**3.4 Photoinduced electron transfer by phosphorus(V) porphyrin triggers the chain reaction for NADH decomposition**

The electron transfer mechanism can contribute to oxidation other various biomolecules. For example, nicotinamide adenine dinucleotide (NADH), an important endogenous reductant, becomes an important targeting molecule [50]. The S1 states of **Por3** and **Por4** easily extract electron from NADH, resulting in the formation of

#### **Figure 5.**

*Structures of NADH and its oxidized form, and the electron transfer-triggered chain reaction of NADH decomposition.*

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

NAD• , a radical. Further oxidation leads to the irreversible decomposition of NADH to NAD<sup>+</sup> (**Figure 5**). The total quantum yield of NADH decomposition (ΦD) is expressed as follows:

$$
\boldsymbol{\Phi}\_{\rm D} = \boldsymbol{\Phi}\_{\rm ET} \times \boldsymbol{\Phi}\_{\rm FR} \,\,\,\,\,\tag{5}
$$

where ΦET is the quantum yield of the initial process (electron transfer) and ΦFR is that of the further reaction to form NAD+ . Analysis of the quantum yields, obtained values of ΦFR became much larger than unity. These findings suggest that the electron accepting by the photoexcited **Por3** and **Por4** triggers a chain reaction of NADH oxidation (**Figure 5**). The initial electron transfer to photoexcited **Por3** or **Por4** produces NAD• . The NAD• immediately reacts with molecular oxygen to produce O2 •−:

$$\text{NAD}^\* + \text{O}\_2 \rightarrow \text{NAD}^\* + \text{O}\_2 \text{ \textdegree\text{.}} \tag{6}$$

In the following process, O2 •− oxidizes NADH and hydrogen peroxide (H2O2) is produced [63]:

$$\text{NADH} + \text{O}\_2\text{ }^\text{"} + \text{H}^\text{"} \rightarrow \text{NAD}^\text{"} + \text{H}\_2\text{O}\_2.\tag{7}$$

The electron transfer-mediated reaction induces the chain reaction, resulting in the acceleration of NADH decomposition and secondary generation of reactive oxygen species. In the case of direct photosensitized reaction, ultraviolet photon is required to produce H2O2 [28]. The secondary formed H2O2 may produce hydroxyl radicals (• OH), very strong ROS. These results suggest that electron transfer reaction with visible light irradiation induces a severe toxic effect through a chain reaction and the formation of H2O2, similarly to the ultraviolet radiation.

### **3.5 Photosensitized oxidation of folic acid by phosphorus(V) porphyrin through electron transfer**

Folic acid, a vitamin, is also oxidized through photoinduced electron transfer [64]. Because the fluorescence intensity of folic acid is significantly increased by the decomposition, a fluorometry of folic acid can be used as a convenient indicator to evaluate the photosensitizer activities [65, 66]. For example, photosensitized decomposition of folic acid by **Por2** through electron transfer was reported [49]. Photoexcited porphyrin can produce <sup>1</sup> O2, and folic acid is also oxidized by <sup>1</sup> O2. The contribution of <sup>1</sup> O2-mediated decomposition can be excluded by the effect of <sup>1</sup> O2 quencher and the effect of electron transfer reaction can be evaluated.
