Mixed ligand complexes involving sulphur containing ligands--Part II. Ternary complexes of Zn(II) involving L-cysteine/D-penicillamine/L-cysteic acid and imidazoles.

The chemical equilibria involved in nine mixed ligand systems Zn(II)-L-cysteine (Cys)/D-penicillamine(Pen)/L-cysteic acid(Cya)(A)-imidazole(Him), histamine(Hist) and L-histidine(His)(B) have been investigated in aqueous perchlorate medium by pH titrimetry at 37 degrees and ionic strength, I = 0.15M (NaClO(4)). The mixed ligand complex species of the types ZnABH(2), ZnABH, ZnAB or ZnAB(2) have been detected in addition to various binary species due to ligands A and B. The results obtained for the ZnABH type of species indicate that the site of protonation is the amino group of Cys/Pen ligands in the Zn(II)-Cys/Pen(A)-Him, Hist and His(B) systems, and the amino group of Hist/His secondary ligands in the Zn(II)-Cya(A)-Hist and His(B) systems. In the ZnABH(2) type of species, one proton is attached with the primary ligand (A) and the other with the secondary ligand(B). In both ZnAB and ZnAB(2) type ternary species in all the systems, the primary ligand binds the metal in a bidentate manner and the secondary ligands Him, Hist and His bind the metal, respectively in a uni, bi and terdentate manner.