Complex-formation between polypeptides and metals. 2. The reaction between cupric ions and some dipeptides

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Abstract
Complex-formation between cupric ions and glycylglycine was examined using methods of potentiometric titration, spectrophotometric examination, electrophoresis and copper-electrode potential measurements. The constants characterizing the reaction between cupric ions and the dipeptide were evaluated from the potentiometric measurements. It was shown that the expected titration figures for solutions containing cupric chloride and glycylglycine calculated using these values of the constants agree with the experimental figures. Other experimental observations are also consistent with the theory. Potentiometric measurements were made on solutions containing cupric ions along with either glycyl-L-leucine or glycyl-L-tyrosine in various molecular proportions. Stability constants were evaluated for the complexes formed. These were similar in magnitude to those of the copper-glycyl-glycine complexes. The phenolic hydroxyl group of glycyl-tyrosine did not appear to take part in the reaction between this peptide and cupric ions. It appears that one cupric ion can combine with either one or 2 anions of these peptides producing complexes, each of which either loses a proton or takes up a hydroxyl group. Potentiometric observations on solutions containing cupric chloride and carnosine indicated that one cupric ion combined with one carnosine molecule at 3 points, namely the N atom of the free NH2 group, the basic iminazole N atom and the N or O2 atom of the peptide bond. This produced a dichelate complex in which the co-ordinated iminazole group had no tendency to take up a proton, i.e. it had lost its salt-forming capacity. Even when carnosine was present in excess a 2d molecule was not apparently taken up by the metal.