Unexplored interactions between pyrroloquinoline quinone and β-nicotinamide adenine dinucleotide

The interactions between pyrroloquinoline quinone (PQQ) and β-nicotinamide adenine dinucleotide (NAD(H)) characterized by spectral and voltammetric means are reported in this paper. PQQ exists with six major acid-base derivatives in aqueous solutions; the derivative predominating at pH 3, denoted H₂Q^-, was characterized to be responsible for the electrochemical and photochemical activities of PQQ, such as for the photooxidation of NADH. In contrast, the derivatives predominating at pH > 5, such as HQ^2- and Q^3-, are responsible for the interaction with NAD⁺; the equilibrium constant was estimated to be ∼10⁶. Although the basic derivatives are less active in emission compared to H₂Q^-, incorporating NAD⁺ can enhance their activity in this respect. The titrations of NAD⁺ with HQ^2- and Q^3- confirm the complexation reactivity of PQQ to NAD⁺, and suggest that the reaction sites of PQQ for NAD⁺ are the same as for Ca²⁺ ions. Thanks to those unique properties, PQQ can photochemically sense glucose and ethanol as NAD⁺ and the respective dehydrogenases are incorporated.