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

Department of Chemistry, University College of Science, University of Calcutta,

92, Acharya Prafulla Chandra Road, Kolkata-700 009, India

E-mail: mukherg@rediffmail.com Fax : 91-033-2351-9755/2241-3222

Manuscript received 11 January 2010, revised 24 February 2010, accepted 25 February 2010

Combined spectrophotometric and computer-based pH-metric investigations on the mixed-ligand complex formation equilibria or Cu^II with histidine [(C₃H₃N₂)CH₂CH(NH₂)COOH, hisH\( \pm\)] and biguanide [NH₂C- (=NH)NHC(=NH)NH₂, Bg] in aqueous solution at a constant ionic strength (I= 0.1 mol dm^-3) and 25 ± 1ºC, provided evidence of binary [Cu(hism)⁺], [Cu(hism-H)], (Cu(hlsm)₂], (Cu(hism)(hism-H)^-], [Cu(bism-H)₂^2- ]and (Cu(Bg)²⁺] and ternary [Cu(bism)(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.