INTERACTION OF TYROSINE AND TYROSINE DIPEPTIDES WITH CU2- A FLUORESCENCE STUDY( IONS )

The interaction of tyrosine (Tyr) and the dipeptides glycyltyrosine (G ly-Tyr) and tyrosylglycine (Tyr-Gly) with copper ions has been studied through the use of fluorescence quenching due to the binding of the p aramagnetic metal to the fluorophores. The pK(a) values for the proton ation of the tyrosine derivatives were estimated from fluorescence dat a being 2.30 and 10.28 for Tyr, 3.00, 8.44 and 10.70 for Gly-Tyr and 2 .98, 8.00 and 10.51 for Tyr-Gly. In the case of the pure amino acid th e pK(a) of carboxylate and phenolic hydroxy groups were resolved, whil e for the dipeptides the pK for the amino group was additionally obtai ned. A simple equilibrium binding model allowed estimates of the assoc iation constants of 10(-7) M(-1) to be obtained for the formation of t he complexes of dipeptides with copper. Stoichiometry of binding was 1 :1 for the dipeptide-metal complex. In the case of Gly-Tyr and acidic pH values quenching of fluorescence was due to complexation of the car boxylate group with CU2+ while for Tyr-Gly some quenching involving ot her sites was also observed. Additional ESR experiments suggested the involvement of the peptide and amino nitrogens in complex formation. O ur data are in agreement with recent X-ray data for the crystal struct ure of Gly-Tyr-Cu2+ complex [I. Dey et al., J. Cryst. Spect. Res., 23 (1993) 1]. The metal is coordinated to amino and amide peptide nitroge ns, carboxylate and to an oxygen from a water molecule.