Photoinduced Electron Transfer between DNA and Water-Soluble Porphyrins.

Photophysical properties of five kinds of porphyrins (H2TMPyP, ZnTMPyP, PdTMPyP, H2TPPS, and ZnTPPS) complexed with model DNAs (ctDNA and dGMP) have been investigated using steady-state absorption, circular dichroism (CD), and femtosecond transient absorption spectroscopy. Upon addition of ctDNA (or dGMP), larger hypochromism and red shifts are observed for H2TMPyP and PdTMPyP compared to the other samples. The steady-state measurements have suggested that the binding modes of H2TMPyP-ctDNA and PdTMPyP-ctDNA are partial intercalation and full intercalation, respectively, while ZnTMPyP-ctDNA shows outside groove binding. No significant interaction was observed between both H2TPPS and ZnTPPS with two kinds of DNA. Upon excitation of the porphyrins into the higher excited state S2 (Soret band), the appearance of the transient absorption from ∼500 to ∼620 nm at about 0.05 ps in H2TMPyP-ctDNA, H2TMPyP-dGMP, and PdTMPyP-dGMP indicates the occurrence of the electron transfer (ET) from guanine to H2TMPyP and PdTMPyP. The forward ET are extremely fast (kf ≥ 1.0 × 1013 s-1), and the backward ET rates are ∼5.6 × 1012 and ∼4.0 × 1012 s-1, respectively. The complexation with DNA may lead to the shorter lifetime of the fluorescence of H2TMPyP and PdTMPyP.