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

The photophysical parameters (fluorescence quantum yield and lifetime, triplet state formation and lifetime) of a cationic porphyrin, i.e. 5 ,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (H-2(TMpy-P4)), were de termined 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), i nteraction induced an efficient fluorescence quenching and decrease in the tripler state quantum yield. No such effects were observed in [po ly(dA-dT)](2)-porphyrin complexes (mostly non-intercalative). The case of DNA was intermediate between the other two. On the basis of the re dox properties of the various nucleic bases, it was shown that these o bservations can be interpreted in terms of fast reversible intermolecu lar 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 l eading to the photodynamic activity of cationic free base porphyrins u sed as photosensitizers in the process of photoinduced DNA strand brea ks. (C) 1997 Elsevier Science S.A.