Optical absorption and fluorescence spectroscopy studies of ground state melanin—cationic porphyrins complexes

Abstract

Optical absorption and fluorescence spectroscopies were employed in the study of the interaction between synthetic l-dopa (dihydroxyphenylalanine) melanin and the cationic porphyrins tetrakis(4- N-methylpyridyl) porphyrin (TMPyP), tetrakis(4- N-benzylpyridyl)porphyrin (TBzPyP), zinc tetrakis(4- N-methylpyridyl)porphyrin (ZnTMPyP) and zinc tetrakis (4- N-benzylpyridyl)porphyrin (ZnTBzPyP). Optical absorption and fluorescence properties of the porphyrins were dependent on the symmetry of the central ring. No evidence was found for dimerization of the porphyrins in phosphate buffer, pH 7, in the concentration range between 4 × 10 −8 to 5 × 10 −5 M. Addition of l-dopa melanin red shifted the optical absorption spectra of porphyrins, concomitant to broadening and reduction in intensity of the bands. l-Dopa melanin also strongly quenched the fluorescence of the porphyrins. Time resolution of the fluorescence decay of porphyrins showed at least two lifetimes that were only slightly modified in the presence of melanin. The interaction between melanin and porphyrin resulted in the formation of non-fluorescent ground state complexes. It was found that there are two different classes of binding sites in melanin for complexation with cationic porphyrins and the values of dissociation constants are of the order of 10 −8 M. These values and the number of binding sites are dependent on the nature of the porphyrins. It was shown that the binding has electrostatic origin, but it is also affected by metal coordination and hydrophobic interaction.