Ternary Coordination Complexes Between Copper(II) and Amino Acids, and an Appraisal of the Enhancement of Ternary Complex Stability

Abstract

The equilibria involved in the ternary systems Cu(II)–A–B′ and Cu(II)–A–B″ (A = L-histidine, B′ = L-serine, B″ = L-glutamine) have been investigated by potentiometry in aqueous solution (0.15 M NaCl, 25°). In the ternary system Cu(II)–A–B′, three mixed complexes (MHAB′, MAB′, and MH−1AB′) were detected in addition to the binary complex species (MHA, MA, MH2A2, MHA2, MA2, MH−1A2, MB′, MB′2). The results observed in the Cu(II)–A–B″ system were quite similar to the system Cu(II)–A–B′ except that it did not show the presence of the species MH−1AB″. The stability constants were refined with a general least squares treatment. In the case of an equimolecular mixture of the constituents, a maximum of about 75% of Cu(II) is present as MAB′ at pH 5.5–9 and about 60% of Cu(II) as MAB″ at pH 5–10 when the concentration of each constituent is set equal to 10−2 M. A theoretical analysis and derivations of the expected stability constants of the complexes are presented on the basis of successive formation constants of the individual complexes carrying identical ligands. In each case an enhancement of the stability was observed with the ternary coordination complexes. Attempts have been made to explain the enhanced stability of the ternary coordination complexes.