Abstract
The voltammetric and spectroscopic properties of pyrroloquinoline quinone (PQQ) have been investigated in the pH range from 5 to 13.5. PQQ gives a reversible cyclic voltammogram at the mercury electrode, which is ascribed to a two-step one-electron redox reaction of the o-quinone moiety. Digital simulation analysis of the reversible voltammogram made it possible to evaluate the standard redox potentials of the PQQ/pyrroloquinoline semiquinone (PQQ sem) and PQQ sem/pyyroloquinoline quinol (PQQ red) couples. From the redox potential vs. pH relationships, the acid dissociation constants of PQQ, PQQ sem, and PQQ red have also been determined. The absorption and electron spin resonance spectroscopic properties are explained by the acid-base and redox equilibria proposed here.
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