MOLYBDENUM(V) SITES IN XANTHINE-OXIDASE AND RELEVANT ANALOG COMPLEXES- COMPARISON OF O-17 HYPERFINE COUPLING

Electron paramagnetic resonance at 2-4 and 9 GHz has been used to exam ine O-17 (I = 5/2) hyperfine coupling in signals from Mo(V) centers in milk xanthine oxidase (Rapid Type 1, Rapid Type 2, Slow) and in the s ynthetic species [MoOXL]- and [MoO(XH)L] (X = O, S; LH-2 = thyl-N,N'-b is(2-mercaptophenyl)-1,2-diaminoethane) generated in solution. Compari son of the new 170 data with available H-1, O-17, S-35, and Mo-95 info rmation indicates that three ligands can be defined in each of the Mo( V) centers derived from active enzyme: Very Rapid, [Mo(V)OS(OR)] (OR, product anion); Rapid Type 1 and Type 2, [Mo(V)O(SH)(OH)]. Formulation of the oxidized resting center as fac-[Mo(VI)OS(OH)] allows derivatio n of a catalytic cycle consistent with known properties of the enzyme.