C-4 and C-5 adducts of cofactor PQQ (pyrroloquinolinequinone). Model studies directed toward the action of quinoprotein methanol dehydrogenase

Methanol addition to the trimethyl ester of cofactor PQQ (PQQTME) was investigated in detail to obtain information on the action of quinoprotein methanol dehydrogenase. The hemiacetal-type adduct was easily isolated from a methanol solution of PQQTME. The crystal structure of the adduct was determined by X-ray diffraction for the first time, showing that methanol addition occurred at the S-position (C-S) of the quinone as in the case of the acetone adduct formation. On the other hand, treatment of PQQTME in methanol under acidic conditions gave the dimethyl acetal derivative as a major product for which the addition position of methanol was determined to be C-4 by X-ray crystallographic analysis