Complex-formation between polypeptides and metals. 3. The reaction between cupric ions and diglycylglycine

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Abstract
Potentiometric titrations and spectrophotometric observations were made with solutions containing cupric chloride (0.005M) and diglycylglycine in the ratio of 1:1, 1:2, 1:4 and 1:6. The observations were interpreted on the assumption of the existence in these solutions of the copper-diglycylglycine complexes ML+2, ML, ML", ML2", symmetrical ML2 2- and asymmetrical ML2 2-. Values were calculated for the stability and dissociation constants of these complexes, and it was shown that these values are consistent with the experimental observations. A characteristic absorption spectrum was assigned to each of these complexes. Results support the view that in the formation of such complexes as ML, a link is established between the amide group and the Cu atom, with the loss of a hydrogen ion. The relative acidities of various glycine-contain-ing peptides for cupric ions are compared with each other and with that of glycine itself, at the approximate physiological pH of 7.3.

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