Interaction of tyrosine and tyrosine dipeptides with Cu2+ ions: A fluorescence study

The interaction of tyrosine (Tyr) and the dipeptides glycyltyrosine (Gly-Tyr) and tyrosylglycine (Tyr-Gly) with copper ions has been studied through the use of fluorescence quenching due to the binding of the paramagnetic metal to the fluorophores. The pKa values for the protonation of the tyrosine derivatives were estimated from fluorescence data 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 the pKa of carboxylate and phenolic hydroxy groups were resolved, while for the dipeptides the pK for the amino group was additionally obtained. A simple equilibrium binding model allowed estimates of the association constants of 10−7 M−1 to be obtained for the formation of the 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 carboxylate group with Cu2+ while for Tyr-Gly some quenching involving other sites was also observed. Additional ESR experiments suggested the involvement of the peptide and amino nitrogens in complex formation. Our data are in agreement with recent X-ray data for the crystal structure 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 nitrogens, carboxylate and to an oxygen from a water molecule.