Ternary complexes of Cu(II) involving histidine and another amino acid or dipeptide

577-580The stability constants of ternary complexes of the type (CuLA) where A= histidine; and L= glycylglycine (L1), glycyl--alanine(L2), glycyl--leucine(L3), tryptophan (L4), phenylalanine (L5), tyrosine (L6) or 3,4-dihydroxyphenylalanine (L-dopa) (L7) have been determined potentiometrically in aqueous medium at 1= 0.2M (NaC1O4) and 25°C. In the ternary complexes of copper (II) with histidine and dipeptides, where histidine acts as an ambidentate ligand coordinating through amino acid end with ptotonated imidazole or histamine end with weakly coordinated carboxylate, three types of species CuLAH, CuLA and CuL(-H)A have been detected. The species CuLAH is found to be the most stable due to hydrogen bonding between protonated imidazole and free COO- group of the dipeptide. In the case of ternary complexes involving histidine and another amino acid(L), two types of species, CuLAH and CuLA, have been detected. (CuLAH) is stabilized due to intramolecular interligand stacking interaction and CuLA due to H-bonding between COO- of histidine and the non-coordinating OH or NH groups of the amino acid