[[abstract]]Pyrroquinoline quinone (PQQ) is a unique cofactor of several important dehydrogenases, such as glucose dehydrogenase (GDH) and methanol dehydrogenase (MDH), playing important roles in biological energy conversion processes. To elucidate the roles of PQQ in this and other aspects, we carried out investigations on PQQ regarding its fundamental properties and application potential in electrochemical, photochemical and catalytic reactions. In the PQQ-mediated transduction processes, Ca2+ ion has been evidenced to play an important role in stabilizing the reaction intermediate of PQQ (semiquinone) and the transfer of electrons from the substrate to the physiological electron acceptor in dehydrogenase. Cyclic voltammetry, fluorescence spectral analysis, UV-Vis absorption spectral analysis, and X-ray absorption spectral analysis including XPS and XANES, confirm this point, and in addition, show that PQQ can interact with many other metal ions, like Pb2+, Cd2+ and Cu2+, as well as with NAD+ and NADH. PQQ can be regarded as a five-proton Lewis acid. This explains the photosensitivity of PQQ to pH, and the reactivity to other metal ions and NAD+. PQQ can also function as a powerful oxidant as it is brought to its electronically excited state. The formal potential (Eo’*) is estimated to be about 3.2 V vs. SCE. Thanks to this property, PQQ shows a photosensitivity to NADH and other reducing agents like EDTA as it is exposed to light irradiation. To get deeper insight into the electrochemical properties of PQQ, we also employed surfactants like didodecyldimethylammonium bromide (DDAB) to fabricate PQQ-modified electrodes. The immobilized PQQ shows similar behaviors as the solution counterpart. Because of this, we have successfully used it to elucidate the transduction mechanisms of GDH and ADH as glucose and alcohol are incorporated.
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