ENOLIMINE AND GEMINALDIAMINE FORMS IN THE REACTION OF PYRIDOXAL-PHOSPHATE WITH ETHYLENEDIAMINE - AN ELECTROCHEMICAL AND SPECTROSCOPIC CONTRIBUTION

The enol-keto tautomerism of the Schiff bases formed by pyridoxal 5'-p hosphate (PLP) with ethylenediamine (Etd) and ethylamine (Et) was stud ied by electrochemical and spectrophotometric methods. The spectroscop ic results revealed differences between the two reactions, i.e. the en olimine/ketoenamine ratio observed in PLP-Etd mixture is higher than t hat of PLP-Et. The differences observed in the electroreduction mechan ism and stability of the Schiff bases formed provide additional eviden ce for an unusually high concentration of enolimine in PLP-Etd in buff ered aqueous solutions. The results are consistent with a cyclic struc ture such as that previously proposed by Robitaille et al. [J. Am. Che m. Soc. 111, 3034-3047 (1989)] on the basis of spectroscopic data. Pro tonation of the terminal amino group of the Etd moiety in the Schiff b ase involves the formation of the cyclic species. The low basicity of ethylenediamine favours the formation of similar concentrations of the enol and keto tautomers of the Schiff base at neutral pH. This behavi our, also observed in the Schiff base formed by PLP and polylysine, ma y be involved in coenzyme-protein linkages.