Transition-metal complexes of carcinine, a peptide-type derivative of histamine

Abstract

The acid–base properties and copper(II), nickel(II) and cobalt(II) complexes of carcinine {3-amino-N-[2-(imidazol-4-yl)ethyl]propanamide} and of N-tert-butoxycarbonylcarcinine have been studied at 25 °C by pH-metric, spectrophotometric and, in part, ¹H NMR and EPR methods, and compared to those of histidine-containing dipeptides. Complexes of the type M(HL), ML and ML₂(charges omitted) are found in the cobalt(II) and nickel(II) systems, with the additional presence of ML₂H_–1 for the latter system. Copper(II) forms CuL_n(n⩽ 4) complexes with N-tert-butoxycarbonylcarcinine molecules acting as monodentate ligands. The copper(II)–carcinine system in the range pH 3–9 shows the expected series of 1 : 1 complexes, more or less deprotonated depending on the pH, of type Cu(HL), CuL and CuLH_–1, and, for an excess of ligand, the unusual 1 : 4 complex CuL₄H₂ involving four N(3) inidazole nitrogens equatorially co-ordinated. At higher pH (> 9) the monomeric CuLH_–2 and polynuclear Cu₄L₄H_–8 complexes are formed depending on the total concentration. The tetrameric species probably involves co-ordination of deprotonated N(1)-pyrrole nitrogens, with consequent formation of imidazolate bridges. The co-ordination ability of the peptide nitrogen in histidyl- and histamine-peptides is shown to depend on the size of the chelate rings formed and the existence of a neighbouring carboxyl group in the ligand.