Imbalanced Transition States from α-H/D and Remote β-Type N-CH/D Secondary Kinetic Isotope Effects on the NADH/NAD+ Analogues in Their Hydride Tunneling Reactions in Solution

2019 
The α-H/D (if available) and remote β-type N-CH3/CD3kinetic isotope effects (KIEs) on 10-methylacridine (MAH), 9,10-dimethylacridine (DMAH), 1,3-dimethyl-2-phenylbenzimidazoline (DMPBIH) and on the oxidized forms MA+ and DMA+, in their hydride transfer reactions with several hydride acceptors/donors in acetonitrile, were determined. The corresponding equilibrium isotope effects (EIEs) were computed. Hammett correlations of several closely related hydride transfer reactions were constructed using the literature data. The α-2° KIEs on both MAH and MA+ are inflated relative to the semiclassical prediction on the basis of the KIE/EIE comparison and Hammond’s postulate. This together with previously published unusual 1° and 2° KIE behaviors strongly suggest a H-tunneling mechanism. By comparing with the EIEs, the α-2° KIEs were used to analyze the rehybridization of the reaction center C and the N-CH3/CD3 2° KIEs to calculate the charge distribution on the structure containing N during H-tunneling. The reh...
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