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 as 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.