KINETICS OF OXYGEN-ATOM TRANSFER IN AN ANALOG REACTION SYSTEM OF THE MOLYBDENUM OXOTRANSFERASES

A newly developed analogue reaction system of the molybdenum oxotransferases, MoO2(tBuL-NS)2 (2) + X reversible MoO(tBuL-NS)2 (3) + XO (tBuL-NS = bis(4-tert-butylphenyl)-2-pyridylmethanethiolate(1-)), exhibits broad substrate reactivity. The kinetics of the reactions with X = Et3P and XO = Me2SO, Ph2SO, (CH2)4SO, Ph3AsO, Ph2SeO, and pyridine N-oxide and three substituted variants have been measured in DMF solutions at 298 K. Activation parameters were determined for seven substrates over temperature ranges of at least 40 K including 298 K. All reactions follow second-order kinetics. In contrast to a previous analogue reaction system, kinetics parameters for reduction are sensitive to substrate. These were selected to cover a range of X-0 bond dissociation energies, basicities, and steric factors. For reduction of substrates without large steric impediments, rate constants and DELTAH(double dagger) values cover a range of 2.2 to 1 x 10(-4) M-1 s-1 and 9.9-14.2 kcal/mol, respectively. Activation entropies (-21 to -33 eu) are consistent with an associative transition state. Rates are controlled by substantial contributions of both DELTAH(double dagger) and TDELTAS(double dagger) to DELTAG(double dagger); no one property of the reductant 3 or substrate XO can be demonstrated to dominate relative reaction rates. Principal contributions to the activation enthalpies include major structural rearrangement of 3 (necessitated by the structural differences between 3 and 2) and substrate binding and stabilization of the rearranged form of 3 (5), which can act as an oxo acceptor with a minimum of further rearrangement. A reaction scheme is proposed in which a species [5...OX]double dagger is the transition state. The extent of X-O bond weaking in this state cannot be discerned because DELTAH(double dagger) values do not correlate convincingly with X-O bond energies.