| Abstract |
Combined spectrophotometric and computer-based pH-metric investigations on the mixed-ligand complex formation equilibria of Cu-II with histidine [(C3H3N2)CH2CH(NH2)COOH, hisH(+/-)]and biguanide [NH2C-(=NH)NH2, Bg] in aqueous solution at a constant ionic strength (l = 0.1 mol dm(-3)) and 25 +/- 1 degrees 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 [(NH2, N-imidazole i.e. (hism(-))] and mixed histamine-like glycine-like [(NH2, N-imidazole) (NH2, COO-) i.e. (hism(-))(gly(-))] chelation, barring simple glycine-like (gly(-)) chelation. Binding affinity of Cu-II for (NH2, =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. |
