Electron spin resonance study of copper(II)–amino-acid complexes: evidence for cis and trans isomers and the structures of copper(II)–histidinate complexes in aqueous solution

Electron spin resonance spectra are reported for copper(II)–amino-acid complexes CuLn+ and CuL2m+(for 12 α-amino-acids) and Cu(HL)L+(HL = L-histidine). Buffer solutions of aqueous copper(II) and ligand were prepared to give optimum concentrations of these complexes. Isotropic g and A values are in the ranges 2.144–2.157 and 5.5–6.5 mT respectively for CuLn+ and 2.118–2.127 and 6.4–7.1 mT respectively for CuL2m+. The high-field absorption for CuL2m+ complexes showed nitrogen hyperfine structure, which for 63Cu and solvent D2O could be resolved in the second derivative into two overlapping quintet components characteristic of cis(N, N, O–, O–) and trans(N, O–, N, O–) isomers respectively. Spectra have been fitted to this model by computer simulation. From observed nitrogen hyperfine structure, and by comparison with spectra of other amino-acid complexes and that of bis(histamine)copper(II), it is deduced that the histidinate complex Cu(HL)L+ has three nitrogen atoms co-ordinated to copper in the planar co-ordination sites, whereas CuL2 appears to consist of a mixture of structures having three and four nitrogen atoms bound to the copper.