Photophysics of cationic 5,10,15,20-tetrakis-(4- N-methylpyridyl) porphyrin bound to DNA, [poly(dA-dT)] 2 and [poly(dG-dC)] 2: on a possible charge transfer process between guanine and porphyrin in its excited singlet state

Abstract

The photophysical parameters (fluorescence quantum yield and lifetime, triplet state formation and lifetime) of a cationic prophyrin, i.e. 5,10,15,20-tetrakis(4- N-methylpyridyl) porphyrin (H 2(TMpy-P4)), were determined directly in phosphate buffer solution in the native state and in complexes with calf thymus DNA, [poly(dA-dT)] 2 and [poly(dG-dC)] 2. In [poly(dG-dC)] 2-porphyrin complexes (mostly intercalative), interaction induced an efficient fluorescence quenching and decrease in the triplet state quantum yield. No such effects were observed in [poly(dA-dT)] 2-pophyrin complexes (mostly non-intercalative). The case of DNA was intermediate between the other two. On the basis of the redox properties of the various nucleic bases, it was shown that these observations can be interpreted in terms of fast reversible intermolecular electron transfer from the guanine moiety to the porphyrin in the excited state, when the porphyrin is intercalated with an appropriate geometry within the DNA duplex. These observations and interpretations support a type I (electron transfer) mechanism as the primary event leading to the photodynamic activity of cationic free base porphyrins used as photosensitizers in the process of photoinduced DNA strand breaks.