Replication of the Enzymatic Temperature Dependencyof the Primary Hydride Kinetic Isotope Effects in Solution: Causedby the Protein-Controlled Rigidity of the Donor–Acceptor Centers?

The change from temperature independence of the primary (1°) H/D kinetic isotope effects (KIEs) in wild-type enzyme catalyzed H-transfer reactions (∆Ea = EaD - EaH ~ 0) to strong temperature dependence with the mutated enzymes (∆Ea >> 0) have recently been frequently observed. This has prompted some enzymologists to develop new H-tunneling models to correlate ∆Ea with the donor-acceptor distance (DAD) at the tunneling-ready state (TRS) as well as the protein thermal motions/dynamics that sample the short DADTRS’s for H-tunneling to occur. While evidence supporting or disproving the thermally activated DAD sampling concept has extensively emerged, comparable study on the simpler bimolecular H-tunneling reactions in solution has not been carried out. Especially, small ∆Ea’s (~ 0) have not been found. In this paper, we report a study of the hydride transfer reactions from four NADH models to the same hydride acceptor in acetonitrile. The ∆Ea’s were determined, which are 0.37 (small), 0.60, 0.99, and 1.53 kca...