Potentiometric titration behavior of polylysine and copolymer of lysine with alanine prepared by thermal polycondensation

Abstract

The dissociation behavior of polyamino acids prepared by thermal polycondensations of free l-lysine and of a mixture of free l-lysine and l-alanine were studied by means of potentiometric titrations at various ionic strengths. The dependence of apparent dissociation constant against the degree of dissociation and the Henderson-Hasselbalch plots were investigated. These results indicated that thermally prepared polylysine and copoly(lys, ala) behave as a branched random-chain polymer. On the other hand, the intrinsic dissociation constants (p K 0) of amino groups attached to lysyl residues and the species of the amino groups were evaluated by the analyses of titrational data. It was found that the α-amino groups, which were characterized by p K 0 values of 7.1–7.4, contain about 93–96% of the titratable amino groups in the polymers. Furthermore, the ϵ-amino groups, which were characterized by p K 0 values of 10.4–10.7, contain about 4–7%. The p K 0 values of α- and ϵ-amino groups mentioned above were similar to those of proteins such as ribonuclease and bovine plasma albumin. The results obtained here are discussed in connection with possible peptide chain propagation by the thermal polycondensation of lysine.