The plant photosensitizer alpha-terthienyl (αT) is toxic toward a variety of organisms, and normally requires exposure to ultraviolet-A radiation for activation and singlet molecular oxygen formation. However, some toxicity has also been reported to occur in the dark. One hypothesis that has been proposed to account for this light-independent toxicity is that the sensitizer becomes activated by energy transfer from the excited-state products of enzymatic reactions. We have investigated this hypothesis using the horseradish peroxidase (HRP)-catalyzed oxidation of indole-3-acetic acid (IAA), which generates indole-3-aldehyde in an excited triplet state. Light is emitted during the IAA/HRP reaction at acidic pH, is increased by inclusion of αT and is not observed with heat-denatured HRP. The rates of both the oxidation of IAA and the subsidence of light emission are more rapid in the IAA/αT/HRP system than with IAA and HRP alone, indicating that the presence of αT accelerates the reaction. Bleaching occurs at the wavelength of maximal αT absorbance and is promoted by the inclusion of IAA. Readdition of both IAA and αT to a spent reaction mixture is required to restore light emission after it has subsided, further suggesting that both are consumed in the reaction. We were unable to detect measurable quantities of singlet molecular oxygen formation in this system. These results do not support the energy transfer hypothesis, but instead are more compatible with a model proposed by Krylov and Chebotareva [Krylov, S. N. and A. B. Chebotareva (1993) FEBS Lett. 324, 6–8] for the co-oxidation of IAA and xanthene dyes.

Singlet-oxygen quenching constants were measured for 19 cyanine dyes in acetonitrile. The most efficient quenchers were 1-butyl-2-[2-[3-[(1-butyl-6-chlorobenz[cd]indol-2(1H)-ylidene)ethylidene]-2-chloro-1-cyclohexen-1-yl]ethenyl]-6-chlorobenz[cd]indolium and 6-chloro-2-[2-[3-(6-chloro-1-ethylbenz[cd]indol-2(1H)-ylidene)ethylidene]-2-phenyl-1-cyclopenten-1-yl]ethenyl]-1-ethylbenz[cd]indolium, having quenching constants with diffusion-controlled values of 2.0 ± 0.1 × 10¹⁰ and 1.5 ± 0.1 × 10¹⁰ M⁻¹ s⁻¹, respectively. There was a trend toward increased quenching constants for cyanine dyes with the absorption band maxima at longer wavelengths. However, the quenching constants correlated better with the oxidation potentials of the cyanine dyes, suggesting that quenching proceeds by charge transfer rather than energy transfer. The quenching constants for 1,1′,3,3,3′,3′-hexamethylindotricarbocyanine perchlorate and 1,1′-diethyl-4,4′-carbocyanine iodide were measured in several solvents as well as in aqueous solutions of detergent micelles. In different solvents, the quenching constants varied by as much as a factor of 50. The quenching constants were largest in solvents with the highest values on the π^* scale of Kamlet, Abboud, Abraham and Taft. This was consistent with quenching occurring by charge transfer. Within cells, cyanine dyes concentrate in membrane-bound organelles. The quenching constants were substantial within detergent micelles. To the extent that micelles are models for biological membranes, cyanine dyes may be effective biological singlet-oxygen quenchers.

Irradiation of the red-light absorbing dye, methylene blue (MB), in the presence of the metal complex, cis-Rh(phen)₂Cl₂ (BISPHEN), leads to irreversible photobinding of both reagents to DNA. Evidence from absorption and emission spectroscopy indicates that the dye is strongly complexed to the DNA at the concentrations used in the experiments and that this complex is unaffected by the presence of BISPHEN. The level of covalent binding is proportional to the absorbed light dose, with the quantum efficiency for covalent binding of BISPHEN to the DNA with 633 nm light equal to 3.5 × 10⁻⁴. Electrospray ionization mass spectrum of a mixture of DNA fragments created by enzymatic degradation of DNA isolated following irradiation indicates that purine adducts are formed with both BISPHEN and the dye. In addition, UV–Vis and high-performance liquid chromatography analyses of the irradiated MB/BISPHEN/DNA mixture and isolated adducts show extensive conversion of the dye and metal complex to the corresponding N-demethylated and aquated derivatives, respectively. Triplet quenchers for MB, for example oxygen and benzoquinone, inhibit both the photoconjugation and the photochemistry of BISPHEN. A mechanism for the synergistic interaction is proposed that involves photoconjugation of both partners to the DNA following oxidation and reduction via electron transfer between ¹MB^*/DNA and ³MB^*/BISPHEN.

The composite fluorescence emission from the two tryptophans (W₇ and W₁₄) of horse heart apomyoglobin was explored by fluorescence quenching experiments. The fluorescence of the W₇ residue is the only one involved in the quenching by iodide or trichloroethanol (TCE) titration. The fluorescence contribution of W₇ is 49% of the total apomyoglobin emission, and its spectrum is red-shifted compared to the W₁₄ emission. The fluorescence decay of Trp residues gives an average fluorescence lifetime of 2.06 ns for W₁₄ and 2.84 ns for W₇. The static fluorescence quenching by TCE was used to monitor the individual motions of the two tryptophans in apomyoglobin. The short correlation time of W₇ (ρ = 3 ns) explains why this residue can experience various environments without having to assume the existence of several protein conformations occurring during its lifetime emission.

α,ω-Diphenylpolyenes have attracted a great deal of attention as models of retinyl polyenes that are related to natural photoreceptors involved in energy and sensory phototransductions. Of particular interest have been the topics of their excited state electronic structure and spectroscopic properties. However, the exact nature of the lowest excited state in terms of their structure and energetics is not clearly known. Examination of the photophysics and photochemistry of donor–acceptor diphenylpolyenes can aid in understanding the excited states and photoprocesses of linear polyenes. In this paper are described the absorption, fluorescence and photoisomerization studies of donor–acceptor diarylbutadienes, namely: p-(N,N-dimethylamino)-p′-cyano-1,4-diphenylbuta-1E,3E-diene (1), p-(N,N-dimethylamino)-p′-nitro-1,4-diphenylbuta-1E,3E-diene (2), p-(N,N-dimethylamino)-m′-nitro-1,4-diphenylbuta-1E,3E-diene (3), p-(N,N-dimethylamino)-o′-nitro-1,4-diphenylbuta-1E,3E-diene (4). Absorption properties are affected as expected due to mesomeric stabilization by the substituent; however, solvent polarity does not significantly affect the absorption properties of these dienes. In contrast, a pronounced solvatochromic fluorescence behavior of these dienes in organic solvents is observed. Time-resolved fluorescence is characterized by a single exponential fluorescence decay with generally increasing lifetime in polar solvents. The fluorescence quantum yields are very low, particularly in polar solvents, but do not show any clear trend. Irradiation of 1E,3E- 1–4 in organic solvents yields the corresponding 1E,3Z-isomer due to one-photon–one-bond isomerization of the C=C double bond lying closer to the acceptor group. The photoisomerization also depended on the solvent polarity and on the concentration of diene. The photoisomerization efficiency of dienes 1 and 2 under direct irradiation condition is greater than dienes 3 and 4. In comparison to the efficiency of photoisomerization under direct irradiation condition, the photosensitized isomerization efficiency is much less, particularly for dienes 1 and 2. The results are discussed in terms of the involvement of excited-state intramolecular charge transfer and conformationally relaxed polar excited states in the photoprocesses of linear polyenes.

The results of a study of the effect of pH on the photophysics and photochemistry of di-sulphonated aluminum phthalocyanine (AlPcS₂) in aqueous solution are presented. The pH dependence of the triplet quantum yield, fluorescence quantum yield, singlet-oxygen quantum yield, triplet lifetime, fluorescence lifetime and apparent dimerization constants is investigated and the results interpreted in terms of the pH dependence of the nature of the axial ligands. Evidence that the aluminum–axial ligand bond strength, rather than dimer binding energy that determines the extent of dimerization is provided by semi-empirical and ab initio calculations. Possible dimer structures obtained using ab initio calculations are discussed.

The photophysical properties of Piroxicam, a nonsteroidal anti-inflammatory drug (NSAID), were investigated at different pH_ext values in reversed micelles of Aerosol-OT (AOT) in iso-octane, using both steady-state and picosecond time-resolved fluorescence spectroscopy. In contrast with the very complex data obtained in aqueous media, where several prototropic species are in equilibrium, the reversed micellar system essentially favors two species. The absorption spectra shows only one isosbestic point at λ = 348 nm. Excited-state intramolecular proton transfer (ESIPT), also detected in water, is promoted at low water pool contents measured by ω₀ = [H₂O]/[AOT]. A strongly shifted (λ_em = 470 nm) tautomeric emission is found. Upon the gradual increase of ω₀, striking differences with pH_ext are found. At pH_ext = 4, the drug preferentially locates itself in the interfacial region partitioning between a hydrophobic and a hydrophilic domain. Global analysis was applied to the decay data and the results were interpreted by the “two-state excited-state” formalism. At pH_ext = 7, the anionic species is prevalent and the probe locates itself deeper inside the water core of the reversed micelles. Thus, a strong dependence on water content is detected, approaching a behavior similar to that observed in free aqueous solutions.

Several cyanine dyes were found to protect K562 leukemia cells against toxicity mediated by cis-di(4-sulfonatophenyl)diphenylporphine (TPPS₂) and light. Most cyanine dyes derived from dimethylindole were better photoprotectors than cyanine dyes with other structures. This correlated with the fact that cyanine dyes derived from dimethylindole were predominately monomeric at millimolar concentrations within K562 cells, while other cyanine dyes formed aggregates. For cyanine dyes that are derived from dimethylindole and have absorption band wavelengths greater than 700 nm, fluorescence-energy transfer from TPPS₂ to the cyanine dye was the most important mechanism for photoprotection. There was no spectroscopic evidence for complex formation between the cyanine dyes and TPPS₂. The dimethylindole derivative, 1,1′,3,3,3′,3′-hexamethylindodicarbocyanine, was an excellent photoprotector, but a poor quencher of TPPS₂ fluorescence and a relatively poor singlet-oxygen quencher. This cyanine dye may act by quenching excited triplet TPPS₂. Singlet-oxygen quenching may contribute to the photoprotection provided by cyanine dyes not derived from dimethylindole. Differences in the subcellular distribution of the various cyanine dyes studied may have contributed to the different apparent mechanisms of photoprotection.

The alkaline and neutral comet assays have been widely used to assess DNA damage and repair in individual cells after in vivo or in vitro exposure to chemical or physical genotoxins. Cells processed under neutral conditions generate comets primarily from DNA double strand breaks, whereas under alkaline conditions, comets arise from DNA single and double strand breaks and alkali-labile lesions. A modified version of the alkaline comet assay, as described here, used silver stain to visualize the comets and a ^™Gelbond base to facilitate the manipulation and processing of samples. To demonstrate how these modifications improve the assay, fibroblasts derived from both normal and Xeroderma pigmentosum (Xp) individuals were exposed to simulated solar radiation and the resulting DNA damage and repair evaluated and compared with results from the relevant literature. Comets from normal fibroblasts reached their maximum length at about an hour after irradiation. Dose-dependent increases in comet length were observed up to at least 360 mJ/cm². In contrast, comet lengths from repair deficient Xp fibroblasts were shorter than normal cells reflecting their reduced capacity to generate single strand breaks by the excision of DNA dimers. For incubation times of more than 1 h, comet lengths from normal fibroblasts underwent a time-dependent decrease, supporting the contention that this change was related to the ligation step in the DNA repair process. These changes were compatible with the model of DNA damage and repair established by others for ultraviolet radiation.

The self-tanning agent dihydroxyacetone (DHA) was applied to human skin samples, and its effect on light absorption was followed in time to study the DHA influence inside the different layers of skin. Application of DHA shows increased light absorption in the visible light region, as evidenced by skin tanning. The tanning effect is enhanced by UV irradiation and appears localized in the near-stratum corneum layer as revealed by depth analysis of the time signal. As a reference, application of an emulsion containing titanium dioxide particles shows clear surface stability and a screening of light penetration beyond the stratum corneum.

Ultraviolet (UV) sunlight actinometers were developed based on the photolysis of nitrate and nitrite. Photon exposures (i.e. time-integrated irradiances) were quantified from the photochemical production of salicylic acid (SA) or p-hydroxybenzoic acid (pHBA) formed by the reaction of the hydroxyl radical with benzoic acid. The solar response bandwidth for the nitrate actinometer in quartz tubing was 322 ± 11 nm during the Spring of 1999, while the response bandwidth of the Mylar D-filtered nitrite actinometer was 355 ± 25 nm. Intercomparisons of the nitrate and nitrite actinometers with a Smithsonian Environmental Research Center SR-18 scanning UV-B radiometer (SERC SR-18) and an Optronics OL-754 spectroradiometer (OL-754) were performed during the summer of 1998, and the winter and spring of 1999. Photon exposures determined by the nitrate actinometer were in excellent agreement with the SERC SR-18, with a slope (95% confidence interval [CI]) of 0.98 ± 0.01 based on SA production and 0.94 ± 0.02 based on pHBA production. Excellent agreement was also found between the nitrite actinometer and the OL-754, with a slope (95% CI) of 1.00 ± 0.01 using SA production and 1.00 ± 0.02 using pHBA production. These actinometers are well suited for use in the water column and are sufficiently sensitive to determine photon exposures below the 0.1% UV light-level.

Light-dependent oxygen uptake was observed and studied in thylakoids from early greening barley in comparison to oxygen uptake in chlorophyll solutions and in thylakoids from fully green leaves. Substantial oxygen uptake was observed in chlorophyll solutions supplemented with tryptophan, histidine, ascorbic acid or linoleic acid. This uptake was diminished by adding azide, β-carotene and α-tocopherol, which are specific singlet-oxygen quenchers. Illuminated thylakoids from greening barley also exhibited marked oxygen uptake that, likewise, was strongly quenched by azide. In comparison, azide was found not to affect oxygen uptake that is associated with the methyl viologen-catalyzed Mehler reaction. It is reasoned that in the first two cases the oxygen uptake arises from chlorophyll-photosensitized activation of oxygen to the singlet state and its consumption by exogenous or endogenous substrates. In greening, we propose that disorganized chlorophyll photosensitizes the oxygen uptake.

Synthesis of δ-aminolevulinic acid (ALA) derivatives is a promising way to improve the therapeutic properties of ALA, particularly cell uptake or homogeneity of protoporphyrin IX (PpIX) synthesis. The fluorescence emission kinetics and phototoxic properties of ALA-n-pentyl ester (E1) and R,S-ALA-2-(hydroxymethyl) tetrahydrofuranyl ester (E2) were compared with those of ALA and assessed on C6 glioma cells. ALA (100 μg/mL), E1 and E2 (10 μg/mL) induced similar PpIX-fluorescence kinetics (maximum between 5 and 7 h incubation), fluorescence being limited to the cytoplasm. The 50% lethal dose occurred after 6 h with 45, 4 and 8 μg/mL of ALA, E1 and E2, respectively. ALA, E1 and E2 induced no dark toxicity when drugs were removed after 5 min of incubation. However, light (25 J/cm²) applied 6 h after 5 min incubation with 168 μg/mL of each compound induced 85% survival with ALA, 27% with E1 and 41% with E2. Increasing the incubation time with ALA, E1 and E2 before washing increased the phototoxicity, but E1 and E2 remained more efficient than ALA, regardless of incubation time. ALA-esters were more efficient than ALA in inducing phototoxicity after short incubation times, probably through an increase of the amount of PpIX synthesized by C6 cells.

Ultraviolet (UV) radiation is a potent activator of human immunodeficiency virus (HIV) gene expression in a HeLa cell clone having stably integrated copies of an HIVcat (cat gene under control of the HIV promoter) reporter construct, whereas ionizing radiation is ineffective. UV-activated HIV gene expression is completely blocked by the specific p38 mitogen-activated protein (MAP) kinase inhibitor SB203580 and by expression of a kinase-inactive p38 mutant that interferes with normal p38 function, suggesting that this stress-activated protein kinase plays an important role in UV-mediated transcriptional activation of HIV. In support of these findings, we show here that Western blot analysis demonstrated rapid and significant activation of p38 MAP kinase by UV. On the other hand, γ-radiation activated p38 MAP kinase very poorly in HeLa cells at both low and high doses at times (5–30 min) when UV radiation was effective. UV radiation also activated HIV gene expression (≤9-fold) in 1G5 Jurkat T-cells stably transfected with a luciferase reporter gene under control of the HIV promoter. In these cells, γ-radiation stimulated HIV gene expression but to a lesser extent (≤3-fold) and with different kinetics than after UV radiation, and this response was obliterated by the incubation of cells with the mitogen-activated protein kinase/Erk kinase (MEK)-½ inhibitor PD98059. This result suggests that in these cells signaling in response to γ-radiation is transduced through the MEK-½/p42/44 MAP kinase pathway to increase HIV gene expression. All combined, these results suggest that activation of p38 MAP kinase is necessary for efficient HIV gene expression triggered by DNA damaging agents, and, in a cell type-specific manner, activation of the MEK-½/p42/44 MAP kinase pathway is important for triggering a response to γ-radiation. Thus, it appears as if UV signaling leading to HIV gene expression requires the p38 MAP kinase pathway whereas activation by γ-radiation requires the MEK-½/p42/44 MAP kinase pathway.

Sulfonated meso-tetraphenylporphyrins of different hydrophilicity were microspectrofluorimetrically examined in endothelial cells using total internal reflection (TIR) illumination or epi-illumination. Since the penetration depth of the evanescent field during TIR illumination is limited to a few hundred nanometers, photosensitizers were almost selectively examined in close vicinity to the plasma membrane. Pronounced fluorescence signals during TIR illumination were observed for the hydrophilic compounds meso-tetraphenylporphyrin tetrasulfonate (TPPS₄) and meso-tetraphenylporphyrin trisulfonate (TPPS₃), whereas the more lipophilic compounds meso-tetraphenylporphyrin disulfonate (TPPS_2a) and meso-tetraphenylporphyrin monosulfonate (TPPS₁) could only be detected under epi-illumination. Irradiation of TPPS₁ and TPPS_2a in the Soret band led to an increase in fluorescence intensity and formation of a photoproduct with an emission maximum around 610 nm, which was limited to intracellular compartments. In contrast, fluorescence spectra of TPPS₃ and TPPS₄ obtained by TIR and epi-illumination remained almost unchanged after irradiation in the Soret band. Extralysosomal location of TPPS₃ and TPPS₄ in close proximity to the plasma membrane was deduced from experiments with the lysosomal markers acridine orange (AO) or lysotracker yellow (LY), which were not detectable under TIR illumination. In conclusion, these results provide for the first time direct evidence for a plasma membrane-associated fraction of the hydrophilic compounds TPPS₃ and TPPS₄ in living cells.

A link between bone mineral density and skin color has been reported recently, and pigmentation has been shown to affect cutaneous vitamin D production. In the present study, we investigated the relationship between phototype, global self-assessed sun exposure, geographical location and vitamin D serum levels in 1191 French adults. When the factors were analyzed separately, individuals with lower phototypes as well as those with lower sun exposure showed significantly lower levels of vitamin D than those with darker phototypes or those with higher sun exposure. However, when factors were analyzed as a whole, the vitamin D status was no longer linked with the phototype, but with sun exposure and geographical location. Since phototypes and global self-assessments of sun exposure were positively linked, our data suggest that lower vitamin D levels in fair-skinned individuals are due to their sun exposure behavior.

α-Crystallin, a major protein of the mammalian lens, plays a vital role in maintaining the structural stability and transparency of the lens. It performs this function through chaperone-like activity; it has recently been reported that heating α-crystallin enhances this ability. The present studies, using both time-resolved and steady-state fluorescence methods, were carried out to compare the conformational changes that result from heating with those that result from increasing protein concentration (up to 70 mg/mL). The relative fluorescence quantum yield from tryptophan (Trp) present in α-crystallin increases and then decreases with a concomitant shift of the emission maximum to longer wavelengths when either heating times or protein concentrations are increased. The time profile of fluorescence decay was resolved into three components with lifetimes of ca 0.5, 3 and 7 ns and emission maxima of ca 340, 342 and 350 nm, respectively. With longer heating time or increasing concentrations the contribution from the longer-lived component increases at the expense of the shorter-lived species. These data indicate that with heating or at higher concentrations the internal Trp residues move to the surface of the protein giving a more hydrophobic exterior and possibly explain the reported increased chaperone activity upon heating. As a result of the concentration studies, α-crystallin may be more efficient in its chaperone activity in vivo than has been determined by in vitro experiments.

This paper investigates the fast photocurrent components, B1 and B2, from oriented bacteriorhodopsin (BR) membrane films at low pH, under pulsed laser excitation. Adding chloride ion changes the acid-blue BR to its acid-purple form. In the presence of chloride, the acid-purple BR shows a positive B2 component in the same direction as that of BR at neutral pH, indicating a rapid intramolecular charge transfer. In the absence of chloride, the acid-blue BR shows only a negative B1 with multi-components, indicating a rapid charge separation process associated with retinal photoisomerization. The multi-components in B1 are possibly formed due to the heterogeneity of the acid-blue BR. In addition, BR mutants, D85N and D115N, at low pH and in the presence of chloride, generate the B2 component as well. The observation of chloride-dependent B2 component in various cases at low pH, is in favor of a possible transient chloride ion transfer, although the nature of the charge being transferred cannot be identified so far.

The plant receptor phytochrome A (phyA) mediates responses like hypocotyl growth inhibition and cotyledon unfolding that require continuous far-red (FR) light for maximum expression (high-irradiance responses, HIR), and responses like seed germination that can be induced by a single pulse of FR (very-low-fluence responses, VLFR). It is not known whether this duality results from either phyA interaction with different end-point processes or from the intrinsic properties of phyA activity. Etiolated seedlings of Arabidopsis thaliana were exposed to pulses of FR (3 min) separated by dark intervals of different duration. Hypocotyl-growth inhibition and cotyledon unfolding showed two phases. The first phase (VLFR) between 0.17 and 0.5 pulses·h⁻¹, a plateau between 0.5 and 2 pulses·h⁻¹ and a second phase (HIR) at higher frequencies. Reciprocity between fluence rate and duration of FR was observed within phases, not between phases. The fluence rate for half the maximum effect was 0.1 and 3 μmol·m⁻²·s⁻¹ for hourly pulses of FR (VLFR) and continuous FR (HIR), respectively. Overexpression of phytochrome B caused dominant negative suppression under continuous but not under hourly FR. We conclude that phyA is intrinsically able to initiate two discrete photoresponses even when a single end-point process is considered.

This work studied the effect of light-stressors, irradiance and photoperiod length on the status of hemolymph glutathione in two species of crayfish, Procambarus clarkii and Procambarus digueti. Adult animals of each species were submitted to two experimental approaches: (1) two batches of each species were placed under low or high light irradiant conditions of light–dark (LD) 24 h cycles of two different photoperiod lengths, one normal LD 12:12 and one extreme LD 20:4 low and high irradiance for 10 weeks. Time-dependent light changes on hemolymph glutathione concentration were determined throughout the entire experimental period; and (2) three batches of the two species were submitted to independent treatments consisting of the same LD 12:12 cycles of high and low irradiance and 20:4 high-irradiance LD cycles. Reduced and oxidized glutathione hemolymph concentrations were determined and total glutathione was calculated. In addition midgut glutathione reductase activity in both species was determined. The two species showed different hemolymph glutathione reactivity and glutathione status for the two light parameters. Dissimilar responses of both species, as well as the rate of mortality of P. digueti represent specific differences in the metabolic responses, as well as tolerance to photo-oxidative stress produced by light. The role of glutathione in the tolerance of crayfish to photo-oxidative stress is discussed.

We present evidence for the presence and nature of a UVB-specific photoreceptor in the cyanobacterium Chlorogloeopsis PCC 6912. The photoreceptor mediates at least the photosensory induction of mycosporine-like amino acid (MAA) synthesis. Because MAA synthesis in this organism can also be induced under salt stress, we could distinguish between the photosensory and the purely biochemical requirements of MAA synthesis. Neither visible light nor UV radiation was necessary for the biosynthetic process, thus indicating that the UVB (280–320 nm) dependence of biosynthesis is based on a UV photosensory capacity of the organism. An action spectrum of the MAA synthesis showed a distinct peak at 310 nm tailing down into the UVA (320–400 nm) region with no detected activity above 340 nm. We found that radiation below 300 nm caused significant inhibition of synthesis of MAAs indicating that the action spectrum at these wavelengths may not have been satisfactorily resolved. We propose that a pterin is a good candidate for a photoreceptor chromophore as (1) reduced pterins present absorption spectra congruent with the action spectrum obtained; and (2) an inhibitor of the biosynthetic pathway of pterins and an antagonist of excited states of pterins, both depressed the photosensory efficiency of induction but not its chemosensory efficiency.