Characterization of topa quinone cofactor.

Electrochemical characterization of topa quinone (6-hydroxydopa quinone), the organic cofactor of copper-containing amine oxidases, has been performed with the aid of spectroscopy and ab initio energy minimization technique. Topa quinone exhibits a totally reversible cyclic voltammogram at a mercury electrode, which is ascribed to a two-step one-electron conversion between topa quinone and topa via topa semiquinone intermediate. Digital simulation of the reversible wave has afforded the separated estimation of each one-electron redox potential. The acid-dissociation constants of the phenolic hydroxyl groups of topa quinone, topa semiquinone and topa have been evaluated electrochemically and supported by electronic and electron spin resonance spectra. At pH 7.0, topa quinone is acid-dissociated and has a two-electron redox potential of 0.079 V vs. NHE coupled with a three-proton transfer. Redox catalytic activity of topa quinone for the oxidation of amines and NADH was not observed over conventional voltammetric time periods. Energy minimization calculation of acid-dissociated topa quinone anion indicates an intermediate electronic structure between the p- and o-quinone types with three almost equivalent carbonyl groups. The lack of the redox catalytic activity of free topa quinone appears to be attributable to the partial contribution of the p-quinone-type structure.