The Metal Complexes of Peptides and Related Compounds: III. Copper(II) Complexes of Glycylglycylglycine in 3.0 m (Na)ClO4 Medium

Abstract

The copper(II) equilibria of triglycine have been investigated in 3.0 m (Na)ClO4 medium at 25° with glass and copper amalgam electrodes. The range of B, the total concentration of copper(II) ions, was 1 to 100 m m ; the range of A, the total concentration of triglycine, was 2 to 250 m m ; and the pH range was 1.5 to 11. The dipolar ion form of triglycine = HA, Y = (H - h)/A, h = [H+], and b = [Cu2+]. Two sets of data (Y against log h at constant A, B) and (log B/b against A at constant h, B) were recorded. They were treated independently, first by the use of various graphical methods assuming the existence of any species of the type CupHqAr(2p+q-r)+ and then by the use of the general (least squares) minimizing computer program, Letagrop (19). The mean composition of the complexes and the “best” sets of equilibrium constants are tabulated. It is concluded that mononuclear species CuHqA(1+q)+, having q = 1, 0, −1, −2, predominate at low B and A. At higher concentrations (≥10 m m ), there is also evidence for some CuHqA2q+ and Cu2HqA2(2+q)+. Of these, species with (p, q) equal to (1, 0), (1, −1), (1, −2) and (2, 1), (2, 0) (2, −2), (2, −4) give the best agreement with the experimental data. Deviations can largely be described to small analytical errors but may also indicate some minor species, if so probably CuH2A22+ and Cu2H2A24+.