On the Stability of the Schiff Bases of Pyridoxal 5′‐Phosphate with Polypeptides Containing L‐Lysine

We determined the apparent equilibrium constant of formation, KpH, of the Schiff bases of pyridoxal 5′-phosphate (PLP) and poly- and copolymers containing L-lysine, as a function of pH at 25° and a constant ionic strength of 0.1 M. The KpH values obtained at acidic and neutral pH were larger that those reported for Schiff bases of PLP and hexylamine. We determined calorimetrically ΔH of formation of Schiff bases of PLP and poly(L-lysine) (−4.5′kcal/mol), and PLP and hexylamine (−3.4 kcal/mol) at pH 7.00. Semi-empirical theoretical calculations (INDO and AMI methods) of a model compound of Schiff base of PLP and polypeptide containing L-lysine show the capability of specific interactions between groups of PLP and the peptide skeleton.