Species absorbing in the 500-nm region in the reactions of pyridoxamine with pyrroloquinoline quinone and phenathrolinequinones

Quinonoid species absorbing in the 500-nm region, which should serve as a model for the key intermediate in reactions catalyzed by pyridoxal phosphate enzymes, was formed in the reactions of pyridoxamine (PM) with pyrroloquinoline quinone (PQQ), 1,7-phenathroline-5,6-quinone and 4,7- phenathroline-5,6-quinone in alkaline methanol at 25δC. The band at around 500 nm appeared gradually and its intensity reached a maximum about 6h after initiation of the reaction, then decreased gradually and disappeared. 1,10- Phenathroline-5,6-quinone which lacks pyridine nitrogen peri to the o-quinone carbonyl groups did not react with primary amines under the conditions used. Crystalline product was prepared from PM and PQQ. Absorption spectra of its methanol and water solutions were similar to the spectrum formed from the reaction of PM and PQQ with 500 nm band assigned to the quinonoid. The results of FAB mass were consistent with the formula of the ketimine and the quinonoid formed from PM and PQQ.