Photophysical studies of excited-state characteristics of meso-tetrakis (4-N-methyl-pyridiniumyl) porphyrin bound to DNA.

The interaction of meso-tetrakis (4-N-methyl-pyridiniumyl) porphyrin (TMPyP) with DNA has been investigated at ionic strength (IS) values of 0.01 and 0.20 M (pH 6.8) using the flash-photolysis technique along with optical absorption and fluorescence data. It is found that the aggregation of TMPyP observed at the porphyrin excess reduces its total fluorescence intensity (JT) and the T-T absorption. At low IS the JT and the T-T absorption for the TMPyP monomers bound to the GC DNA sites are lower than those for the free ones, whereas binding to the AT sites (high IS) increases JT. At low IS the triplet decay of TMPyP is mono-exponential, the lifetime increasing with the [DNA] increase, while at high IS the addition of DNA transforms this profile to a bi-exponential form with lifetimes of the components independent of [DNA]. Binding to DNA reduces the quenching constants of the porphyrin triplet states by molecular oxygen (kq), the effect depending on the site and mode of binding. So, at low IS the kq value for the TMPyP externally bound to the GC sites (3.0 x 10(8) M-1 s-1) is five times lower than that for the free porphyrin and twice as high as that for the intercalated one. At high IS the TMPyP binding reduces the kq three-fold for the AT sites in the minor groove and 16-fold in the major groove as compared with the free one (approximately or equal to 1.6 x 10(9) M-1 s-1).