PHOTOPHYSICS OF THE CATIONIC 5,10,15,20-TETRAKIS(4-N-METHYLPYRIDYL) PORPHYRIN BOUND TO DNA, [POLY(DA-DT)](2) AND [POLY(DG-DC)](2) - INTERACTION WITH MOLECULAR-OXYGEN STUDIED BY PORPHYRIN TRIPLET-TRIPLET ABSORPTION AND SINGLET OXYGEN LUMINESCENCE

Interaction between molecular oxygen and the cationic free-base 5, etr akis(4-N-methylpyridyl)porphyrin(H(2)TMpyP(4+)) complexed with [poly(d A-dT)](2), [poly(dG-dC)](2) and calf thymus DNA, has been monitored in air-saturated heavy water solutions through porphyrin trip]et-triplet absorption and singlet oxygen luminescence. Three different rate cons tants of porphyrin triplet state quenching have been found which corre spond to different accessibilities of molecular oxygen to porphyrins e mbedded in the duplexes. The longest triplet state lifetime (30 mu s), found for porphyrin bound to [poly(dG-dC)](2), corresponds to molecul es well protected from oxygen. This supports the hypothesis of an inte rcalative binding mode of the porphyrin between GC base-pairs ('type A ' sites). The fraction f(Delta)(T) of the porphyrin triplet states que nched by molecular oxygen with singlet oxygen generation, is unity. In [poly(dA-dT)](2)-porphyrin complexes, two sites ('type B' and 'C' sit es of interaction) an involved, yielding very different tripler state lifetimes (5.5 mu s and 20.5 mu s) and efficiencies of singlet oxygen generation (f(Delta)(T) = 0.50 and 0.82). The f(Delta)(T) decreases ca n likely be explained in terms of competition between energy and elect ron transfer from the porphyrin excited triplet state to molecular oxy gen. All three types (A, B and C) of interaction sites can be expected in porphyrin-DNA complexes. (C) 1998 Elsevier Science S.A. All rights reserved.