Stable Photoproducts Research Articles

Studies on the UV photodestruction of purine free base in aqueous solutions at 77 and 298K.

— The photodestruction of purine free base used as a model for the purine bases in DNA has been studied in order to better understand the effect of UV light on these molecules. Photodestruction yields have been determined in glassy aqueous solutions at 77 K and at room temperature at different pH's. The yield decreases in the order of 0.04, 0.01, 0.001 in 8 M NaOH, 8 M NaCIO4 and 6 M H3PO4, respectively, while at room temperature the highest destruction yield is 0.005 for the unbuffered neutral solution. These yields have been determined by measuring the initial decrease in the purine absorption maximum as a function of irradiation time. During the illumination stable photoproducts, as well as reactive intermediates, such as trapped electrons, radical anions and cations, are formed and have been characterized from their absorption spectra. The addition of triplet quenchers and an electron scavenger resulted in a decrease in the yield, implying the participation of the purine triplet state and a radical anion in the reactions leading to the photodestruction of purine.

Enhanced optoelectronic properties of UV-light-induced photodegraded TPD

We report UV-light-induced photodegradation of N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’ diamine (TPD) dye. When a halomethane solution of TPD is exposed to UV light it first oxidizes TPD and subsequently with further exposure leads to irreversible transformation of TPD to an acridine derivative as photoproduct. Exclusive use of halomethane solvents like chloroform and dichloromethane suggests solvent radical initiated photochemical transformation of TPD to a stable acridine-based photoproduct. The resultant photoproduct of TPD showed broad-band dual (both blue and green) emissions in various solvents and also from the sublimed thin film. Based on solvent- and concentration-dependent photoluminescence (PL) of the photoproduct, PL excitation and lifetimes of blue and green bands, the origin of emission in the green region is due to an intermolecular excimer/aggregate of acridine-based photoproduct. A film of acridine-based photoproduct dispersed in polystyrene also showed substantial improvement in electrical conductivity compared to the parent compound TPD. The technique of UV-light-induced molecule structure transformation and material processing under specific environmental conditions may be considered as an efficient means of obtaining materials with enhanced optoelectronic properties.

Chlorpromazine Binding to Na+, K+‐ATPase and Photolabeling: Involvement of the Ouabain Site Monitored by Fluorescence

This work reports the results of ultraviolet irradiation on the interaction of the phototoxic antipsychotic drug chlorpromazine (CPZ) with the sodium pump Na+, K+-ATPase. The study was performed by monitoring the fluorescence modifications of CPZ itself and of the specific probe anthroylouabain (AO). CPZ association with Na+, K+-ATPase was found to modify the kinetics of CPZ-photodegradation. It was demonstrated that UV irradiation produces a stable fluorescent photoproduct of CPZ covalently bound to Na+, K+-ATPase. The fluorescent probe AO, which specifically binds to the extracellular ouabain site of the pump, was used to localize the CPZ binding site. UV-irradiation of AO-labeled Na+, K+-ATPase treated with CPZ at concentration about 20 microM produced dose-dependent modifications of the AO fluorescence, e.g. increased quantum yield and blue shift. The results demonstrated that CPZ binds near the ouabain site. The photo-induced reaction of CPZ with AO-labeled Na+, K+-ATPase protected the ouabain site from the aqueous environment. It was also found that UV irradiation of CPZ-treated enzyme obstructs the binding of AO, which suggested occlusion of the ouabain site. This effect can be evaluated for a potential use of CPZ in photochemotherapy.

Photochromism ofAnabaenaSensory Rhodopsin

Protein-controlled photochemical reactions often mediate biological light-signal and light-energy conversions. Microbial rhodopsins possess all-trans or 13-cis retinal as the chromophore in the dark, and in the light-driven proton pump, bacteriorhodopsin (BR), the stable photoproduct at the end of the functional cycle of the all-trans form is 100% all-trans. In contrast, a microbial rhodopsin discovered in Anabaena PCC7120 is believed to function as a photochromic sensor. For Anabaena sensory rhodopsin (ASR), the photoreaction is expected to be not cyclic, but photochromic. The present low-temperature UV-visible spectroscopy of ASR indeed revealed that the stable photoproduct of the all-trans form in ASR is 100% 13-cis, and that of the 13-cis form is 100% all-trans. The complete photocycle for the proton pump in BR and the complete photochromism for the chromatic sensor of ASR are highly advantageous for their functions. Thus, the microbial rhodopsins have acquired unique photoreactions, in spite of their similar structures, during evolution.

N,N-Di(4-halophenyl)nitrenium Ions: Nucleophilic Trapping, Aromatic Substitution, and Hydrogen Atom Transfer

The reactive intermediates N,N-di(4-chlorophenyl)nitrenium ion and N,N-di(4-bromophenyl)nitrenium ion were generated through photolysis of the corresponding N-amino(2,4,6,-collidinium) ions. The behavior of these diarylnitrenium ions was characterized by laser flash photolysis, analysis of the stable photoproducts, and ab initio calculations with density functional theory. The latter predict these species to have singlet ground states. The halogenated diarylnitrenium ions are significantly longer lived than the unsubstituted diphenylnitrenium ion. Specifically, cyclization to form carbazole derivatives occurs negligibly, if at all, with the halogenated derivatives. They do, however, carry out most of the characteristic reactions of singlet arylnitrenium ions, including combining with nucleophiles on the aryl rings, adding to arenes, and accepting electrons from readily oxidized traps. Interestingly these species also abstract H atoms from 1,4-cyclohexadiene and various phenol derivatives. The implication of the latter process in relation to the computed singlet-triplet energy gaps of ca. -12.5 kcal/mol is discussed.

Photochemical behaviour of triclosan in aqueous solutions: Kinetic and analytical studies

The mechanism of the direct photolysis of the anti-microbial triclosan in aqueous solutions was investigated by using steady state and laser flash photolysis. Quantum yields were determined for the disappearance of triclosan and formation of chloride anions in steady state irradiations in the absence and in the presence of oxygen as well as a function of pH. The photoreactivity was found to be efficient with the anionic form and in the absence of oxygen. Following laser flash photolysis (226 nm), three transients were found (triclosan triplet state, solvated electron and phenoxyl radical). Several primary and secondary stable photoproducts were elucidated by means of LC/MS/MS data. They were found to arise from four main photochemical processes: isomerisation, cyclization (leading to the formation of dioxin derivatives), dimerisation of the phenolic moiety and hydrolysis. The ionic chromatography showed that the loss of chloride anion in triclosan phototransformation represents an important degradation pathway. The formation of oligomeric products was also observed for prolonged irradiation time. A detailed mechanism for the formation of the primary products is proposed and discussed. The very important photocyclization reaction is more likely involving the triplet state pathway and the homolytic dissociation of the ether bridge occurs from the singlet excited state pathway.

Photochemical Ligand Substitution Reactions of fac-[Re(bpy)(CO)3Cl] and Derivatives

Excitation by high-energy light, such as that of 313 nm wavelength, induces a photochemical ligand substitution (PLS) reaction of fac-[Re(bpy)(CO)3Cl] (1a) to give the solvento complexes (OC-6-34)- and (OC-6-44)-[Re(bpy)(CO)2(MeCN)Cl] (2 and 3) in good yields. The disappearance quantum yield of 1a was 0.01+/-0.001 at 313 nm. The products were isolated, and X-ray crystallographic analysis was successfully performed for 2. Time-resolved IR measurements clearly indicated that the CO ligand dissociates with subpicosecond rates after excitation, leading to vibrationally hot photoproducts, which relax within 50-100 ps. Detailed studies of the reaction mechanism show that the PLS reaction of 1a does not proceed via the lowest vibrational level in the 3MLCT excited state. The PLS reaction gives 2 and (OC-6-24)-[Re(bpy)(CO)2(MeCN)Cl] (5) as primary products, and one of the products, 5, isomerizes to 3. This type of PLS reaction is more general, occurring in various fac-rhenium(I) diimine tricarbonyl complexes such as fac-[Re(X2bpy)(CO)3Cl] (X2bpy=4,4'-X2-bpy; X=MeO, NH2, CF3), fac-[Re(bpy)(CO)3(pyridine)]+, and fac-[Re(bpy)(CO)3(MeCN)]+. The stable photoproducts (OC-6-44)- and (OC-6-43)-[Re(bpy)(CO)2(MeCN)(pyridine)]+ and (OC-6-32)- and (OC-6-33)-[Re(bpy)(CO)2(MeCN)2]+ were isolated. The PLS reaction of rhenium tricarbonyl-diimine complexes is therefore applicable as a general synthetic method for novel dicarbonyls.

In Vitro Phototoxicity of Phenothiazines: Involvement of Stable UVA Photolysis Products Formed in Aqueous Medium

This paper reports the results of an in vitro evaluation of the phototoxic potential of stable photoproducts formed by UVA photolysis of three phenothiazines, perphenazine, fluphenazine, and thioridazine, in a water environment. Perphenazine gave a single product due to dechlorination. From thioridazine, the two major products formed; the endocyclic sulfoxide and the endocyclic N-oxide in which the 2-SCH3 substituent was replaced by a hydroxy group were tested. From fluphenazine, two products have been examined as follows: an exocyclic N-piperazine oxide and a carboxylic acid arising from hydrolysis of the 2-CF3 group. The phototoxicity of the isolated photoproducts has been studied in order to determine their possible involvement in the photosensitizing effects exhibited by the parent drugs, using hemolysis and 3T3 fibroblasts viability as in vitro assays. As fluphenazine, perphenazine, and thioridazine did, some photoproducts proved phototoxic. In particular, the perphenazine dechlorinated photoproduct and the thioridazine N-oxide were found to exert phototoxic properties similar to the parent compounds. Therefore, our data suggest that some phenothiazine photoproducts may play a role in the mechanism of photosensitivity of these drugs. Because some of these photoproducts correspond to metabolic products of phenothiazines found in humans, it cannot be ruled out that metabolites of phenothiazines can be phototoxic in vivo.

Putative phototautomerization of 4-thiouridine in the S 2 excited state revealed by fluorescence study using picosecond laser spectroscopy

The stationary and dynamic properties of S 2 → S 0 fluorescence of 2′,3′,5′-tri- O-acetyl-4-thiouridine (TU) in acetonitrile solution at room temperature have been studied. In the emission spectra of the sample, measured upon excitation with picosecond laser pulses, changes in fluorescence intensity and emission band shape have been observed with increasing time of excitation. The fluorescence intensity increases and the band maximum moves to the shorter wavelength until the steady-state condition is achieved. These changes are fully reversible indicating that they are not related to the formation of stable photoproducts. No such changes have been observed in the fluorescence spectrum recorded upon excitation of the same sample of TU with continuous light from xenon lamp. Under the latter conditions the fluorescence is very weak ( ϕ F = 1 × 10 −4) and the band exhibit a maximum at λ max F = 420 nm . The decay kinetics was measured using a two-stage method in order to estimate the number of emitting species and determine the parameters characterizing their emission. Analysis of the experimental fluorescence decays at λ F = 400, 430 and 450 nm measured upon various time intervals (<20 min) of the sample excitation with laser pulses has shown that the best fit of experimental to theoretical curve is obtained assuming the presence of three emitting species with lifetimes of τ 1 = 5 ± 1 ps, τ 2 = 80 ± 20 ps and τ 3 = 600 ± 100 ps. In contrast to the lifetimes the values of fractional coefficients depend on λ F and the time of measurement. The results of the fluorescence study of TU with picosecond laser spectroscopy are discussed in terms of phototautomerization of TU (2-keto-4-thione) in the S 2 state leading to its two possible tautomers: 4-thione, 2-hydroxy form in the S 2 state and 4-thiol, 2-keto form in the S 1 state. The models of the tautomers of TU were synthesized and their absorption and emission features were determined.

Inhibition of Nucleotide Excision Repair by High Mobility Group Protein HMGA1

The mammalian non-histone "high mobility group" A (HMGA) proteins are the primary nuclear proteins that bind to the minor groove of AT-rich DNA. They may, therefore, influence the formation and/or repair of DNA lesions that occur in AT-rich DNA, such as cyclobutane pyrimidine dimers (CPDs) induced by UV radiation. Employing both stably transfected lines of human MCF7 cells containing tetracycline-regulated HMGA1 transgenes and primary Hs578T tumor cells, which naturally overexpress HMGA1 proteins, we have shown that cells overexpressing HMGA1a protein exhibit increased UV sensitivity. Moreover, we demonstrated that knockdown of intracellular HMGA1 concentrations via two independent methods abrogated this sensitivity. Most significantly, we observed that HMGA1a overexpression inhibited global genomic nucleotide excision repair of UV-induced CPD lesions in MCF-7 cells. Consistent with these findings in intact cells, DNA repair experiments employing Xenopus oocyte nuclear extracts and lesion-containing DNA substrates demonstrated that binding of HMGA1a markedly inhibits removal of CPDs in vitro. Furthermore, UV "photo-foot-printing" demonstrated that CPD formation within a long run of Ts (T(18)-tract) in a DNA substrate changes significantly when HMGA1 is bound prior to UV irradiation. Together, these results suggest that HMGA1 directly influences both the formation and repair of UV-induced DNA lesions in intact cells. These findings have important implications for the role that HMGA protein overexpression might play in the accumulation of mutations and genomic instabilities associated with many types of human cancers.

Two-Photon Absorption of Bacteriorhodopsin: Formation of a Red-Shifted Thermally Stable Photoproduct F 620

By means of high-intensity 532 nm laser pulses, a photochemical conversion of the initial B 570 state of bacteriorhodopsin (BR) to a stable photoproduct absorbing maximally at ≈620 nm in BR suspensions and at ≈610 nm in BR films is induced. This state, which we named F 620, is photochemically further converted to a group of three products with maximal absorptions in the wavelength range from 340 nm to 380 nm, which show identical spectral properties to the so-called P 360 state reported in the literature. The photoconversion from B 570 to F 620 is most likely a resonant two-photon absorption induced step. The formation of F 620 and P 360 leads to a distinguished photo-induced permanent optical anisotropy in BR films. The spectral dependence of the photo-induced anisotropy and the anisotropy orientations at the educt (B 570) and product (F 620) wavelengths are strong indicators that F 620 is formed in a direct photochemical step from B 570. The chemical nature of the P 360 products probably is that of a retro-retinal containing BR, but the structural characteristics of the F 620 state are still unclear. The photo-induced permanent anisotropy induced by short laser pulses in BR films helps to better understand the photochemical pathways related to this transition, and it is interesting in view of potential applications as this feature is the molecular basis for permanent optical data storage using BR films.

Photochemistry of carboxylic acids containing the phenyl and thioether groups: Steady-state and laser flash photolysis studies

The mechanisms for the direct photolysis of phenylthioacetic acid (PTAA) and S-benzylthioglycolic acid (SBTGA) in acetonitrile were investigated using steady-state and laser flash photolysis. For both compounds, a variety of stable photoproducts were found under steady-state, 254 nm irradiation of acetonitrile solutions. The products from the direct photolysis of PTAA included carbon dioxide (photodecarboxylation), acetic acid, diphenyl disulfide, diphenyl sulfide, thiophenol, thioanisole, di(phenylthio) methane, and S-phenyl benzenethiosulfate. The products from the direct photolysis of SBTGA included carbon dioxide, toluene, dibenzyl, dibenzyl sulfide, dibenzyl disulfide, thioglycolic acid, benzyl mercaptan, benzyl alcohol, and benzaldehyde. These stable photoproducts were identified and characterized using HPLC, GC, GC–MS, and UV–vis methods. Quantum yields were determined for the formation of the various stable products following steady-state irradiations in the absence and in the presence of oxygen. In laser flash photolysis (266 nm Nd:YAG laser) experiments, a variety of transients (e.g., phenylthiyl radical, benzyl radical, etc.) was found. For both substrates (PTAA and SBTGA), photoinduced C S bond cleavage was the main primary process. It was also found that for both acids, photoinduced C C bond cleavage occurred, but as a minor process relative to C S bond cleavage. Detailed mechanisms of the primary and secondary processes are proposed and discussed. The validity of these proposed mechanisms was supported by an analysis of the quantum yields of stable products and their transient precursors. Supplementary observations on reactions between the radicals (e.g., C 6H 5–S , C 6H 5– CH 2) and oxygen are also consistent with the proposed mechanisms.

Photochemistry of 1,3,5-trithianes in solution: Steady-state and laser flash photolysis studies

In this work, we characterized the direct photochemistry of a set of five structurally-related 1,3,5-trithianes. The compounds were 1,3,5-trithiane, the α- and β-isomers of the 2,4,6-trimethyl derivatives, and the α- and β-isomers of the 2,4,6-triphenyl derivatives. Under steady-state, 254-nm irradiation of acetonitrile solutions of all five trithianes, dithioesters of the form RC( = S)SCH(R)SCH 2R were identified and shown to be primary photoproducts (R = H, CH 3, or C 6H 5). Shorter dithioesters, RC( = S)SCH 2R, were also identified and shown to be secondary products. The presence of the dithioesters could be monitored by their strong absorption bands in the region of 310 nm. This same band was evident following the laser flash photolysis of the five trithianes. The laser-induced transient spectra showed another absorbing species ( I) in all five trithianes. This species was not stable and showed a complementary decay that matched the growth of the stable photoproducts at 310 nm. This suggested that the intermediates ( I) are the precursors of the corresponding dithioesters, RC( = S)SCH(R)SCH 2R. These correlated processes were related to monophotonic events. However, in the laser flash photolysis experiments in the triphenyl derivatives, there was an additional pathway for the formation of the dithioesters, and this was biphotonic. When the biphotonic formation of products was compensated for, RC( = S)SCH(R)SCH 2R formation quantum yields from steady-state and laser flash photolysis matched within experimental error. The absorption band of ( I) varied systematically with substituents, 320 nm in 1,3,5-trithiane, 340 nm in the 2,4,6-trimethyl derivatives, and 420 nm in the 2,4,6-triphenyl derivatives. The nature of these intermediates ( I) were discussed as resulting from C S bond cleavage, probably heterolytic.

Photolysis of 1-pyrenemethylamine ion-exchanged into a zirconium phosphate framework

The photolysis of the luminescence probe 1-pyremethylamine (PYMA) ion-exchanged into a layered zirconium phosphate (ZrP) framework has been studied. The ZrP used was an hydrated form of α-ZrP with six water molecules per formula unit and with an interlayer distance of 10.3 Å. The luminescence spectrum for a PYMA ion-exchanged in ZrP (PYMA-exchanged ZrP) colloidal suspension in aqueous solution, after irradiation, shows a rapid decrease of the most intense peak at 376 nm and an increase of the peak at 383 nm of the PYMA molecule. The photolysis of a PYMA aqueous solution proceeds slowly to give a luminescence spectrum similar to that observed upon photolysis of PYMA-exchanged ZrP. We have identified pyrene, 1-hydroxypyrene, 1,6-pyrenedione, and 1,8-pyrenedione as stable photoproducts using steady-state luminescence spectroscopy, high performance liquid chromatography, and UV–vis spectrophotometry. The XRPD pattern for PYMA-exchanged ZrP, after photolysis, shows that the photoproducts remain entrapped between the layers. The influence of polarity of the solvent and the presence of air, nitrogen, or oxygen on the formation of the photoproducts have been investigated. Based on the photoproduct identification a reaction mechanism is proposed involving the formation of radicals.

Photochemistry of 4-(methylthio)phenylacetic acid: Steady-state and laser flash photolysis studies

The mechanism for the direct photolysis of 4-(methylthio)phenylacetic acid (4-MTPA) in acetonitrile was investigated using steady-state and laser flash photolysis. A variety of primary stable photoproducts was found under steady-state, 254 nm irradiation of acetonitrile solutions including carbon dioxide (photodecarboxylation), phenylacetic acid, dimethyl disulfide, methyl p-tolyl sulfide, 4-(methylthio)benzaldehyde, 4-(methylthio)benzyl alcohol and secondary stable photoproducts including toluene, benzyl alcohol, benzaldehyde, 1,2-diphenylethane. These stable photoproducts were identified and characterized using HPLC, GC, GC–MS, and UV–Vis methods. Quantum yields were determined for the formation of the various stable products in steady-state irradiations in the absence and in the presence of oxygen. Following laser flash photolysis (266 nm Nd:YAG laser) a variety of transients (e.g., the 4-MTPA triplet state and the sulfur-centered radical cation, H 3C S + C 6H 4 CH 2 COOH) was found. A detailed mechanism of the primary and secondary processes is proposed and discussed. The photoinduced C C bond cleavages are shown to result from an excited-singlet reaction pathway, and the C S bond cleavages follow a triplet pathway. The validity of this proposed mechanism was supported by an analysis of the quantum yields of stable products and their transient precursors. The results from excited-state quenching by oxygen are also consistent with the proposed mechanism.

Generation of Thiyl Radicals by the Photolysis of 5-Iodo-4-thiouridine

The photochemistry of 2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine (3) in deoxygenated 1:1 CH(3)CN-H(2)O pH 5.8 (phosphate buffer) solution has been studied by means of steady-state and nanosecond laser flash photolysis methods. Under steady-state irradiation (lambda > or = 334 nm), the stable photoproducts were iodide ion, 2',3',5'-tri-O-acetyl-4-thiouridine (4), and two disulfides. The disulfides were the symmetrical bis-(2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine) (5) and unsymmetrical 6, which contains both 4-thiouridine and 5-iodo-4-thiouridine residues. The formation of the dehalogenated photoproduct suggests that C(5)-I bond cleavage is a primary photochemical step. Attempts to scavenge the resulting C(5)-centered radical by suitable addends, bis-(N-alpha-acetyl)cystine-bis-N-ethylamide or benzene, were unsuccessful. Analysis of the photoproducts formed under these conditions showed that the S-atom is the reactive center. The photoproduct 4, obtained by irradiation of 3 in CD(3)CN-H(2)O, followed by reversed-phase HPLC isolation using nonlabeled eluents, did not contain deuterium. An analogous experiment performed in CH(3)CN-D(2)O gave deuterated product 4-d with 88% of the deuterium incorporated at C(5). Transient absorption observed upon laser excitation (lambda= 308 nm) of 3 was assigned to the 4-uridinylthiyl radical on the basis of the similarity of this spectrum with that obtained upon laser photolysis of the disulfide: bis-(2',3',5'-tri-O-acetyl-4-thiouridine) 14. On the basis of the results of steady-state and laser photolysis studies, a mechanism of the photochemical reaction of 3 is proposed. The key mechanistic step is a transformation of the C(5)-centered radical formed initially by C(5)-I bond cleavage into a long-lived S-centered radical via a 1,3-hydrogen shift. Theoretical calculations confirmed that the long-lived S-centered radical is the most stable radical derived from the 4-thiouracil residue.

Photochemistry of Ru(etp)(CO)H2 (etp = PhP(CH2CH2PPh2)2): Fast Oxidative Addition and Coordination Following Exclusive Dihydrogen Loss

The photochemistry of Ru(etp)(CO)H2 (1, etp = PhP(CH2CH2PPh2)2) has been studied by UV/vis spectroscopy following nanosecond laser flash photolysis and by NMR and IR spectroscopy following steady-state irradiation. Steady-state irradiation under CO, C2H4, and Et3SiH yields Ru(etp)(CO)2, Ru(etp)(CO)(C2H4), and Ru(etp)(CO)(SiEt3)H, respectively. Laser flash photolysis (laser wavelength 308 nm) of 1 in cyclohexane generates the 16-electron transient Ru(etp)(CO). In the absence of additional ligands, Ru(etp)(CO) decays by reaction with photoejected dihydrogen, regenerating 1. When flash photolysis was performed in the presence of added ligands, the transient decays by pseudo-first-order kinetics with second-order rate constants on the order of 108 dm3 mol-1 s-1. However, the fastest reaction rate (H2) is only a factor of ca. 4 greater than the slowest (Et3SiH). Activation parameters for the reaction of 1 with Et3SiH were determined as ΔG298⧧ = 25.7 ± 0.1 kJ mol-1, ΔH⧧ = 11 ± 1 kJ mol-1, and ΔS⧧ = −49 ± 4 J mol-1 K-1. The evidence from the UV/vis spectrum of the transient and from the structures of the stable photoproducts indicates that Ru(etp)(CO) adopts a nonplanar geometry.

Photo-degradation of bacteriochlorophyll c in intact cells and extracts from Chlorobium tepidum

The photo-stability of bacteriochlorophyll c (Bchlc) in an acetone–methanol–water solution extracted from Chlorobium tepidum and in the chlorosomes of intact Chlorobium tepidum cells is studied. The photo-stability of Bchlc in the chlorosomes is high (quantum yield of photo-degradation φ D≈8×10 −7). In air-saturated acetone–methanol–water solution the photo-degradation of Bchlc is caused by chemical reaction with the dissolved oxygen, where triplet Bchlc produces singlet oxygen, and singlet oxygen reacts with Bchlc to form stable oxidized linear tetrapyrroles as well as intermediate oxidized bacteriochlorophylls. The intermediate photoproducts degrade to linear tetrapyrroles by the catalytic action of triplet oxygen. The initial quantum yield of photo-degradation is direct proportional to the concentration of Bchlc giving φ D,0≈0.011 for a 7×10 −6 M solution at an excitation wavelength of 672 nm. In nitrogen-bubbled solution the photo-degradation is dominated by direct photoproduct formation in the triplet state of Bchlc. Long-wavelength absorbing intermediate photoproducts degrade slowly in the dark to short-wavelength absorbing stable photoproducts. The quantum yield of photo-degradation of a de-oxygenated solution was found to be φ D,0≈0.0012 for excitation at 672 nm.

Bistable UV pigment in the lamprey pineal.

Lower vertebrates can detect UV light with the pineal complex independently of eyes. Electrophysiological studies, together with chromophore extraction analysis, have suggested that the underlying pigment in the lamprey pineal exhibits a bistable nature, that is, reversible photoreaction by UV and visible light, which is never achieved by known UV pigments. Here we addressed the molecular identification of the pineal UV receptor. Our results showed that the long-hypothesized pigment is a lamprey homologue of parapinopsin, which exhibits an absorption maximum at 370 nm, in the UV region. UV light causes cis-trans isomerization of its retinal(2) chromophore, forming a stable photoproduct having an absorption maximum at 515 nm, in the green region. The photoproduct reverts to the original pigment upon visible light absorption, showing photoregeneration of the pigment. In situ hybridization showed that parapinopsin is selectively expressed in the cells located in the dorsal region of the pineal organ. We successfully obtained the hyperpolarizing responses with a maximum sensitivity of approximately 380 nm from the photoreceptor cells at the dorsal region, in which the outer segment was clearly stained with anti-parapinopsin antibody. These results demonstrated that parapinopsin is the pineal UV pigment having photointerconvertible two stable states. The bistable nature of the parapinopsin can account for the photorecovery of the pineal UV sensitivity by background green light in the lamprey. Furthermore, we isolated the parapinopsin homologues from fish and frog pineal complexes that exhibit UV sensitivity, suggesting that parapinopsin is a common molecular basis for pineal UV reception in the vertebrate.

Development of a Test System for Mutagenicity of Photosensitizers Using Drosophila melanogaster¶

ABSTRACTIn the past few years, there has been an increase in the application of photosensitizers for medical purposes. A good standardized test system for the evaluation of the mutagenic potentials of photosensitizers is therefore an indispensable device. In the standard Ames test, white light itself was proven to be mutagenic and the result influenced by the light source. Lack of a reliable positive control is another problem in many genotoxicity test systems used for the evaluation of mutagenicity of photosensitizers. Based on the validated somatic mutation and recombination test, known as SMART, and using Drosophila melanogaster, we developed the Photo‐SMART and demonstrated that methylene blue, known to induce photomutagenicity, can act as a positive control in the presented test system. The SMART scores for the loss of heterozygosity caused predominantly by homologous mitotic recombination. The Photo‐SMART can be used to detect photogenotoxicity caused by short‐lived photoproducts or by stable photoproducts or both. We demonstrated the Photo‐SMART to be a good standardized test system for the evaluation of mutagenic potentials of the photosensitizer 5,10,15‐tris(4‐methylpyridinium)‐20‐phenyl‐[21H,23H]‐porphine trichloride (TPP). We demonstrated that TPP was mutagenic using the Photo‐SMART. For hematoporphyrin, the results of the Photo‐SMART indicate the absence of mutagenicity.