L-GAMMA-GLUTAMYL-L-EPSILON-LYSINE AND L-GLUTAMYL-L-EPSILON-LYSINE COORDINATION PROPERTIES TOWARDS COPPER(II) AND NICKEL(II) IN AQUEOUS-SOLUTION

The complex formation between copper(II) or nickel(II) and the alifati c dipeptides L-gamma-glutamyl-L-epsilon-lysine and L-glutamyl-L-epsilo n-lysine has been studied in aqueous solution at 25 degrees C and ioni c strength 0.1 mol . dm(-3) by means of pH-metric and spectroscopic me asurements. In L-gamma-glutamyl-L-epsilon-lysine, two alpha-aminoacidi c residues are located at a long distance from each other, and are so far from peptide nitrogen atom that the involvement of dissociated pep tide group in coordination is not favored. As a consequence, a small n umber of complexes is formed, either with copper(II) or nickel(II), an d the ML degrees species predominates over the others. In L-glutamyl-L -epsilon-lysine, the amino group of glutamyl residue is adjacent to th e peptide group whose dissociation occurs in the presence of both copp er(II) and nickel(II). Either potentiometric or spectroscopic data sug gest the formation of a high number of complexes with the absence of a dominant species both for copper(II) and nickel(II): MLH(2)(2+), MLH( +), M(2)L(2)H(+), M(2)L(2) degrees, M(2)L(2)H(-1)(-), M(2)L(2)H(-2)(2- ), ML(2)H(2) degrees, ML(2)H(-), ML(2)(2-), M(2)L(2+), and M(2)LH(-1)( +) (M = copper(II) or nickel(II)). Since both of the ligands can form homobinuclear complexes with each of the metal ions investigated, mixt ures containing both metal ions and one of the peptide ligands have be en studied too, but the relevance of heterobinuclear species is not re markable. The spectroscopic data obtained for single species in soluti on allowed us to propose the probable structure in solution for severa l copper(II) complexes.