Complexing properties of [(glycylamino)methyl]phosphinic acids towards Co2+, Ni2+, Cu2+ and Zn2+ in aqueous solutions

The complexing properties of the phosphinic acid analogues of glycylglycine ([(glycylamino)methyl]phosphinic acids) NH2CH2C(O)-NHCH2P(O)(R)(OH) [GlyGl y(P-R)] (R = Ph, Me and tert-Bu) towards Co2+, Ni2+, Cu2+ and Zn2+ ions in aqueous solutions are investigated using potentiometry, and absorption and EPR spectroscopies. The potentiometric results indicate different propertie s of the Cu2+-GlyGly(P-R) systems in comparison with glycylglycine and its phosphonic acid analogue NH2CH2C(O)-NHCH2P(O)(OH), [GlyGly(P)]. The phosphi nic dipeptides GlyGly(P-R) readily form complexes with Cu2+/ligand molar ra tio 1:2 exhibiting the (N-amine, N-amide)(2) coordination mode above pH 9 e ven in solutions with only a slight excess of the ligand. In the system wit h GlyGly, the Cu2+ complex is observed at pH 12-13 and a metal:ligand ratio of 1:500. The metal-induced deprotonation of the amide nitrogen and the ex tent of formation of the 1 : 2 complexes depend on the acidity of the phosp hinic acid group which is influenced by the substituent at the phosphorus a tom. The models based on potentiometry are confirmed by spectroscopic measu rements. Stability constants of the phosphinic dipeptides GlyGly(P-R) with the other metal ions are the same as for glycylglycine and, therefore, the same binding pattern through the amine group and peptide oxygen is proposed . The presence of protonated. complexes is assumed in all the systems studi ed. In the protonated complexes, the peptide is bound through the phosphini c acid group whereas the amino group remains protonated.. The presence of t he species in the system containing the phosphonic acid analogue of glycylg lycine [GlyGly(P)] with Cu2+, which was investigated by potentiometry previ ously, is confirmed by EPR spectroscopy.