**5.1 Electron transfer control by pH**

The biomolecule oxidation activity of photosensitizer through electron transfer can be controlled by using changeable electron donor. **Por13** was designed and synthesized to control the photodynamic activity of phosphorus(V) porphyrin photosensitizer (**Figure 7**) [48]. As an electron-donor, 6-methylpyridine was used. The photoexcited **Por13** is quenched through intramolecular electron transfer and this quenching is suppressed by protonation of the methylpyridine moiety, an electron donor. The p*K*a of protonated methylpyridine moiety was about 7, and fluorescence lifetime of **Por13** was lengthened under an acidic condition by

**Figure 7.** *Scheme of the activity control of photosensitizer, Por13, by pH and the relaxation processes of photoexcited state.*

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

suppression of the quenching through intramolecular electron transfer by methylpyridine. The quantum yields of photosensitized <sup>1</sup> O2 generation and biomolecule oxidation through electron transfer mechanism were also increased under acidic condition. NADH oxidation by **Por13** through photoinduced electron transfer was successfully enhanced under acidic conditions. However, photosensitized protein damage (oxidative damage of HSA) through electron transfer was decreased under an acidic condition, and relatively strong protein damage was observed under a neutral condition. It is explained by the fact that a relatively weak association between protein and **Por13** under an acidic condition due to electrostatic repulsion. Protonated protein under acidic condition decreases the association with cationic porphyrin, resulting in the suppression of the electron transfer from the amino acids. Furthermore, the hydrophobic environment of protein inhibits the electron transfer-quenching of **Por13**. This study shows the difficulty of activity control of photosensitizers by pH, because other factors significantly affect the photoinduced electron transfer.
