Direct Correlation between Donor–Acceptor Distance and Temperature Dependence of Kinetic Isotope Effects in Hydride-Tunneling Reactions of NADH/NAD⁺ Analogues

Abstract

Recent study of structural effects on primary kinetic isotope effects (1° KIEs) of H-transfer reactions in enzymes and solution revealed that a more rigid reaction system gave rise to a weaker temperature dependence of 1° KIEs, i.e., a smaller isotopic activation energy difference (ΔE a = E aD – E aH). This has been explained within the contemporary vibrationally assisted activated H-tunneling (VA-AHT) model in which rigidity is defined according to the density of donor–acceptor distance (DADTRS) populations at the tunneling ready state (TRS) sampled by heavy atom motions. To test the relationship between DADTRS and ΔE a in the model, we developed a computational method to obtain the TRS structures for H-transfer reactions. The method was applied to three hydride transfer reactions of NADH/NAD+ analogues for which the ΔE a’s as well as secondary (2°) KIEs have been reported. The 2° KIEs computed from each TRS structure were fitted to the observed values to obtain the optimal TRSs/DADTRS’s. It was found that a shorter DADTRS does correspond with a smaller ΔE a. This appears to support the VA-AHT model. Moreover, an analysis of hybridizations at the bent TRS structures shows that rehybridizations at the donor–acceptor centers are much more advanced than predicted from the classical mechanism. This implies that more orbital preparations are required for the nonclassical H-tunneling to take place.

Solvent Effects on the Temperature Dependence of Hydride Kinetic Isotope Effects: Correlation to the Donor–Acceptor Distances

“…We have reported the structural effects on the T -dependence of KIEs of several hydride-transfer reactions of NADH/NAD + analogues in solution. − We found that a smaller Δ E a accompanies a more reactive donor/acceptor that forms stronger charge-transfer (CT) complexations. , We also found that in some of the systems, the more crowded reaction centers did appear to correspond with a smaller Δ E a . , These results support our hypothesis. In these studies, the short tunneling DADs would be mainly sampled by the CT complexation thermal vibrations.…”

“…27 We have attempted to use the 1,3-N,N-2CH 3 / 2CD 3 γ-2°KIEs on DMPBIH for its hydride transfer to 9-(para-substituted)phenylxanthylium ions to obtain the substituent effect on DAD TRS 's. 46,47 Here in this study, we were able to determine the N-CH 3 /CD 3 2°KIEs on both DMPBIH and MA + to evaluate the electronic structure and the rigidity order of the TRSs of their reactions in acetonitrile vs chloroform. For the reaction of HEH, however, no such 2°K IE on HEH is available.…”

“…We have proposed a hypothesis on the basis of the above explanations for enzymes. The hypothesis is that the more rigid the reaction system, the more difficult for the DAD to change with temperature, and the weaker the T -dependence of KIEs will be. − Here, a rigid TRS can be a tightly associated one with strong electronic attractions between the donor and acceptor, and/or with steric factors that minimize the flexibility of the two reaction centers, and/or with strong solvation effects that stabilize the TRS and thus strengthen the donor–acceptor attraction in it. Investigation of the hypothesis is expected to provide insight into the role of protein motions in catalysis and opens a new research direction that studies the relationship between structure and T -dependency of KIEs for general H-transfer reactions.…”

Correlation of temperature dependence of hydride kinetic isotope effects with donor–acceptor distances in two solvents of different polarities

“…We have recently developed a method to compute the degenerate [D–H A] ‡ and [D H–A] ‡ structures for the TRS structures. 46 In our method, we use the classical transition state (TS) structure to construct the initial guess of the TRS structure. The dihedral angles with respect to the reaction centers at D and A in [D–H A] ‡ at a selected DAD TRS are scanned to form the 2-dimentional potential energy surface (2D-PES) for reactant state and the same is done for the [D H–A] ‡ product state structure.…”

“…44 We have used the electronic, steric, and solvent effects to control the system rigidity for the study and have found that the hypothesis appears to be supported. 44–47…”