Primary Kinetic Isotope Effects on Hydride Transfer from Heterocyclic Compounds to NAD~+Analogues

摘要

Primary kinetic isotope effects (KIEs), kilki), have been determined spectrophotometrically for the reactionsof NAD+ analogues (acridinium ions, la—e+, and quinolinium ion, 2+) with heteroaromatic compounds suchas 3-methyl-2-phenylbenzothiazoline, 3H(D), and 1,3-dimethyl-2-phenylbenzimidazoline, 4H(D) in a mixedsolvent containing four parts 2-propanol and one part water at 25.0 + 0.1 °C. The KIEs decrease from 6.24to 3.93 as the equilibrium constant, K, is increased from about 1 to 1012 by the structural variation in thehydride acceptor. The Marcus theory of atom transfer in a linear, triatomic model of the reaction, with tunneling,can explain the variation of KIE with K. The Marcus theory is based on a model involving no high-energyintermediates, leading to a one-step mechanism. The present system satisfies this condition.