Complexes of copper(II) ion with histamine-containing dipeptides: formation constants and structure

The acid–base properties and copper(II) complexes of glycyl- and sarcosyl-histamine (histamine = imidazole-4-ethanamine) have been studied by pH-metric, spectrophotometric and EPR methods, and compared to those of analogous histidine-containing dipeptides on the one hand and of carcinine (β-alanylhistamine) on the other. At pH 4–9 the predominant species is the 3N-co-ordinated complex CuLH–1(charges omitted) together with minor quantities of CuLH and CuL. The pK for metal ion-promoted deprotonation of the peptidic nitrogen is exceptionally low (pK= 3.20 and 3.66, respectively). The bis complexes CuL2 and CuL2H–1 also form in the presence of ligand in excess. Around pH 9–11 the monomeric 3N-co-ordinated hydroxo-complex CuLH–1(OH) and a polynuclear 4N-co-ordinated species are in equilibrium. The latter is assumed to be tetrameric Cu4L4H–8, with the imidazole rings as bridging bidentate units through co-ordination to both N3 and N1-pyrrolic nitrogens. At high pH (≈ 11) a further deprotonation results in the production of the monomeric 3N-co-ordinated hydroxo-complex CuLH–2(OH) with a pendant deprotonated N1-pyrrolic nitrogen.