Charge at the migrating hydrogen in the transition state of hydride transfer reactions from CH groups to hydride acceptors. Dynamics of the redox-couple NADH-NAD(+)

摘要

We consider the controversial conclusions of the charge at the migrating hydrogen in the transition state of hydride-transfer reactions from CH-groups to hydride acceptors. Quantum chemical calculations were performed on elementary organic reactions involving carbenium ions, which can be considered as hydride acceptors. We also discuss the biochemical hydride-transfer reactions in which the coenzyme NADH-NAD(+) plays an important role. With the calculations and the experimental model systems, an answer is given for the stereospecificity of the hydride transfer. Generally, the hydride transfer occurs via a trigonal pyramidal geometry in which the transferred hydride of the CH-group is located in the axial position. In the case of the coenzyme NADH-NAD', the hydride transfer is coupled with an out-of-plane orientation of the carboxamide group of the pyridinium moiety, resulting in an increased stereospecificity. (C) 2004 Wiley Periodicals, Inc.