Mechanism of formamide hydroxylation catalyzed by a molybdenum-dithiolene complex: A model for xanthine oxidase reactivity

The intrinsic reactivity of the molybdenum center in xanthine oxidase has b een studied by electronic structure calculations of a molybdenum-dithiolene model, based on the crystal structure of the closely related aldehyde oxid oreductase from Desulfovibrio gigas. Using first-principles electronic stru cture calculations (the HF/DF B3LYP method at the LanL2DZ level), we find t hat the reoxidation of the substrate-reduced molybdenum center proceeds thr ough hydration, followed by subsequent loss of two electrons. The oxidation is likely to be coupled with loss of protons first from water coordinated to Mo(IV) species to give Mo-v-OH and second from the A Mo-SH ligand of the Mo(V) species to give a Mo-VI=S species. Starting with the structure of th e oxidized center and formamide as a substrate, and using (V)MP2/LanL2DZ, f ormalism, we identify the reaction transition state as a planar S=Mo-O ... C complex formed upon nucleophilic attack of metal-bound hydroxide on the s ubstrate carbon atom that is to be hydroxylated. Following sp(2) --> sp(3) rehybridization of this carbon atom to create an R-chiral tetrahedral cente r, the transition state breaks down via hydride transfer from the substrate carbonyl carbon to the Mo=S as the dominant reaction coordinate. The react ion is completed by product dissociation and replacement by water in the me tal coordination sphere. Alternative transition states, involving molybdenu m-carbon bond formation, are found to be energetically and stereochemically prohibitive.