TRANSITION STRUCTURES OF HYDRIDE TRANSFER-REACTIONS OF PROTONATED PYRIDINIUM ION WITH 1,4-DIHYDROPYRIDINE AND PROTONATED NICOTINAMIDE WITH 1,4-DIHYDRONICOTINAMIDE

Hydride transfer reactions of protonated pyridinium ion with 1,4-dihydropyridine and protonated nicotinamide with 1,4-dihydronicotinamide have been studied with ab initio molecular orbital calculations. There is a strong preference for a syn or stacking approach of the two pyridine rings in transition structures. The pyridine rings are slightly puckered into boat conformations in the transition structures. When in a cis conformation, the 3-amide group of nicotinamide slightly increases the activation energy for the hydride transfer. When the group is in a trans conformation, it significantly reduces the activation energy for the hydride transfer. There is a preference for the trans amide group to be out-of-plane, with the carbonyl group directed toward the transferring hydride. The relevance of these findings to enzymatic systems involving NAD(+) and NADH is also discussed.