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

Abstract

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 ( J T) and the T-T absorption. At low IS the ( J T) 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 ( J T). 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 ( κ q), the effect depending on the site and mode of binding. So, at low IS the κ q value for the TMPyP externally bound to the GC sites (3.0 × 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 κ q threefold for the AT sites in the minor groove and 16-fold in the major groove as compared with the free one (≅ 1.6 × 10 9 M −1 s −1).