Experimental Evidence For Hydrogen Tunneling When The Isotopic Arrheniusprefactor (a_h/a_d) Is Unity

摘要

Elevated values for primary deuterium kinetic isotope effects (k_H/k_D), deviations of the temperature dependence of k_H/k_D from semiclassical predictions1 and the breakdown of multiple isotope effects from simple reduced mass relationships (Swain-Schaad relationship) have been used extensively in the detection of hydrogen tunneling near room temperature. In particular, the temperature dependence of k_H/k_D has emerged as an important diagnostic of H-tunneling, with isotopic Arrhenius prefactor ratios (A_H/A_D) that are distinct from unity providing strong evidence for tunneling. However, the case is ambiguous when the value of A_H/ A_D approaches unity (range of 0.8 to 1.4) and is generally interpreted as a reaction involving no tunneling. Herein, we report experimental evidence for an isotopic Arrhenius prefactor ratio of unity in an enzymatic reaction that undergoes extensive tunneling of both protium and deuterium. Such an observation has been predicted within the environmentally coupled tunneling model put forth by Knapp et al.