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

• Published in: Journal of organic chemistry 2009-09, Vol.74 (17), p.6503-6510
• Main Authors: Lu, Yun, Qu, Fengrui, Zhao, Yu, Small, Ashia M. J, Bradshaw, Joshua, Moore, Brian
• Format: Article
• Language: English
• Subjects: 2-Propanol - chemistry; Acetonitriles - chemistry; Alcohol Dehydrogenase - chemistry; Alcohols; Catalytic Domain; Deuterium - chemistry; Hydroxyl Radical; Isotopes; Kinetics; Models, Chemical; NAD - chemistry; Solvents - chemistry; Temperature
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo9007628

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% H2O/20% AN) were determined over a temperature range from 36 to 67 °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 H2O obs/k D2O obs = 0.54), the larger than unity equilibrium isotope effect (K(H2O)/K(D2O) = 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 °C: KIEα-D H = 3.2(AN), 3.2(H2O); KIEβ-D6 H = 1.05(AN), 1.16(H2O); KIEOD H = 1.08(AN), 1.04(H2O). These KIE values are consistent with the presence of the positively charged alcohol moiety in the transition state (TS) for cleavage of the α-C−H bond, the delocalization of the positive charge over the α-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.