Enolimine and geminaldiamine forms in the reaction of pyridoxal phosphate with ethylenediamine. An electrochemical and spectroscopic contribution

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Abstract
The enol–keto tautomerism of the Schiff bases formed by pyridoxal 5'‐phosphate (PLP) with ethylenediamine (Etd) and ethylamine (Et) was studied by electrochemical and spectrophotometric methods. The spectroscopic results revealed differences between the two reactions, i.e. the enolimine/ketoenamine ratio observed in PLP–Etd mixture is higher than that of PLP–Et. The differences observed in the electroreduction mechanism and stability of the Schiff bases formed provide additional evidence for an unusually high concentration of enolimine in PLP–Etd in buffered aqueous solutions.The results are consistent with a cyclic structure such as that previously proposed by Robitaille et al. [J. Am. Chem. Soc. 111, 3034–3047 (1989)] on the basis of spectroscopic data. Protonation of the terminal amino group of the Etd moiety in the Schiff base 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 behaviour, also observed in the Schiff base formed by PLP and polylysine, may be involved in coenzyme–protein linkages.