Acid Dissociation Constants of Some Histidine-containing Peptides and Formation Constants of Their Metal Complexes

Glycyl-l-histidine, l-histidylglycine, glycylglycyl-l-histidine, glycyl-l-histidylglycine, and l-histidylglycylglycine were synthesized by the carbobenzoxy azide procedure. The acid dissociation constants of these peptides and their related compounds were determined from the potentiometric titration curves of 0.01 M solutions at 21°C. The dissociation of one ionization group is greatly influenced by the charge on the other groups. The formation constants of metal complexes of l-histidylglycine, l-histidylglycylglycine, and l-3-benzylhistidine were calculated by the least-squares method from the titration curves of the solution containing the ligand (0.01 M) and metal in the molar ratio 2: 1, assuming the formation of 1: 1 and 2: 1 normal complexes (ML+ and ML2), and the results were compared with those of l-histidine-metal complexes. For any metal ion, the order of formation constants was found to be histidine≈3-benzylhistidine>histidylglycine>histidylglycylglycine, in agreement with that of the basicity of amino and imidazole groups. In these metal complexes, metal ion is considered to bind through amino, imidazole, and carboxyl or carbonyl groups of histidine residue.