Synthesis and crystal structure of pyrroloquinoline quinol (PQQH2) and pyrroloquinoline quinone (PQQ)

Pyrroloquinoline quinone (PQQ) is a water-soluble quinone compound first identified as a cofactor of alcohol- and glucose-dehydrogenases (ADH and GDH) in bacteria. For example, in the process of ADH reaction, alcohol is oxidized to the corresponding aldehyde, and inversely PQQ is reduced to pyrroloquinoline quinol (PQQH 2 ). PQQ and PQQH 2 molecules play an important role as a cofactor in ADH and GDH reactions. However, crystal structure analysis has not been performed for PQQ and PQQH 2 . In the present study, the synthesis of PQQH 2 powder crystals was performed under air, by utilizing vitamin C as a reducing agent. By reacting a trihydrate of disodium salt of PQQ (PQQNa 2 ·3H 2 O) with excess vitamin C in H 2 O at 293 and 343 K, yellowish brown and black powder crystals of PQQH 2 having different properties were obtained in high yield, respectively. The former was PQQH 2 trihydrate (PQQH 2 ·3H 2 O) and the latter was PQQH 2 anhydrate (PQQH 2 ). Furthermore, sodium-free red PQQ powder crystal (a monohydrate of PQQ, PQQ·H 2 O) was prepared by the reaction of PQQNa 2 ·3H 2 O with HCl in H 2 O. Single crystals of PQQH 2 and PQQ were prepared from Me 2 SO/CH 3 CN mixed solvent, and we have succeeded in the crystal structure analyses of PQQH 2 and PQQ for the first time.