Kinetics of the hydride transfer reactions from three aliphatic alcohols to two NAD+ models (9-phenylxanthylium ion (PhXn+) and xanthylium ion (Xn+)) were determined.  2o KIEs on both hydride donors and acceptors for both hydride transfer and deuteride transfer were determined.  Both of them are close to unity and significantly far from the corresponding EIEs.  2o KIE on 9-H/D of Xn+ in some systems are even larger than unity outside of the normal range from EIE to unity.  Moreover, 2o KIEs are slightly larger in hydride transfer than in deuteride transfer in some systems.  These findings suggest a nonclassical hydride tunneling mechanism involving an imbalanced tunneling ready state where rehybridization of both the hydride donor and the acceptor lags behind the H-tunneling.  This is in sharp contrast to the H-tunneling mechanism found in the corresponding enzymatic reactions (e.g., the ADH reactions) in which rehybridization precedes H-tunneling.  The latter observations in enzymes have been used to suggest the role of enzyme motions in organizing favorable reaction geometries for H-tunnelling, which should contribute to the enzymatic rate enhancement.  The findings in this work lend strong support to this important proposition.

Imbalanced tunneling ready states in alcohol dehydrogenase model reactions: Rehybridization lags behind H-tunneling

245th National Meeting (2013)

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