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Biomolecular photo-damaging activity of a watersoluble cationic porphyrin was examined using human serum albumin (HSA), a water-soluble protein as a target biomolecule model by a fluorometry. Dichlorophosphorus(V) tetraphenylporphyrin (Cl2P(V)TPP), was synthesized and used as a photosensitizer. This porphyrin could bind to HSA and cause the photosensitized oxidation of HSA through the singlet oxygen generation and the oxidative photo-induced electron transfer (ET). Near infrared emission spectroscopy demonstrated the photosensitized singlet oxygen generation by this porphyrin. Decrement of the fluorescence lifetime of Cl2P(V)TPP by HSA supported the ET mechanism. Furthermore, the estimated Gibb's energy indicated that the ET mechanism is possible in the terms of energy. Because oxygen concentration in cancer cell is relatively low, ET mechanism is considered to be advantageous for photosensitizer of photodynamic therapy. Porphyrins have been used as the drugs for photodynamic therapy (PDT), which is a less invasive treatment of cancer [1,2]. The oxidation of biomolecules is the major mechanism of PDT. It is triggered by singlet oxygen (1O2) generation or photo-induced electron transfer (ET). Biomolecule oxidation by 1O2 is considered as the main mechanism of PDT, however, the low concentration of oxygen in cancer cells restricts the PDT effect [3,4]. Therefore, ET mechanism might become a more important mechanism of PDT in the future. The ET mechanism requires highly oxidative activity (a lower reduction potential) in the photoexcited state of the photosensitizer. Larger excitation energy is advantageous for the lower reduction potential of the photosensitizer in the photoexcited state. Indeed, ultra-violet photosensitizers mainly induce biomolecule photodamage through the ET mechanism, whereas a visible-light photosensitizer is not appropriate for this mechanism [5]. Therefore, an appropriate molecular design to achieve ET mediated biomolecule damage using a visible-light photosensitizer is important. Since highvalent porphyrin complexes demonstrate a lower reduction potential in their photoexcited state than free-base or low-valent metal complexes, these porphyrins are advantageous for the oxidative ET reaction [5-7]. For example, in this research, a synthesized porphyrin, Cl2P(V)TPP (Figure1) was used. Figure 1. Structure of Cl2P(V)TPP used in this research. The protein damage photosensitized by Cl2P(V)TPP was examined by the method of the previous reports [8,9]. The sample solution containing 10 μM porphyrin and 10 μM human serum albumin (HSA), a water-soluble protein, in a sodium phosphate buffer (pH 7.6) was irradiated with a light-emitting diode (LED) (519 nm, 1 mW cm, CCS Inc., Kyoto, Japan). The intensity of the LED light source was measured with a photo-power meter (8230E, ADCMT, Tokyo, Japan). Protein damage by porphyrins was evaluated by measurement of the fluorescence intensity from the amino acid residues as previously reported (excitation wavelength: 298 nm, dete ction wavelength: 350 nm). The concentration of damaged HSA, [Damaged HSA], is determined as follows:

Photosensitized oxidative damage of human serum albumin by water-soluble dichlorophosphorus(V) tetraphenylporphyrin