Mixed-ligand complex formation equilibria of CuII with histidine and biguanide

Abstract

Combined spectrophotometric and computer-based pH-metric investigations on the mixed-ligand complex formation equilibria of Cu II with histidine [(C 3 H 3 N 2 )CH 2 CH(NH 2 )COOH, hisH ± ] and biguanide [NH 2 C-(=NH)NHC(=NH)NH 2 , Bg] in aqueous solution at a constant ionic strength (I = 0.1 mol dm ―3 ) and 25 ± 1°C, provided evidence of binary [Cu(hism) + ], [Cu(hism-H)], [Cu(hism) 2 ], [Cu(hism)(hism-H) ― ], [Cu(hism-H) 2 2― ] and [Cu(Bg) 2+ ] and ternary [Cu(hism)(Bg) + ], [Cu(hism-H)(Bg)], [Cu(Bg)(OH) + ] and [Cu(Bg-H)(OH)] complexes. Histidine provides both histamine-like [(NH 2 , N-imidazole i.e. (hism ― )] and mixed histamine-like glycine-like [(NH 2 , N-imidazole) (NH 2 , COO-) i.e. (hism ― )(gly ― )] chelation, barring simple glycine-like (gly ― ) chelation. Binding affinity of Cu II for (NH 2 , =NH) bidentate chelating biguanide is so strong that the histidine-imidazole deprotonated mixed-ligand complex, [Cu(hism-H)(Bg)] decomposes at pH > 10 to biguanide (=NH) deprotonated ternary hydroxo complex, [Cu(Bg-H)(OH)], setting free the his- ligand ion in solution, a phenomenon of great biological significance, in regard to medicinal applications of the title ligands and their metal derivatives.