Histidine-Copper (II) Complexes

Abstract

STABILITY constants are known for copper complexes of most of the naturally occurring α-amino-acids. However, although it is of considerable biochemical interest, the copper–histidine system has not been interpreted in detail. The system is anomalous, probably because, although the metal can co-ordinate to the amino, imidazole or carboxyl groups in histidine, an assumed planar configuration of the cupric complexes allows only two sites in any histidine molecule to be bound simultaneously¹. In this way both the histidine anion (L⁻) and the zwitterion (HL⁺⁻) can act as chelating species, giving the five different complexes, CuL⁺, CuHL⁺⁺, CuL₂, CuHL²⁺ and Cu(HL)²⁺⁺. Because the concentration ratio of L⁻ to HL⁺⁻ is dependent on pH, the proportions of the complexes present will vary with both ligand concentration and pH.