Kinetics of the Hydride Reduction of an NAD+ Analogue by Isopropyl Alcohol in Aqueous and Acetonitrile Solutions: Solvent Effects, Deuterium Isotope Effects, and Mechanism

Abstract

The rate constants of the hydride-transfer reactions from isopropyl alcohol (i-PrOH) to an NAD(+) model, 9-phenylxanthylium ion (PhXn(+)), in acetonitrile (AN) and in water containing AN (80% H(2)O/20% AN) were determined over a temperature range from 36 to 67 degrees C. The reactions follow second-order rate laws. In the latter solution, formation of the water adduct of PhXn(+) was observed as a side-equilibrium (K). The observed inverse solvent kinetic isotope effect (k(H(2)O)(obs)/k(D(2)O)(obs) = 0.54), the larger than unity equilibrium isotope effect (K(H(2)O)/K(D(2)O) = 2.69), and the results of acid effect on the observed rate constants of the reactions are consistent with the "side-equilibrium mechanism". Kinetic isotope effects at all three H/D positions of i-PrOH for the net hydride-transfer process were determined in both solutions at 60 degrees C: KIE(alpha-D)(H) = 3.2(AN), 3.2(H(2)O); KIE(beta-D6)(H) = 1.05(AN), 1.16(H(2)O); KIE(OD)(H) = 1.08(AN), 1.04(H(2)O). These KIE values are consistent with the presence of the positively charged alcohol moiety in the transition state (TS) for cleavage of the alpha-C-H bond, the delocalization of the positive charge over the alpha-C-OH group, and the stepwise hydride and proton transfer processes. Comparison of the activation parameters for the reactions in the two solvent systems as well as those in the i-PrOH/AN (1:1 v/v) reported earlier suggests that the AN medium promotes the reaction by activating the ground-state alcohol reactant through weak interactions with the electron pairs on alcohol O, while water and parent alcohol media facilitate the reaction by H-bonding stabilization of the alcohol moiety of the TS. Results suggest that in the alcohol dehydrogenases without a Zn(II) cofactor in the active sites alcohols would be oxidized via hydride transfer to NAD(+) coenzyme followed by the rapid deprotonation to the nearby basic species in the active site of the enzymes.