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Donor–Acceptor Separation Augments Temperature Dependence of Kinetic Isotope Effects in NADH Model Hydride Transfer Reactions: Mimicking Enzyme versus Mutant Dynamics
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DOI：10.1021/acs.jpcb.6c02719.png)
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Abstract
En 中文
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 ΔEa = EaD – EaH), we design hydride-transfer reactions in solution. Our hypothesis is that a looser nucleus-transfer system exhibits a larger ΔEa value. Herein, the ΔEa’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. ΔEa 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, ΔEa increases with the size of the R substituent. The unusually high ΔEa’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.
Keywords:
Isotope effects
Peptides and proteins
Reaction mechanisms
Solvents
Transfer reactions
Journal
T
IF:
2.9
Papers:
767
Citations:
2
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Cited Papers