High Resolution Image Download MS PowerPoint Slide Observed shift from temperature (T)-independent hydrogen tunneling kinetic isotope effects (KIEs) in enzymes to T-dependent KIEs in mutants has been attributed to the donor(D)–acceptor(A) separation effects caused by the weakened protein dynamical compression of D–A distances (DADs) in mutants. To examine the relationship between D–A separations (DADs) and T-dependence of KIEs (represented by Δ E a = E aD – E aH ), we design hydride-transfer reactions in solution. Our hypothesis is that a looser nucleus-transfer system exhibits a larger Δ E a value. Herein, the Δ E a ’s were determined for three series of apparent hydride-transfer reactions of NADH models in acetonitrile. These include hydride-transfers (1) from Hantzsch ester to 10-methyl-9-substituted(R)acridinium ions (RMA + ), (2) from the reduced RMA + (RMAH) to a benzoquinone structure, and (3) from RMAH to the (BnTPEN)Fe(IV)═O 2+ complex. Reactions (2) and (3) use multistep electron–proton–electron sequential transfer mechanisms. Δ E a increases from reactions (1) (0.94–1.19 kcal/mol) to (2) (1.14–1.60 kcal/mol) to (3) (3.05–5.05 kcal/mol), and within each series, Δ E a increases with the size of the R substituent. The unusually high Δ E a ’s observed for the iron(IV)-oxo complex reactions are likely partly attributed to electrostatic repulsion between like-charged RMAH +• acid and (BnTPEN)Fe(III)═O + in the reaction complex. These results support our hypothesis and the proposed role of protein dynamics in barrier compression for enzyme catalysis.
DeGroot et al. (Tue,) studied this question.