title: Theoretical Studies on Models for the Oxo-Transfer Reaction of Dioxomolybdenum Enzymes.
abstract: Patterned after synthetic model systems for dioxomolybdenum enzymes, our theoretical model system produces an energy profile and structures for the various species and oxidation states in the catalytic cycle. A key step in this cycle is the oxo-transfer reaction. Here, our substrate, PMe(3), approaches [Mo(VI)O(2)](2+) at an O-Mo-O-P dihedral angle of 90 degrees, i.e. perpendicular to the MoO(2) plane, crosses over a barrier of 14 kcal/mol, and rotates to an O-Mo-O-P dihedral angle of 0 degrees to form an intermediate, [Mo(IV)O(OPMe(3))](2+), which is 69 kcal/mol more stable than the reactants. The direction of the substrate's attack leaves the two d electrons of this Mo(IV) system in an orbital which is delta with respect to the remaining spectator Mo-O bond, a configuration which allows this O to form a formal triple Mo-O bond. The displacement of the product, OPR(3), by water, H(2)O, proceeds via an associative mechanism with a barrier of only 19 kcal/mol. In our model, [Mo(IV)O(OH(2))](2+) then reacts with [Mo(VI)O(2)](2+) to form [Mo(V)O(OH)](2+), a process which is exothermic by 14 kcal/mol. The addition of O(2) then oxidizes [Mo(V)O(OH)](2+) to [Mo(VI)O(2)](2+) to complete our model catalytic cycle.
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, Texas 77843-3244.
authors.forename: Michelle A.
authors.initials: MA
authors.lastname: Pietsch
authors.forename: Michael B.
authors.initials: MB
authors.lastname: Hall