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 (beta-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 (almost-equal-to 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.