BINDING CHARACTERISTICS OF ZINC PORPHYRIN ELLIPTICINE HYBRID MOLECULES WITH DNA

The affinity for DNA of two hybrid molecules synthesized from zinc mes o-tetrakis(4-N-methylpyridiniumyl)porphyrin linked by a twelve-(hybrid 1) or an eight-bond (hybrid 2) chain to a 9-methoxy-N-2-methylellipti cinium group has been investigated by comparison to that of each moiet y. In the absorption spectra, the band attributed to the ellipticine m oiety exhibits a strong hypochromic effect in the presence of DNA, whi le the intensity of the fluorescence spectrum of the porphyrin moiety is greatly enhanced. Titrations were performed in the presence of CT D NA and poly[d(A-T)](2), and the data were analyzed using McGhee-von Hi ppel one-interaction and Scatchard two-interaction models. The binding of both hybrids involves two interactions, depending on the binding d ensity. At a high binding density (v greater than or equal to 0.1) bot h hybrids bind to CT DNA with an affinity constant of about 10(6) M(-1 ) and a binding site sire of 1 to 2 base pairs. In this situation, the ellipticine moiety is intercalated whereas the porphyrin moiety hangs outside the polymer. At a low binding density (v less than or equal t o 0.1) a stronger interaction is involved, characterized by a high aff inity constant (10(8) to 10(9) M(-1) for hybrids 2 and 1, respectively ) and a large binding site (12 to 14 base pairs). In this case, both c hromophores may participate in the interaction with DNA. No difference in binding was found using CT DNA or poly[d(A-T)](2). The photoreacti vity of the hybrids towards DNA is thus discussed with respect to the binding modes. The hybrids have nearly the same cleavage efficiency as ZnTMPyP. This may be correlated to the high binding constant of these compounds for DNA, resulting in large proportions of bound dye.