Coordination behaviour of glycylglycine with zinc(II) in presence of <img src='http://www.niscair.res.in/jinfo/small.gif' border=0>-histidine, histamine and imidazole ligands

Abstract

Computer-based analysis of the <em>p</em>H titrations data obtained in aqueous perchlorate media at 37ᵒC and <em>1=</em>0.15 mol dm^-3 (NaClO₄) for the zinc(II)-glycylglycine/glycinamide (A)-<img src='http://www.niscair.res.in/jinfo/small.gif' border=0>-histidine and histamine (B) mixed ligand systems shows the presence of the mixed ligand species ZnABH and ZnAB, ZnAB arid ZnAB₂ mixed species are formed in the zinc(II)-glycylglycine/glycinamide (A)-imidazole (B) systems. There is no evidence for the formation of the zinc(II) binary and mixed ligand species containing amide deprotonated dipeptide below <em>p</em>H 7.5. The results indicate that the dipeptide binds the metal through N-amino and O-peptido groups in the ZnABH and ZnAB mixed species as also in the ZnA or ZnA₂ type of binary species. In the ZnABH species the extra proton is attached to the primary amino group of the <img src='http://www.niscair.res.in/jinfo/small.gif' border=0>-histidine or histamine secondary ligand (B). The carboxylate group of <img src='http://www.niscair.res.in/jinfo/small.gif' border=0>-histidine is also involved in coordination in both the ZnAB and ZnABH types of mixed species. It appears that the ZnAB₂ species is more stable than the ZnAB species in the zinc(II)-glycylglycine/glycinamide (A)-imidazole (B).