**6.1 Photosensitized DNA damage through electron transfer**

DNA is a potentially important targeting biomacromolecules for PDT and aPDT [1–3, 28]. In the cases of DNA damage, the generation of reactive oxygen species, such as <sup>1</sup> O2 (Type II mechanism), and the direct oxidation of nucleobases through photoinduced electron transfer (Type I mechanism) are important. In general, O2 •− formation and following H2O2 and/or • OH production (Type II mechanism, minor) require relatively shorter wavelength radiation, such as ultraviolet ray [28, 32, 33]. Therefore, the contribution of the O2 •− generation (Type II minor) mechanism is considered to be small in the aPDT mechanism. As mentioned above, photosensitized 1 O2 generation is the important mechanism of aPDT. Guanine is the selective target of <sup>1</sup> O2, and every guanine is oxidized by <sup>1</sup> O2 in a DNA sequence [28, 33]. Similar to the <sup>1</sup> O2 generation mechanism, guanine is also damaged through electron transfer selectively [28, 32, 33]. However, single guanines in double-stranded DNA and guanine residue in single-stranded DNA are resistant to electron transfer mechanism, in the contrary to the <sup>1</sup> O2 mechanism [28, 33]. Since π-π interaction between consecutive guanines decrease the *E*ox of guanine, the consecutive

#### *Electron Transfer-Supported Photodynamic Therapy DOI: http://dx.doi.org/10.5772/intechopen.94220*

guanines, such as GG and GGG, are selectively oxidized through electron transfer mechanism [77–79]. Similar compounds are produced of guanine oxidation through the both mechanisms of <sup>1</sup> O2 generation and electron transfer [72].

The mechanism of DNA damage photosensitized by Nile Blue (**Figure 9**) has been studied as a potential photosensitizing reaction [96]. The reported value of ΦΔ by Nile Blue is very small (0.005) [66, 97]. Therefore, Nile Blue is an appropriate model to examine the oxygen-independent mechanism. Nile Blue bound to DNA strand through an electrostatic interaction and the fluorescence lifetime was decreased, supporting the electron transfer quenching. Using 32P-5′-end-labeled DNA fragments, DNA damaging mechanism of Nile Blue was examined and consecutive guanine damage was observed. From the analysis of DNA damaging pattern, the contribution of DNA damage through electron transfer mechanism was estimated to be 72% (the contribution of <sup>1</sup> O2 mechanism is 28%). The Δ*G* of electron transfer from guanine to the S1 state of Nile Blue is negative (−0.15 eV) [96], and this value is considered to become smaller in the case of consecutive guanine, as mentioned above [77–79]. The estimated *k*ET value is relatively large (1.0 × 1010 s−1). These values supported the electron transfer-mediated DNA oxidation. The mechanism of DNA damage photosensitized by Nile Blue is shown in **Figure 9**. Relevantly, rhodamine-6G, a fluorescence dye, induces the electron transfer-mediated oxidation of DNA [98] and folic acid [64] with photoirradiation. In general, fluorescence dyes hardly photosensitize 1 O2 generation. On the other hand, photooxidative activity through electron transfer depends on the redox potential of molecules. These results suggest that the electron transfer-oxidation becomes important PDT mechanism for non-<sup>1</sup> O2 generating dyes.
