Solvent dependence of the lifetime of singlet oxygen.
Hydrogen peroxide (H2O2) and singlet oxygen (1O2) are important reactive oxygen species (ROS) for biological and medicinal fields. Oxidation processes of chemical materials by molecular oxygen are important H2O2 source, whereas photochemical reaction is important for 1O2 production. Reactivity and biomolecule damage by these ROS depend on the surrounding conditions and targeting molecules. In this chapter, production mechanisms of H2O2 and 1O2, biomolecule oxidation by these ROS, their detection methods, and production control of 1O2 are briefly reviewed.
- hydrogen peroxide
- singlet oxygen
- DNA damage
- protein damage
Biomolecule damage, for example, oxidation of DNA and/or protein, by reactive oxygen species (ROS) is closely related to carcinogenicity [1, 2, 3] and/or toxicity [4, 5, 6]. Furthermore, oxidative damage to unwanted tissue can be applied to the treatment of disease including cancer treatment [7, 8, 9], and similar reaction is applied to sterilization [10, 11, 12, 13, 14]. Hydrogen peroxide (H2O2) is a relatively long-lived ROS compared with a short-lived ROS such as superoxide anion radicals (O2•−) . One of the most important producing mechanisms of H2O2 is a dismutation of O2•−, which is easily formed though oxidation of various materials by dioxygen molecule (O2). Various carcinogenic chemical compounds produce H2O2 through their oxidation processes. Relationship among molecular oxygen and ROS is shown in Figure 1. Oxygen molecules are easily reduced by surrounding materials, and various ROS and the intermediates are formed (Figure 1A). In the case of photosensitized reaction, excited states of oxygen molecules are produced (Figure 1B). Singlet oxygen (1O2), which is also an important ROS, can be easily generated via photosensitized reaction [16, 17, 18]. The 1∑g+ state (1O2(1∑g+)) is mainly produced through the excitation energy transfer from the excited state, in general triplet excited (T1) state, of photosensitizer [16, 17, 18]. The 1O2(1∑g+) has higher energy, 1.6 eV, corresponding to the ground state of oxygen molecule (3O2). The lifetime of 1O2(1∑g+) is several picoseconds, and 1O2(1∑g+) is rapidly converted to the 1Δg state (1O2(1Δg)) [16, 17, 18]. Because the lifetime of 1O2(1Δg) (several microseconds) is markedly longer than that of 1O2(1∑g+), 1O2(1Δg) is a more important ROS. After that, 1O2 indicates 1O2(1Δg) without explanation in this chapter. Visible light, other than ultraviolet radiation, has sufficient energy to produce 1O2 from the ground state of oxygen molecule. Therefore, 1O2 production is an important mechanism of phototoxicity and/or photo-carcinogenicity under strong light illumination with phototoxic materials. The purpose of this chapter is a review of the ROS-mediated biomolecule damage and the related topics.
2. Hydrogen peroxide
Hydrogen peroxide itself is not strongly ROS. However, other ROS including hydroxyl radicals (•OH) are produced from H2O2. In general, H2O2 is produced from the dismutation of O2•−, and, in vivo, production of H2O2 and O2•− occurs in mitochondria . In this section, H2O2 formation from compounds, specifically artificial materials, is introduced.
2.1. Hydrogen peroxide formation through oxidation of chemical compounds
One of the most important processes of H2O2 production is a dismutation of O2•−. Various chemical compounds or metals can be oxidized by oxygen molecules. In the case of a simple electron transfer-mediated oxidation, O2•− is produced by the electron extraction from chemical compounds or metals. The lifetime of O2•− in aqueous solution is about several milliseconds . The produced O2•− in aqueous media is converted to H2O2 through the dismutation by proton (H+) as follows:
For example, hydroquinone, which is one of the metabolites of benzene, can produce H2O2 through the autoxidation process (Figure 2) . This process is markedly enhanced by the presence of metal ions, specifically Cu2+ ions . In the presence of sacrificial reductants, for example, nicotinamide adenine dinucleotide (NADH), the oxidized form of hydroquinone, p-benzoquinone, is reduced to the parent hydroquinone. Consequently, the redox cycle is formed, leading to the production of H2O2 abundantly. It has been also reported that hydrazine analogues produce H2O2 through their autoxidation processes (Figure 3) [21, 22, 23].
2.2. Hydrogen peroxide production through photochemical processes
Photochemical processes also contribute to the formation of H2O2. Because the reorganization energy of the reduction of small molecule, such as O2 molecules, through electron transfer becomes large due to the Marcus theory [24, 25], the O2•− production through photoinduced electron transfer is energetically difficult [26, 27]. However, ultraviolet radiation to reductive photosensitizer, such as NADH (Figure 4), can produce O2•− as follows :
where NADH* is the photoexcited state of NADH and NAD• is the radical form. NAD• undergoes further oxidation by oxygen molecules to NAD+, the final oxidized product. The formed O2•− is also converted to H2O2 through the dismutation process of Eq. (1).
Photocatalytic reaction can also produce H2O2 [29, 30, 31, 32, 33, 34]. For example, the surface of titanium dioxide (TiO2) can reduce relatively oxidative molecules under ultraviolet A (UVA; wavelength, 315–400 nm) irradiation [29, 30, 31, 32]. Two crystalline forms of TiO2, anatase and rutile with band gap energies of 3.26 and 3.06 eV, respectively, are well-known semiconducting photocatalyst [29, 30, 31, 32]. The adsorbed oxygen molecules on the TiO2 surface is reduced to O2•− by the electron of conduction band, which is excited from the valence band by UVA energy (Figure 5). Similarly to the abovementioned reaction, O2•− is also converted to H2O2 through the dismutation process of Eq. (1). In addition, oxidation reaction of TiO2 photocatalyst also produces H2O2. The formed hole (h+) in the valence band by UVA irradiation oxidizes water molecules on the surface of TiO2 to •OH. The reaction of two •OH species can produce H2O2 as follows:
Although TiO2 particles are barely incorporated into cell nucleus , cellular DNA damage was reported [36, 37, 38, 39]. Because H2O2 has a transparency for nuclear membrane, the cellular DNA damage can be explained by H2O2-mediated mechanism . The activation of H2O2 and DNA damage by H2O2 are described later.
2.3. Secondary formation of hydrogen peroxide through photocatalytic reaction
Photocatalytic reaction can produce oxidized intermediates other than final oxidized products of chemical compounds. For example, photooxidized amino acids  and sugars  by TiO2 photocatalyst produce H2O2 through secondary oxidation reaction in the presence of metal ions (Figure 6). Titanium dioxide can photocatalyze the production of •OH, a strong oxidant, through the decomposition of H2O. The formed h+ in the valence band by UVA irradiation can also oxidize various materials adsorbed on TiO2 surface. Hydroxyl radicals and h+ can oxidize these biomolecules, resulting in the production of oxidized intermediates. The formation of partly oxidized molecules leads to the secondary H2O2 production in the presence of metal ions. This H2O2 production process may cause a remote H2O2 generation in cells.
It has been reported that the photooxidized phenylalanine and tyrosine by TiO2 produce H2O2 in the presence of copper(II) ion . Since TiO2 photocatalysis induces a hydroxylation of aromatic compounds , the formation of benzenediol derivatives from these aromatic amino acids is possible (Figure 7). As mentioned above, hydroquinone can produce H2O2 through the autoxidation (Figure 2) [20, 43]. The amount of H2O2 production from the photooxidized phenylalanine is significantly larger than that from tyrosine . The difference of their autoxidation rates should affect the H2O2 production. It has been reported that the autoxidation rate of 1,4-form of benzenediol is markedly faster than that of 1,2-form . Phenylalanine can be oxidized into various types of benzenediol, including the 1,4-form; however, tyrosine cannot be converted to the 1,4-form. Consequently, phenylalanine can produce relatively large amount of secondary H2O2 through the photocatalysis of TiO2. Furthermore, other amino acids can be also oxidized by TiO2 photocatalyst and induce the secondary ROS production. Specifically, photocatalyzed cysteine by anatase form produces significantly large amount of secondary H2O2. In the case of sugar oxidation by TiO2 photocatalyst, the activity of secondary H2O2 production by anatase form TiO2 is larger than that of rutile form .
3. DNA damage by hydrogen peroxide
Hydrogen peroxide itself barely induces DNA damage; however, it can oxidize nucleobases and cleave sugar-phosphate backbone in the presence of metal ions. In this section, the sequence-specific DNA damage by the H2O2-derived ROS and its biological effect are briefly introduced.
3.1. Sequence-specific DNA damage by hydrogen peroxide
Hydrogen peroxide causes alkali-labile products at guanine, thymine, and cytosine in the presence of cupper ion (Cu2+) . Since copper ions are associated with chromatin  to form stable complexes with DNA [46, 47, 48, 49], Cu2+ can play an important role in the activation of H2O2 in cell nucleus. Polyacrylamide gel electrophoresis studies demonstrated that H2O2 itself cannot cleave and oxidize DNA . However, the incubation of DNA with H2O2 and Cu2+ induce base modifications at guanine, thymine, and cytosine residues. These base modification sites can be cleaved by hot piperidine treatment [20, 21, 22, 44]. The derived reactive species from H2O2, for example, copper-peroxyl species (Cu(I)-OOH), are responsible for this DNA damage:
Cu(I)-OOH is not strongly reactive compared with •OH; however, its lifetime is relatively long to induce DNA base modification. Single-stranded DNA is easier oxidized by these ROS. Therefore, DNA damage by H2O2 is enhanced by denaturation of DNA . Abovementioned chemical compounds, benzenediol  and hydrazine , induce these base modification in the presence of Cu2+. In the case of relatively low concentration of TiO2 particles, similar sequence-specific DNA damage was observed after UVA irradiation with Cu2+ . DNA damage mediated by H2O2 is effectively inhibited by catalase , which is an enzyme to decompose H2O2 to H2O and O2. Chelating molecules for copper ions also effectively suppress this DNA damage. In addition, 3-methylthiopropanal (methional) is an effective inhibitor of Cu(I)-OOH [20, 32, 44]. Cu(I)-OOH cannot be scavenged by free •OH scavengers, such as sugars and alcohols [20, 22, 32, 44]. In the presence of Cu2+, UVA-irradiated NADH also induces DNA damage by the similar process through H2O2 production . In general, photosensitized DNA damage could be explained by 1O2 formation mechanism or electron transfer-mediated oxidation . The H2O2-mediated DNA is a rare case in the photochemical DNA damage.
Hydrogen peroxide and Cu2+ can induce tandem lesion at guanine and thymine residues . Clustered DNA lesions including tandem damage have important mutagenic potential [52, 53, 54]. Furthermore, the repair of such DNA damage is more difficult than single-base damage [55, 56, 57, 58, 59, 60]. Therefore, oxidative DNA damage through H2O2 production may play an important role in carcinogenesis.
In the presence of iron ions (Fe2+), •OH is formed as follows:
Formed •OH induces base oxidation with non-sequence specificity, because •OH can oxidize all nucleobases [44, 61]. In addition, direct cleavage of sugar-phosphate backbone is caused by •OH. Hydroxyl radical-mediated DNA damage was reported by the case of ascorbate with Cu2+ . As mentioned above, in the case of TiO2 photocatalysis, •OH is directly produced from water decomposition [29, 30, 31, 32], and DNA damage without sequence specificity can be induced in the absence of metal ions . Relatively high concentration of anatase form of TiO2 induce non-sequence-specific DNA damage under UVA irradiation without metal ions through •OH production . DNA damage by •OH is effectively inhibited by sugars and alcohols [32, 44]. However, in the presence of metal ions, the addition of •OH scavengers rather enhances DNA damage through the secondary generation of H2O2 from the oxidized products of scavengers themselves by •OH [32, 41]. Base modifications can cause carcinogenesis. Because H2O2 can penetrate into nuclear membrane, DNA modification can be induced by H2O2 originally formed in the sphere of outer cell nucleus through the assistance of metal ions.
3.2. Mutagenicity and cytotoxicity caused by hydrogen peroxide production
As oxidized products of nucleobases by the H2O2-mediated mechanism, 8-oxo-7,8-dihydroguanine (8-oxo-G; oxidized guanine, Figure 8) [63, 64, 65]; 5,6-dihydroxy-5,6-dihydrothymine (OH-thy; oxidized thymine, Figure 9) [58, 66, 67]; 5-hydroxyuracil (OH-Ura; oxidized cytosine, Figure 9) [67, 68]; 5-hydroxyhydantoin (OH-Hyd; oxidized cytosine, Figure 9) , and 5-hydroxycytosine (OH-Cys; oxidized cytosine, Figure 9)  are well-known compounds. In a certain case, oxidative DNA damage induces cell death [69, 70]. As a minor oxidized product of guanine, 2,6-diamino-4-hydroxy-5-formamidopyrimidine (Fapy-G, Figure 10) can be formed by H2O2 and metal ions [63, 71]. Mutagenicity of Fapy-G is low ; however, a related product, methyl-Fapy-G formation, is a lethal lesion . Furthermore, a theoretical study suggested that the formation of Fapy-G contributes to mutation . Cytotoxicity of TiO2 photocatalyst can be explained by oxidative damage of membrane protein [75, 76, 77]. In addition, cellular DNA damage was also reported [78, 79]. Because H2O2 has a transparency for nuclear membrane, the cellular DNA damage by TiO2 photocatalysis can be explained by H2O2 production. The formed H2O2 through TiO2 photocatalysis is incorporated into cell nucleus and activated by endogenous metal ions, leading to oxidative DNA damage . Examples of the mutations caused by the oxidized guanines are described in Section 4.
4. Singlet oxygen
In general, the production mechanism of 1O2 involves photochemical processes. Various photooxidation processes can be explained by 1O2 production. In this section, the production mechanism of 1O2, its application, and biomolecule oxidation by 1O2 are briefly introduced.
4.1. General property of singlet oxygen
Singlet oxygen is an excited state of 3O2, ground triplet state of molecular oxygen [16, 17, 18]. In general, singlet excited (S1) states of O2 are 1Δg and 1∑g+; they have excitation energy of 0.98 eV and 1.63 eV above 3O2, respectively [16, 17, 18]. Because of the short lifetime of 1∑g+ (a few picoseconds), 1Δg, the lower S1 state of O2, plays an important role in various oxidation reactions. In this chapter, 1Δg is denoted throughout as 1O2. The highest occupied molecular orbital (HOMO) of 3O2 is a semi-occupied molecular orbital (SOMO), whereas this molecular orbital of 1O2 becomes the lowest unoccupied molecular orbital (LUMO) (Figure 1B). The oxidative activity of 1O2 is stronger than that of 3O2 due to the vacant molecular orbital. Commonly, 1O2 is produced through photosensitized reaction. Since the excitation energy of 1O2 is relatively small, which corresponds to the energy of photon with the wavelength of 1270 nm (smaller than that of visible light photon), photoexcited states of various dyes can sensitize the generation of 1O2 under visible light or ultraviolet irradiation. Various molecules become photosensitizer (PST) to generate 1O2. In general, the photosensitized reaction of 1O2 generation is an electron exchange energy transfer (the Dexter mechanism) . These processes are presented as follows:
where PST*(S1) and PST*(T1) are the S1 and T1 states of PST, respectively. In general, since the lifetime of PST*(T1) is markedly longer (several microseconds) than that of PST*(S1) (several nanoseconds), 1O2 is produced by PST*(T1). However, the formation of 1O2 by PST*(S1) is not impossible. The lifetime of 1O2 (τΔ) is relatively long (Table 1). Generated 1O2 can oxidize various materials, including biomolecules, within its long lifetime. The τΔ strongly depends on the surroundings, and a solvent deuterium effect on the reactivity of 1O2 is significant (Table 1). For example, the τΔ in deuterium oxide (D2O) is markedly longer than that in H2O, and the biomolecule oxidation by 1O2 is significantly enhanced in D2O compared with that in H2O.
|Water (H2O)||Cationic porphyrin||3.5|||
|Phosphate buffer (pH 7.6)||P(V) porphyrin||3.5|||
|Ethanol (C2H5OH)||Rose bengal||15.4|||
|Ethanol/H2O (1/1)||Rose bengal||6.37|||
|Water (D2O)||Berberine with DNA||72|||
4.2. Photodynamic therapy
One of the most important medicinal applications of 1O2 is photodynamic therapy (PDT) (Figure 11) [7, 8, 9]. Photodynamic therapy is a promising and less invasive treatment for cancer [7, 8, 9] and photosterilization [10, 11, 12, 13, 14]. For cancer PDT, in general, porphyrins are used for photosensitizers, for example, porfimer sodium  and talaporfin sodium . Photosterilization, antimicrobial PDT, is also carried out using dyes, for example, methylene blue (MB) [11, 14, 89]. The important mechanism of PDT processes including photosterilization is oxidation of biomolecules of cancer cell or bacteria through 1O2 production under visible light irradiation. Visible light, especially longer wavelength visible light (wavelength > 650 nm), is less harmful for the human body and can penetrate into the tissue deeply. As mentioned above, 1O2 can be generated by longer wavelength visible light. Administered photosensitizers, porphyrins, or other dyes produce 1O2 through energy transfer to oxygen molecules with relatively large quantum yield (ΦΔ).
4.3. Photocatalytic singlet oxygen generation
As mentioned above, TiO2 photocatalyzes the generation of various ROS. Singlet oxygen can be also produced through the photocatalysis of TiO2 [31, 90, 91, 92, 93, 94, 95, 96]. In general, photogenerated electron in the conduction band reduces the surface-adsorbed oxygen molecules to O2•−. Through the reoxidation of O2•−, 1O2 is formed. The possible reactions of photocatalytic 1O2 productions are as follows:
The photogenerated h+ in the valence band and •OH can act as the oxidants to produce 1O2. In addition, hydroperoxyl radical (•OOH) generated from O2•− and H+ also produces 1O2 as follows:
The reported values of ΦΔ are depending on the experimental condition, for example, around 0.2 (0.2, Degussa P25 in water , and 0.22, rutile particle in chloroform ). Other cases reported relatively small values, for example, 0.003  and 0.02 . In the cases of airborne 1O2, quite small value (10−8–10−9) was reported . It has been reported that the τΔ value of 1O2 produced by Degussa P25 aqueous suspension is 5 μs . Other photocatalytic materials, for example, zinc oxide (ZnO) can photocatalyze 1O2 production through the similar reaction of TiO2 photocatalysis . Recently, carbon quantum dots, which have been paid attention as interesting nano-materials, also photocatalyze 1O2 production .
Singlet oxygen is an important ROS for PDT. Other than 1O2, H2O2 production can be also applied for PDT mechanism. Photocatalytic materials can produce these ROS under photoirradiation. Therefore, application of photocatalysts, specifically TiO2 nanoparticles, for PDT has been also studied [29, 98, 99, 100, 101]. To realize the TiO2-utilized PDT, direct administration of small TiO2 powders into tumor assisted with an optical fiber was proposed . In addition, it was reported that oral-administrated TiO2 nanoparticles are transported into the tumor of nude mouse skin transplanted from a human prostate cancer cell line . As mentioned above, in general, TiO2 nanoparticles can be excited by UVA irradiation. To utilize visible light for TiO2 excitation, upconversion technique was also studied .
4.4. DNA oxidation by 1O2 and mutation
Singlet oxygen can oxidize only guanines without sequence specificity; however, it does not have the ability to induce the oxidation of other nucleobases or to cleave the sugar-phosphate backbone . The main oxidized product of guanine by 1O2 is 8-oxo-G (Figure 8) [63, 64, 65]. Guanines undergo the Diels-Alder reaction by photoproduced 1O2, leading to the formation of [4 + 2] cycloaddition product with the imidazole ring to produce an endoperoxide. Through the subsequent proton transfer, this peroxide is converted to 8-hydroperoxyguanine [102, 103], which becomes 8-hydroxyguanine . The keto-enol tautomerism produces 8-oxo-G from 8-hydroxyguanine. Because single-stranded DNA is easily oxidized by ROS, 8-oxo-G formation by 1O2 is increased by DNA denaturation . The 8-oxo-G formation causes DNA misreplication (Figure 12), which can lead to mutations such as G-C:T-A transversion caused by the stable base-pair formation between 8-oxo-G and adenine [104, 105]. Since 8-oxo-G is more easily oxidized than guanine, 8-oxo-G undergoes further reaction, leading to the formation of imidazolone and oxazolone (Figure 13) [63, 106, 107]. Imidazolone forms more stable base pair with guanine than cytosine [106, 107]. Therefore, guanine oxidation by 1O2 may cause G-C:C-G transversion [108, 109] through imidazolone formation, a further oxidized product of 8-oxo-G. Indeed, it has been reported that UVA can induce these mutations .
4.5. Protein oxidation by 1O2
Protein oxidation is also induced by 1O2. The following amino acids, tryptophan, tyrosine, cysteine, histidine, and methionine, can be oxidized by 1O2 . In the case of tryptophan oxidation by 1O2, N-formylkynurenine (Figure 14) is a major oxidized product [112, 113]. The reported reaction rate coefficient between tryptophan and 1O2 is 3.0 × 107 s−1 M−1 . Oxidation of tryptophan residue in a certain protein can be examined with a fluorometer . For example, human serum albumin (HSA) has one tryptophan residue, and the intrinsic fluorescence of tryptophan at around 350 nm can be diminished by the oxidative damage. Porphyrin phosphorus(V) complexes (Figure 15), of which the ΦΔ is larger than 0.5, can induce oxidative damage to the tryptophan residue of HSA . Photosensitized HSA damage is enhanced in D2O, in which the lifetime of 1O2 is markedly elongated compared in H2O (Table 1). Furthermore, sodium azide (NaN3), a strong physical quencher of 1O2 , effectively suppresses this HSA damage. From the analysis of the effect of NaN3 on the HSA damage, the contribution of 1O2-mediated oxidation to the total quantum yield of protein damage can be determined . Photosensitized 1O2 production by porphyrin phosphorus(V) complexes induces the damage of tyrosinase, which is an enzyme to catalyze the hydroxylation of tyrosine, resulting in the deactivation of tyrosinase . Oxidation of the amino acid residue by 1O2 can cause the deactivation of protein function. The protein oxidation photosensitized by porphyrins through ROS production is an important mechanism of PDT.
Photocatalyzed 1O2 production by TiO2 may not play an important role in the oxidation reaction [31, 94]. Formed 1O2 on the TiO2 surface is quenched by TiO2 itself with relatively large quenching rate coefficient (e.g., 2.4 × 109 M−1 s−1 ). In the presence of bovine serum albumin, 1O2 produced by TiO2 photocatalysis is effectively quenched, suggesting the protein oxidation . However, in the case of TiO2 photocatalyst, other ROS are more important for protein oxidation than 1O2-mediated reaction [29, 30, 31, 32].
5. Detection of ROS
ROS detection is an important theme to investigate a biological effect of ROS or evaluation of the activity of PDT photosensitizers [119, 120, 121, 122]. Fluorometry is one of the most important and effective methods of ROS detection. For example, 5-carboxyfluorescein-based probe has been developed (Figure 16) . This probe can detect H2O2 in the living cell. As an inexpensive method, the fluorometry using folic acid (Figure 17) was reported [23, 119, 124]. Folic acid can be decomposed by H2O2 in the presence of Cu2+, resulting in the fluorescence enhancement. The limit of detection (LOD, at signal/noise = 3) for this method was 0.5 μM H2O2. This method is based on the oxidative decomposition of folic acid by Cu(I)-OOH. In the presence of Fe2+, •OH slightly induces the folic acid decomposition; however, the effect of •OH on this folic acid decomposition is negligibly small because of the very short lifetime [125, 126]. In addition, O2•− does not have the activity of folic acid decomposition. Using folic acid or its analogue, 1O2 can be also detected . Specifically, in D2O, folic acid or methotrexate (Figure 17), an analogue of folic acid, is effectively decomposed by 1O2, resulting in the fluorescence enhancement . Using this method, the values of ΦΔ of various water-soluble photosensitizers can be determined.
6. Control of singlet oxygen production
Control of photosensitized 1O2 is an important theme for biology or medicine, for example, to realize target-selective PDT  or “theranostics” (therapy and diagnosis) . The pH-dependent control  and target-selective control [127, 128, 130, 131, 132] methods have been reported. It has been reported that free base porphyrins were synthesized to control their photosensitized 1O2 generating activity by pH (Figure 18) . The S1 state of this porphyrin is quenched by the electron-donating moiety in neutral or alkali solution. However, protonation of this electron-donating moiety under acidic condition suppresses the electron transfer, leading to the recovery of the 1O2 production activity of porphyrin ring. Because cancer cell is slightly a more acidic condition compared with normal cells [133, 134, 135], this pH-based control of photosensitized 1O2 production can be applied to cancer-selective PDT. DNA-targeting control of photosensitized 1O2 generation has been also reported [127, 128]. For example, electron donor-connecting porphyrins have been studied (Figure 19) [81, 130, 131, 132]. These compounds can be photoexcited by visible light irradiation, and their S1 states are effectively quenched through intramolecular electron transfer. The charge-transfer state energy can be raised through the binding interaction with DNA, an anionic polymer, resulting in the inhibition of the intramolecular electron transfer and enhancement of 1O2 generation.
Hydrogen peroxide is easily produced from the oxidation processes of chemical compounds by oxygen molecules. In addition, UVA-irradiated NADH and semiconductor photocatalytic materials can also produce H2O2. Formed H2O2 in cells can be incorporated into cell nucleus and activated by endogenous metal ions. Copper ion induces Cu(I)-OOH formation from H2O2, whereas •OH is produced from H2O2 and iron ion. These ROS cause base oxidation, and •OH can induce strand break of DNA. Base modifications lead to carcinogenesis or lethal effect. Photoirradiation to various sensitizing materials induces 1O2 production. Visible light has sufficient energy to produce 1O2. Therefore, 1O2 is easily produced by various dyes under photoirradiation. Photocatalytic 1O2 formation through reoxidation of O2•− is also possible. Formed 1O2 can oxidize guanine residues of DNA without sequence specificity and several amino acid residues of protein within its lifetime, which depends on the surroundings. Various detection methods of these ROS have been developed. In addition, the target-selective or condition-selective productions of ROS become important strategies for PDT and cancer “theranostics.”
These works were partially supported by the Grants-in-Aid for Scientific Research (KAKENHI) from the Japan Society for the Promotion of Science (JSPS).