO(SH)L] for comparison with the S-33 coupling observed in a number of EPR signals of xanthine oxidase. Detailed examination confirms [Mo(V)O(SH)], [Mo(V)O(SH)(OH)], and [Mo(V)O(OH)] as the centers responsible for the Rapid Type 1, Rapid Type 2, and Slow EPR signals and provides compelling evidence that [Mo(V)OS] is the source of the Very Rapid signal. A characteristic of the catalysis is a correlated electron-proton transfer from substrate to the molybdenum site. The present work suggests a specific mechanism: reduction of the Mo(VI)OS resting form to Mo(V) or Mo(IV) causes occupation of a highly covalent MoS pi-antibonding orbital with a consequent increase in the basicity of the thio ligand.