The course of the formation of a quinonoid species absorbing in the 500-nm region was studied in reactions of pyridoxal, ethyl methionate and Cu(II) in the presence and absence of chelating ligands, such as ethylenediamine, diethylenetriamine, α,α'-bipyridine and α,α',α''-terpyridine in methanol solvent. The results indicated the formation of six-coordinated mixed chelates of Cu(II) with the quinonoid and the chelating ligand having retarded the transformation of the chelated quinonoid to the aldimine. The reaction system was thought to serve as a model for the key intermediate in reactions catalyzed by pyridoxal phosphate enzymes.

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