**4.2 Electron transfer from DNA to photoexcited cationic porphyrins and microenvironmental effect of DNA on photoinduced electron transfer**

Photoinduced electron transfer between DNA and the cationic porphyrins, H2TMPyP and ZnTMPyP, was analyzed by the fluorescence measurements (**Figure 6**) [74]. Absorption spectrum and circular dichroism measurements showed that H2TMPyP mainly intercalates to calf thymus DNA, whereas ZnTMPyP binds into a DNA groove. An electrostatic interaction with DNA raises their redox potentials of the binding cationic porphyrins. In the presence of DNA, the fluorescence intensity of these porphyrins was almost the same as that without DNA. The *E*ox of H2TMPyP (>1.30 V vs. SCE in water) [27], ZnTMPyP (1.18 V vs. SCE in water) [73], and guanine (1.24 V vs. SCE in acetonitrile) [75, 76] suggested that electron transfer by the S1 state of H2TMPyP is possible in terms of energy. Furthermore, the electron donating character of guanines increased in the double-stranded structure [77–79]. However, the fluorescence measurements indicated that the S1 states of these porphyrins are barely quenched by DNA. These results could be explained by that an electrostatic interaction between cationic porphyrins and an anionic DNA strand should increase the redox potential of porphyrins, leading to the inhibition of the electron transfer. In the cases of their higher excited states, secondary excited singlet (S2) states, the electron transfer from DNA was observed. The lifetime of S2 state is significantly short (a few picoseconds). However, the *E*red value of their S2 states are large (larger *E*red value of the excited state indicates stronger oxidative activity); >2.14 V vs. SCE for H2TMPyP and 1.94 V vs. SCE for ZnTMPyP. Therefore, the S2 states of porphyrins are thermodynamically strong oxidants through electron transfer mechanism.

Photoinduced electron transfer from these porphyrins to benzoquinones, electron acceptors, and that from *N*-(4-aminobenzoyl)-L-glutamic acid (ABG), an electron donor, to these porphyrins were also studied [74]. As mentioned above, the electrostatic interaction with DNA raises the redox potential of cationic porphyrins (*i.e.* decreases the oxidative property of cationic porphyrins). Therefore, the DNA microenvironment inhibited the electron transfer from ABG, an electron-donating quencher, to the binding porphyrins. On the other hand, the electron transfer from the binding porphyrins to benzoquinones, an electron-accepting quencher, was enhanced. A steric effect by the DNA strand was also important. A hydrophobic bulky electron acceptors forms stacking complex with porphyrins, resulting in the strong fluorescence quenching. The interaction with DNA strand cleaves this stacking interaction and inhibit the electron transfer to the benzoquinone. In summary,

the DNA microenvironment significantly affects the electron transfer property of the binding cationic porphyrins through an electrostatic interaction and the steric effect.
