An ESR study of the copper(II)–glycyl-l-serine and copper(II)–l-seryl-glycine systems by the simultaneous analysis of multi-component isotropic spectra. Formation constants and coordination modes

Abstract The formation constants and the isotropic ESR parameters ( g -factors, 63 Cu, 65 Cu, 14 N hyperfine coupling constants and relaxation parameters) of the various species were determined by the simultaneous analysis of a series of spectra, taken in a circulating system at various pH and ligand-to-metal concentration ratio. For both systems the new [CuLH] 2+ complex was identified in acidic solutions. With the glycyl- l -serine ligand below pH 11.5 the same complexes and coordination modes are formed than with simple dipeptides. The side-chain donor group is bound only over pH 11.5 in the complex [CuLH −2 (OH)] 2− , where it is deprotonated and substitutes the carboxylate O in the third equatorial site. For the bis complex [CuLH −1 (L)] − an isomeric equilibrium was shown, where the difference between the isomers was based on which of the donor atoms of the ‘L’ ligand, the peptide O or the amino N, occupies the fourth equatorial position, and which one is coordinated axially. The l -seryl-glycine ligand forms the same species as simple dipeptides and glycyl- l -serine up to pH 8. The only difference is that the axial binding of the alcoholic OH group fairly stabilizes the bidentate equatorial coordination of the ‘L’ ligand through the amino N and peptide O atoms in the [CuL] + complex as well as in the major isomer of the [CuLH −1 (L)] − complex. For this system we showed that (1) proton loss and the equatorial coordination of the alcoholic OH group occurs at relatively low pH (over pH 8–9), which results in the [CuL 2 H −2 ] 2− complex with excess ligand, and also the newly identified species [Cu 2 L 2 H −4 ] 2− : (2) this process is in competition with the proton loss of a coordinated water molecule. For both systems, the ESR-inactive species [Cu 2 L 2 H −3 ] − was also shown.