A p H -metric study, together with some supporting spectroscopy (u.v.-visible, c.d., and e.s.r.), was made on copper( 11) complexes of L-phenylalanyl-L-tyrosine, L-tyrosyl-L-phenylalan ine, L-lysyl-Ltyrosine, and L-tyrosyl-L-lysine at 25 "C and I = 0.2 mol dm-3 (KCI). It was established that in dilute aqueous solutions, besides metal-ligand co-ordination characteristic of simple dipeptides, there are interactions between copper( 11) and the side-chain phenolate group of the tyrosine residue and/or the &-amino group of the lysine residue. In these dimeric species, both the lysine and the tyrosine moieties can behave as bridges between monomeric complexes.The side-chain donor groups of peptide molecules can affect their complex-forming capabilities significantly.' In the case of the transition-metal complexes of di-and tri-peptides the role of the aspartic cysteine,47' histidine,6 and tyrosine 7-9 residues seems to be the most important. The co-ordination properties of the side-chain groups have been found to depend considerably on their position in the peptide molecule. Cysteine or histidine in the N-terminal position hinders the coordination of the peptide-amide group and thus the participation of the C-terminal residue in the co-ordination is subordinate. The carboxylate group of a non-N-terminal aspartic acid in the peptide chain can also block the deprotonation and co-ordination of the remainder of the molecule. Aromatic moieties can generally interact with the empty d orbitals of metal ions. Strongly co-ordinating sidechain donor groups, such as phenolate, imidazolyl, or thiolate, can act as bridges between monomeric species, which leads to the formation of various oligomeric (mainly dimeric) complexes. This is more pronounced if the side-chain group is in the N-terminal position.The role of the lysyl residue in metal-ion binding has not been widely studied. In metal complexes of lysine itself, the &-amino group is not able to co-ordinate directly for steric reasons. Of the papers published so far 3 7 1 0 -1 3 on the complex-forming properties of lysine-containing oligopeptides, only two have suggested the involvement of the lateral Lys-NH, group in metal-ion binding. In both cases the ligands were rather large biomolecules: poly(L-1 ysine),' Gly-Gly-Pro-Lys, or (Gly-Gly-Pro-Lys),.13 Recently, X-ray evidence was obtained of the direct co-ordination of the &-amino group of the lysyl residue of a small dipeptide, Lys-Tyr, to copper(I1) in the solid ~t a t e . ' ~ Accordingly, in this work the complex-formation processes between copper(I1) and various dipeptides containing Tyr and Phe or Lys, such as L-phenylalanyl-L-tyrosine (Phe-Tyr), Lt yrosyl-L-phenylalanine (Tyr-Phe), L-lysyl-L-t yrosine (Lys-Tyr), and L-tyrosyl-L-lysine (Tyr-Lys) have been studied. To determine the stoicheiometries and stabilities of the complexes formed between copper(I1) and these ligands, detailed equilibrium investigations were carried out over a wide pH range, while u.v.-visible, circular dichroism (c.d.), and e.s.r. spectroscopic mea...
