Nonenzymatic Reduction of Tetrachloro-1,4-Benzoquinone by Reduced Nicotinamide Adenine Dinucleotide Phosphate in an Aqueous System

Abstract

NADPH was shown to reduce tetrachloro-1,4-benzoquinone (TCBQ) to tetrachloro-1,4-benzene diol which was identified on the basis of cochromatography with synthetic standard in a reverse phase HPLC system, electronic absorption spectroscopy, and mass spectrometry. Conversely, NADPH was shown to undergo TCBQ-dependent oxidation as evidenced by uv spectroscopic analysis. ESR spectroscopy demonstrated that NADPH-dependent reduction of TCBQ proceeds through the intermediacy of a semiquinone radical intermediate. In addition, molecular mechanics and molecular orbital calculations were carried out on TCBQ to obtain partial atomic charges and the results suggest that the first electron transfer from the reduced nicotinamide moiety to TCBQ occurs at a carbonyl carbon. Kinetic analysis indicated that the rate of NADPH oxidation is first order with respect to [TCBQ], second order with respect to [NADPH], and an Arrhenius plot yielded an activation energy of 26 kJ · mol −1for the rate limiting step. Mechanisms are proposed which are consistent with these results.