Hydride Reduction of NAD+ Analogues by Isopropyl Alcohol: Kinetics, Deuterium Isotope Effects and Mechanism

• Published in: Journal of organic chemistry Vol. 73; no. 13; pp. 4763 - 4770
• Main Authors: Lu, Yun, Qu, Fengrui, Moore, Brian, Endicott, Donald, Kuester, William
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 04.07.2008
• Subjects: 2-Propanol - chemistry; Boranes - chemistry; Chemistry; Deuterium - chemistry; Exact sciences and technology; Heterocyclic compounds; Heterocyclic compounds with o, s, se, te hetero atom and condensed derivatives; Heterocyclic compounds with only one n hetero atom and condensed derivatives; Kinetics; Kinetics and mechanisms; Molecular Structure; NAD - chemistry; NAD - metabolism; Organic chemistry; Oxidation-Reduction; Preparations and properties; Reactivity and mechanisms; Solvents; Deuterium; Water; Equilibrium constant; Organic cation; Nitrogen heterocycle; Stabilization; Alcohol; Hydrides; Alcohol dehydrogenase; Oxygen heterocycle; Chemical reduction; Transfer reaction; NADH; Reaction mechanism; Rate constant; Solvent; Solvent isotope effect; Purine nucleotide; Ether; Enzyme; Activation parameter; Hydride transfer; Reaction rate; Kinetic isotope effect; Cosolvent; Isotope effect; Analog; Oxidoreductases; Kinetics; Hydrogen bond
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo800820u

Observed pseudo-first-order rate constants (k obs) of the hydride-transfer reactions from isopropyl alcohol (i-PrOH) to two NAD+ analogues, 9-phenylxanthylium ion (PhXn+) and 10-methylacridinium ion (MA+), were determined at temperatures ranging from 49 to 82 °C in i-PrOH containing various amounts of AN or water. Formations of the alcohol−cation ether adducts (ROPr-i) were observed as side equilibria. The equilibrium constants for the conversion of PhXn+ to PhXnOPr-i in i-PrOH/AN (v/v = 1) were determined, and the equilibrium isotope effect (EIE = K(i-PrOH)/K(i-PrOD)) at 62 °C was calculated to be 2.67. The k H of the hydride-transfer step for both reactions were calculated on the basis of the k obs and K. The corresponding deuterium kinetic isotope effects (e.g., KIEOD H = k H(i-PrOH)/k H(i-PrOD) and KIEβ-D6 H = k obs(i-PrOH)/k obs((CD3)2CHOH)), as well as the activation parameters, were derived. For the reaction of PhXn+ (62 °C) and MA+ (67 °C), primary KIEα-D H (4.4 and 2.1, respectively) as well as secondary KIEOD H (1.07 and 1.18) and KIEβ-D6 H (1.1 and 1.5) were observed. The observed EIE and KIEOD H were explained in terms of the fractionation factors for deuterium between OH and OH+(OHδ+) sites. The observed inverse kinetic solvent isotope effect for the reaction of PhXn+ (k obs(i-PrOH)/k obs(i-PrOD) = 0.39) is consistent with the intermolecular hydride-transfer mechanism. The dramatic reduction of the reaction rate for MA+, when the water or i-PrOH cosolvent was replaced by AN, suggests that the hydride-transfer T.S. is stabilized by H-bonding between O of the solvent OH and the substrate alcohol OHδ+. This result suggests an H-bonding stabilization effect on the T.S. of the alcohol dehydrogenase reactions.