Add time:09/06/2019         Source:sciencedirect.com

Nicotinamide adenine dinucleotide (NADH) is a coenzyme involved in enzyme-catalyzed oxidation-reduction reactions in which a hydride ion is transferred to the substrate or vice versa As part of a broader theoretical study designed to elucidate the mechanism of the hydride-ion transfer, ab initio calculations were carried out at the SCF/STO-3G, SCF/3-21G and SCF/6-31G levels on the 1-methyl-nicotinamide and 1-methyl-dihydronicotinamide coenzyme analogues. As the orientation of the carboxamide side-chain may play a catalytic role in the hydride-ion transfer, analytic-gradient methods were used to study the geometries, relative energies, energy barriers and electron distributions for the in-plane (cis and trans) and several out-of-plane orientations of the carboxamide group with respect to the planar nicotinamide rings. The results show that the most stable conformer for both reduced and oxidized species is cis planar, with a local shallow minimum at the trans configuration, and that the energy barrier to cis-trans rotation is greater for the reduced species. Some implications for the enzymic mechanism are discussed.

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