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

The formation constants and the isotropic ESR parameters (g-factors, Cu-63( -), Cu-65, N-14 hyperfine coupling constants and relaxation parameters) of the various species were determined by the simultaneous analysis of a serie s of spectra, taken in a circulating system at various pH and ligand-to-met al concentration ratio. For both systems the new [CuLH](2+) complex M as id entified in acidic solutions. With the glycyl-L-serine ligand below pH 11.5 the same complexes and coordination modes are formed than with simple dipe ptides. The side-chain donor group is bound only over pH 11.5 in the comple x [CuLH-2(OH)](2-), where it is deprotonated and substitutes the carboxylat e O in the third equatorial site. For the bis complex [CuLH-1(L)](-) an iso meric equilibrium was shown, where the difference between the isomers was b ased on which of the donor atoms of the 'L' ligand, the peptide O or the am ino N, occupies the fourth equatorial position, and which one is coordinate d axially. The L-seryl-glycine ligand forms the same species as simple dipe ptides and glycyl-L-serine up to pH 8. The only difference is that the axia l binding of the alcoholic OH group fairly stabilizes the bidentate equator ial 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 equatoria l coordination of the alcoholic OH group occurs at relatively low pH (over pH 8-9), which results in the [CuL2H-2](2-) complex with excess ligand. and also the newly identified species [Cu2L2H-4](2-): (2) process is in compet ition with the proton loss of a coordinated water molecule. For both system s, the ESR-inactive: species [Cu2L2H-3](-) was also shown. (C) 2001 Elsevie r Science Ltd. All rights reserved.