C-4 AND C-5 ADDUCTS OF COFACTOR PQQ (PYRROLOQUINOLINEQUINONE) - MODELSTUDIES 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 quinopro tein methanol dehydrogenase. The hemiacetal-type adduct was easily iso lated from a methanol solution of PQQTME. The crystal structure of the adduct was determined by X-ray diffraction for the first time, showin g that methanol addition occurred at-the 5-position (C-5) of the quino ne as in the case of the acetone adduct formation. On the other hand, treatment of PQQTME in methanol under acidic conditions gave the dimet hyl acetal derivative as a major product for which the addition positi on of methanol was determined to be C-4 by X-ray crystallographic anal ysis. Studies of the adduct formation reactions with methanol using a series of PQQ model compounds and the molecular orbital calculations p rovided a clear-cut explanation for the difference in positions betwee n the hemiacetal formation and the acetal formation. Because the C-5 h emiacetal was not very stable, it readily reverted to the quinone in s olution, while the C-4 acetal was reduced to the quinol derivative whe n treated with base. The spectral characteristics and biological signi ficance (particularly in the enzymatic alcohol oxidation mechanism) of the C-4 and C-5 adducts of cofactor PQQ are discussed.