Abstract
The kinetics and reaction mechanism of the hydride transfer reaction between leucomethylene blue (MBH) and 2,5-dihydroxy-p-benzoquinone (H2Q) in buffer solutions were investigated spectrophotometrically by means of the stopped-flow technique. MBH were prepared by photoreduction of methylene blue with ascorbic acid. The rates of formation of MB+ in the presence of a large excess of H2Q obeyed pseudo-first-order kinetics. The pseudo-first-order rate constants (kobsd) were estimated for several concentrations of H2Q and several values of pH at 298 K. The pK1 and pK2 values of H2Q are 2.95 and 4.87 respectively. It was found that the kobsd depends on pH and the kobsd-value has the maximum value at pH=4.0. This shows that the reactivity of HQ- is higher than those of H2Q and Q2-. Inclusion of molecules or ions by cyclodextrins (CDs) sometimes accelerates and sometimes inhibits reactions. The effects of β- and γ-CD on the rates of oxidation were investigated at various pH values. The addition of β-CD decreased the kobsd-value at all pH-values examined. The effects of γ-CD were more complicated. The reaction mechanism was proposed.
