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.

Abstract

The α-H/D (if available) and remote β-type N-CH3/CD3 2° kinetic 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 rehybridization appears to lag behind the charge development in the donor moiety. The charge distribution at the tunneling ready transition state is in agreement with the Hammett correlations; the donor is product-like, and the acceptor is reactant-like, indicative of a partial negative charge borne by the "in-flight" nucleus being "hydridic" in character. Results were compared with the α-2° KIEs on NADH/NAD+ and the Hammett correlations in closely related enzymes. The comparison implicates that the H-tunneling probability would be enhanced by these enzymes.