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

Abstract

Electron spin resonance spectra are reported for copper(II)–amino-acid complexes [CuL]ⁿ⁺ and [CuL₂]^m+(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 [CuL]ⁿ⁺ and 2.118–2.127 and 6.4–7.1 mT respectively for [CuL₂]^m+. The high-field absorption for [CuL₂]^m+ 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.