Solvent and micellar effects upon the reaction of NADH models with methylene blue

Abstract

AbstractThe oxidation of the NADH‐model compounds 1‐R‐1,4‐dihydronicotinamide (R = benzyl, octyl or cetyl) and 1‐benzyl‐1,4‐dihydroquinoline‐3‐carboxamide by methylene blue proceeds via a direct hydride transfer mechanism. In the transition state of this reaction, a considerable fraction of positive charge is developed in the dihydronicotinamide ring. Effects of solvents and effects of NaLS micelles upon the reaction have been investigated. In organic solvents, the oxidation proceeds ca. 1.5–50 times more slowly than in water. Interestingly, the reaction rate in organic solvents increases with decreasing polarity of the solvents. In aqueous NaLS solution, the oxidation of the NADH models proceeds 9‐80 times more rapidly than in water. The kinetic data in micellar solution are analyzed in terms of the pseudophase model, taking into account partitioning of the reactants between micellar and aqueous pseudophase. It appears that the observed rate enhancements in micellar solution are completely due to an increase of the reactant concentrations in the micellar pseudophase. The second‐order rate constants for oxidation in the micellar pseudophase are 35‐150 times smaller than those in the aqueous phase. It is suggested that both a decrease of the micropolarity and a unfavourable orientation of the reactants contribute to the low values of the second‐order rate constants in the micellar pseudophase.