Complexing properties of phosphonodipeptides containing aminomethylphosphonic acid

Abstract

A series of phosphonodipeptides containing glycyl, L-alanyl, L-leucyl, or L-phenylalanyl and a terminal aminomethylphosphonic acid residue was studied pH-metrically at 25 °C and at an ionic strength of 0.1 mol dm^–3(KNO₃), and their protonation and complex-formation stability constants with Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ were determined. The protonation constants show the same dependence on the side chain as those of common dipeptides but the differences are not as large. The stability constants point to the formation of protonated, non-protonated and deprotonated complexes with a metal:ligand molar ratio of 1:1 and, except for zinc, to the formation of 1:2 complexes. Simultaneous deprotonation and co-ordination of the peptide amide bond was confirmed only for Cu²⁺. The same influence of the size of the amino acid side chain as found for common dipeptides was observed but the differences are smaller due to the stronger complexing ability of the phosphonic group.