Complexes of vitamin B 6—XVIII. Ternary and quaternary complexes involving pyridoxamine, glycylglycine and imidazole and some bivalent metal ions

Abstract

The formation constants of ternary and quaternary complexes containing the ligands pyridoxamine, glycylglycine and imidazole with Co(II), Ni(II), Zn(II) and Cd(II) were determined by pH-metric titration in 0.15 M NaNO 3 at 37°C using the MINIQUAD-75 program. It has been shown that the logarithms of the ternary formation constants are linear increasing functions of the sum of the logarithms of the first protonation constants of the ligands, but the log K I (MLH m +ML′H m ⇌ K I MLL′H s ) are linear decreasing functions. It has been shown that deprotonation of the peptidic group is prohibited in ternary and quaternary metal complexes. It is also found that the imidazole moiety favourably ligates the ternary complexes rather than the bare metal ions.