Voltammetric and spectroscopic properties of the ammonia adduct of pyrroloquinoline quinone

Abstract

The interaction of pyrroloquinoline quinone (PQQ ox), pyrroloquinoline semiquinone (PQQ sem), and pyrroloquinoline quinol (PQQ red) with ammonia has been investigated by cyclic voltammetry, absorption spectroscopy, and electrochemical electron spin resonance (ESR) spectroscopy. The voltammetric and spectroscopic measurements show that PQQ ox reacts with ammonia, yielding 5-imino-PQQ ox, which is reduced reversibly to 5-amino-PQQ red via a 5-aminyl PQQ sem radical by a two-step one-electron mechanism. Each of the one-electron redox potentials has been determined by a digital simulation analysis of the reversible cyclic voltammogram. The ESR spectrum of 5-aminyl PQQ sem was recorded successfully. Theoretical analyses of the ammonia concentration dependence of the redox potentials and the absorbance give the constants of the ammonia adduct formations of PQQ ox, PQQ sem, and PQQ red as 6.9 × 10, 6.5 × 10 2, and 9.3 × 10 2 M −1, respectively, at pH 9.2.