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

The method of spectrophotometric analysis is therefore considered the best of the three methods used here for testing the results derived from potentiometric titration experiments.It proved to be of particular value in a later study of the com- bination between cupric ions and diglycylglycine, where the reaction constants ofthe system could not be evaluated from the titration data alone.SUMMARY 1.The following methods have been studied with a view to their application to the reactions between metals and polypeptides in aqueous solution: (a) potentiometric titration, (b) copper-electrode potential measurements, (c) electrophoresis, and (d) spectrophotometric observations.2. When these methods were applied to the study of complex formation between glycine and cupric ions they confirmed the view that two com- plexes ML+ and ML2 are formned.3.Even in alkaline solution and with an excess of glycine there was no evidence from the electro- phoretic observations of the formation of a complex with a negative charge such as would be found if one cupric ion combined with more than two glycine anions.4. Absorption curves in the visible range of the spectrum have been calculated for the copper- glycine complexes.We wish to acknowledge a maintenance grant from the Department of Scientific and Industrial Research to one of us (H.D.) and to thank Professor A. Albert of the National University of Australia for help in developing the potentiometric titration technique.On the assumption that combination between cupric ions and glycylglycine involves the following steps:-+ H+, where Grepresents the glycylglycine anion, mathematical equations may be derived to obtain, from the potentiometric-titration data, values for the following equilibrium constants: K1= [CuG+]/[Cu2+] [G-], K2= [CuG2 ]/[CuG] [G ], K.= [CuG] [H+]/[CuG+], K,= [CuG(OH) ] [H+]/[CuG], Kcff = [CuG22-] [H+]/[CuG2 ].VoI. 59