Structural Effects on the Temperature Dependence of Hydride Kinetic Isotope Effects of the NADH/NAD Model Reactions in Acetonitrile: Charge-Transfer Complex Tightness Is a Key.

It has recently frequently been found that the kinetic isotope effect (KIE) is independent of temperature () in H-tunneling reactions in enzymes but becomes dependent on in their mutants. Many enzymologists found that the trend is related to different donor-acceptor distances (DADs) at tunneling-ready states (TRSs), which could be sampled by protein dynamics. That is, a more rigid system of densely populated short DADs gives rise to a weaker dependence of KIEs. Theoreticians have attempted to develop H-tunneling theories to explain the observations, but none have been universally accepted. It is reasonable to assume that the DAD sampling concept, if it exists, applies to the H-transfer reactions in solution, as well. In this work, we designed NADH/NAD model reactions to investigate their structural effects on the dependence of hydride KIEs in acetonitrile. Hammett correlations together with N-CH/CD secondary KIEs were used to provide the electronic structure of the TRSs and thus the rigidity of their charge-transfer complexation vibrations. In all three pairs of reactions, a weaker dependence of KIEs always corresponds to a steeper Hammett slope on the substituted hydride acceptors. It was found that a tighter/rigid charge-transfer complexation system corresponds with a weaker dependence of KIEs, consistent with the observations in enzymes.