TRANSITION-METAL COMPLEXES OF CARCININE, A PEPTIDE-TYPE DERIVATIVE OF HISTAMINE

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-degrees-C by pH-metric, spectrophotometric and, in part, H-1 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 less-than-or-equal-to 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.