**6.2 Photosensitized protein damage through electron transfer**

Photosensitized protein damage by Methylene Blue and its analogues (**Figure 10**) were studied [99]. Similar to the cases of phosphorus(V) porphyrin photosensitizers, HSA was used as the targeting biomacromolecules. DNA binding through electrostatic force of these cationic compounds are well-known [40, 71, 74, 96, 100]. However, the interaction between these cationic dyes and HSA is small and a hydrophobic

#### **Figure 9.**

*Structure of Nile Blue and the proposed mechanism of guanine decomposition through photoinduced electron transfer.*

#### *Photodynamic Therapy - From Basic Science to Clinical Research*

#### **Figure 10.**

*Structures of Methylene Blue and its analogues. Binding constants with HSA were examined in a 10 mM sodium phosphate buffer (pH 7.6). QET: The quantum yield of HSA oxidation through electron transfer mechanism. QSO: The quantum yield of HSA oxidation through 1 O2 generation.*

interaction (not electrostatic interaction) may be a driving force of the association with HSA [58]. The reported binding constant, which were estimated by the Benesi-Hildebrand Equation [101] are shown in **Figure 10**. Fluorometry of HSA tryptophan residue demonstrated the photosensitized oxidation through both mechanisms, electron transfer and 1 O2 generation [99]. The analyzed quantum yields through these mechanisms are shown in **Figure 10**. Fluorescence decay of these dyes was complex. From the analysis of their observed fluorescence decay, the estimated *k*ET values were order of 109 s−1, supporting the electron transfer mechanism. Furthermore, this result suggests the existence of markedly fast electron transfer species, much faster than the detection limit of this study (within ~50 ps) [99]. DFT calculation also supported the electron transfer mechanism. The energy gap between the highest occupied molecular orbital (HOMO) of amino acids and that of photosensitizers are important for the electron transfer mechanism. The plot between the HOMO values of these cationic dyes and the protein damaging quantum yield through electron transfer demonstrated a relatively good relationship. Furthermore, the relationship between the ΦΔ and the damaging quantum yield through 1 O2 generation is also observed. These results shown that the electron transfer mechanism is also important for photosensitized protein oxidation by Methylene Blue and its analogues, as 1 O2 generation mechanism does. The electron transfer mechanism is not completely independent of oxygen molecule, because oxygen support the electron transfer by removing the excess electron from the reduced photosensitizer. However, other endogenous oxidative agents, such as metal ions, may support the electron transfer mechanism, in *vivo*, the electron transfer mechanism may play an important role in the aPDT under hypoxic condition.

#### **7. Conclusions**

This chapter reviewed the several topics about the photosensitizers, which play electron transfer-supported mechanism. 1 O2 is the important reactive species in PDT and aPDT. However, hypoxic condition in biological environment is not appropriate for reactive oxygen-dependent mechanism. Electron transfer is not completely independent of oxygen; however, this mechanism does not absolutely require oxygen. Endogenous oxidative substances other than oxygen can support the electron

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

transfer mechanism. In the study of PDT photosensitizer for cancer, phosphorus(V) porphyrins showed the selectivity for cancer cell and relatively strong PDT effects. Most important property of these photosensitizers is strong photooxidative activity through electron transfer under long-wavelength visible light irradiation. Furthermore, the photosensitizing activity of phosphorus(V) porphyrins through electron transfer mechanism can be controlled by surroundings, such as pH. In the processes of aPDT, the electron transfer mechanism may be important. For developing the effective drugs for aPDT, molecular design based on the electron transfer is also useful as well as that based on the 1 O2 generating activity. The activity of electron transfer oxidation depends on the redox potential, and a long lifetime of photoexcited state is advantageous. For PDT photosensitizers, relatively strong response to long-wavelength radiation is required. In the molecular design of PDT photosensitizers including phosphorus(V) porphyrins, the calculations of HOMO energy level and the excitation energy are important as the initial steps.
