Voltammetric determination of acid dissociation constants of pyrroloquinoline quinone and its reduced form under acidic conditions

Abstract The voltammetric properties of the coenzyme pyrroloquinoline quinone (PQQ ox ) have been investigated in the pH range from 0 to 8. PQQ ox gives a quasi-reversible cyclic voltammogram at a platinum electrode, which is ascribed to a two-electron redox reaction of PQQ ox to pyrroloquinoline quinol (PQQ red ). The standard redox potential ( E o ′) of the PQQ ox /PQQ red couple has been determined using cyclic voltammetry. From the analysis of the E o ′ vs. pH relationship, the four macroscopic acid dissociation constants ( K a ) of both PQQ ox and PQQ red have been determined. The p K a s have been assigned to the pyridyl nitrogen and three carboxyl groups. The E o ′ of the PQQ ox /PQQ red couple at pH 7.0 has been determined as −0.175 V vs. SCE (a saturated calomel electrode) with a semiquinone (PQQ sem ) formation constant of 0.02, where PQQ ox , PQQ red , and PQQ sem are present as trianions with the three carboxylate anion substituents. The pH dependence of the absorption spectra of PQQ ox , has been interpreted in terms of the p K a values.