Mechanism of the Oxidation of NADH by Quinones. Energetics of One-Electron and Hydride Routes

Article abstract of DOI:10.1021/ja00288a035

The kinetics of NADH oxidation by 7 o-benzoquinones and 14 p-benzoquinones were studied by using buffered aqueous solutions and UV/vis spectroscopy.For each quinone the rate law was first order in NADH and first order in quinone.The rate constants varied from 0.0745 to 9220 M^-1s^-1.Variation of the pH from 6 to 8 gave no change in rate.The use of 4-D and 4,4-D2NADH revealed kinetic isotope effects.The dideutero data gave k_H/k_D in the range 1.6-3.1 for p-quinones and 4.2 for 3,5-di-tert-butyl-o-quinone.When p-quinones were used, the log k was a linear function of E^o for the quinone/hydroquinone monoanion (Q/QH(1-)) couple with a slope of 16.9 V^-1. o-Quinones reacted about 100 times more rapidly, but the same linear relationship with a slope of 16.4 V^-1 was observed.Comparisons to data for one-electron-transfer reactions indicate that such mechanisms are not involved.A hydride-transfer mechanism accommodates all the data, and rate-limiting hydrogen atom transfer followed by electron transfer cannot be ruled out.