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PubMed2004

Molecular design of the photosystem II light-harvesting antenna: photosynthesis and photoprotection.

The photosystem II (PSII) light-harvesting system carries out two essential functions, the efficient collection of light energy for photosynthesis, and the regulated dissipation of excitation energy in excess of that which can be used. This dual function requires structural and functional flexibility, in which light-harvesting proteins respond to an external signal, the thylakoid DeltapH, to induce feedback control. This process, referred to as non-photochemical quenching (NPQ) depends upon the xanthophyll cycle and the PsbS protein. In nature, NPQ is heterogeneous in terms of kinetics and capacity, and this adapts photosynthetic systems to the specific dynamic features of the light environment. The molecular features of the thylakoid membrane which may enable this flexibility and plasticity are discussed.

Adaptation, Physiological↗

PubMed1992

Transformation of a bop-hop-sop-I-sop-II-Halobacterium halobium mutant to bop+: effects of bacteriorhodopsin photoactivation on cellular proton fluxes and swimming behavior.

We have transformed Pho81, a Halobacterium halobium mutant strain which does not contain any of the four retinylidene proteins known in this species, with the bop gene cluster to create Pho81BR, a BR+HR-SR-I-SR-II-strain. The absorption spectrum, pigment reconstitution process, light-dark adaptation and photochemical reaction cycle of the expressed protein are indistinguishable from those of native bacteriorhodopsin (BR) in purple membrane of wild type strains. Strain Pho81BR permits for the first time characterization of effects of BR photoactivation alone on cell swimming behavior and energetics in the absence of the spectrally similar phototaxis receptor sensory rhodopsin I (SR-I) and electrogenic chloride pump halorhodopsin (HR). A non-adaptive upward shift in spontaneous swimming reversal frequency occurs following 3 s of continuous illumination of Pho81BR cells with green light (550 +/- 20 nm). This effect is abolished by low concentrations of the proton ionophore carbonylcyanide m-chlorophenylhydrazone. Although BR does not mediate phototaxis responses in energized Pho81BR cells under our culture conditions, proton pumping by BR in Pho81BR cells partially deenergized by inhibitors of respiration and adenosine triphosphate synthesis results in a small attractant response. Based on our measurements, we attribute the observed effects of BR photoactivation on swimming behavior to secondary consequences of electrogenic proton pumping on metabolic or signal transduction pathways, rather than to primary sensory signaling such as that mediated by SR-I. Proton extrusion by BR activates gated proton influx ports resulting in net proton uptake in wild-type cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacteriorhodopsins↗

PubMed1991

[A simple setup for measuring scattering in testing the photodynamic effectiveness of dyes].

An experimental setup for the determination of the photodynamic activity of dyes on the basis of scattering measurements is described. With its aid, photochemically induced morphological changes and the process of cell lysis can be registered, and action spectra measured by varying the irradiation wavelength. Different photoproducts of the photosensitizer hematoporphyrin derivative (HPD) were tested in human erythrocyte suspensions. The photodynamic activity of these products was found to depend on absorption behavior, but is lower than that of HPD.

Cells, Cultured↗

PubMedDate not supplied

Photosystem II heterogeneity of in hospite zooxanthellae in scleractinian corals exposed to bleaching conditions.

Increased ocean temperatures are thought to be triggering mass coral bleaching events around the world. The intracellular symbiotic zooxanthellae (genus Symbiodinium) are expelled from the coral host, which is believed to be a response to photosynthetic damage within these symbionts. Several sites of impact have been proposed, and here we probe the functional heterogeneity of Photosystem II (PSII) in three coral species exposed to bleaching conditions. As length of exposure to bleaching conditions (32 degrees C and 350 micromol photons m(-2) s(-1)) increased, the QA- reoxidation kinetics showed a rise in the proportion of inactive PSII centers (PSIIx), where QB was unable to accept electrons. PSIIx contributed up to 20% of the total PSII centers in Pocillopora damicornis, 35% in Acropora nobilis and 14% in Cyphastrea serailia. Changes in Fv/Fm and amplitude of the J step along fast induction curves were found to be highly dependent upon the proportion of PSIIx centers within the total pool of PSII reaction centers. Determination of PSII antenna size revealed that under control conditions in the three coral species up to 60% of PSII centers were lacking peripheral light-harvesting complexes (PSIIbeta). In P. damicornis, the proportion of PSIIbeta increased under bleaching conditions and this could be a photoprotective mechanism in response to excess light. The rapid increases in PSIIx and PSIIbeta observed in these corals under bleaching conditions indicates these physiological processes are involved in the initial photochemical damage to zooxanthellae.

Animals↗

PubMed2004

[Photosynthetic characteristics and photoprotective mechanisms during leaf development of soybean plants grown in the field].

Gas exchange, chlorophyll a fluorescence and HPLC analysis were used to explore photosynthesis and dissipation of excited energy in soybean leaves from emergence to full development. During leaf development, both photosynthetic rate and stomata conductance increased gradually, whereas the increase in stomata conductance significantly lagged behind that of photosynthetic rate. Considering stomatal limiting value, it can be easily deduced that photosynthesis was considerably limited by low stomatal conductance during leaf development. Though the maximum quantum yield of photosystem II (PSII) photochemistry (F(v)/F(m)) was quite high at the stages of leaf development, it is appreciably lower than that in fully developed leaves. Reversible decrease in F(v)/F(m) occurred caused by high irradiance during daily courses at all stages of leaf development, indicating that no severe photoinhibition occurred when exposed to high light. Under high irradiance, a substantial increase in the actual PSII efficiency (Phi(PSII)) together with a marked decreased in non-photochemical quenching (NPQ) occurred during the process of leaf development. Compared with the values obtained at 9:00 AM, Phi(PSII) in developing leaves was drastically down-regulated after midday, whereas NPQ was enhanced significantly. Compared with fully expanded leaves, those developing leaves, with higher xanthophyll pool size, exhibited a much higher ratio of zeaxanthin (Z) + antheraxanthin (A) to Chl under irradiation. In addition, the relative xanthophyll pool size (V+A+Z)/Chl was found to decrease with leaf development. Our experiments revealed that the decline of xanthophyll cycle pool during leaf development was due to the fact that the chlorophyll content increased faster than the xanthophyll cycle pigment content. We suggest that as soon as leaf emerged, photoprotective mechanism was developed preferentially which can effectively protect leaves against excessive irradiance. Thermal dissipation depending on xanthophyll cycle is a very important photoprotective mechanism for dealing with high irradiance during leaf development.

Chlorophyll↗

PubMedDate not supplied

[Effect of erythrocyte membrane structure on the dose dependence of photohemolysis].

The influence of different factors modifying erythrocyte membrane structure on the value of power index of doze dependence of photohemolysis of hematoporphyrin-sensitized erythrocytes has been investigated. It has been established that thermoinduced spectrin aggregation results in increase the index, whereas vesiculation under metabolic exhaustion of erythrocytes greatly decreases value n. A more considerable decrease of the parameter was observed when photohemolysis of erythrocytes was occurred in alkaline medium. It has been shown that the observed decrease of the power index in the latter case is not caused by pH-induced changes of photochemical reactions but by some dark processes. The decrease is possibly brought about by pH-induced conformational changes of protein band 3.

Anion Exchange Protein 1, Erythrocyte↗

PubMed2006

Application of the photo-Fenton process to the treatment of wastewaters contaminated with diesel.

The application of the photo-Fenton process for the treatment of wastewaters contaminated with diesel oil was investigated. This particular process has been widely studied for the photochemical degradation of highly toxic organic pollutants. Experiments were performed according to a factorial experimental design at two levels and two variables: H(2)O(2) concentration (5-200 mM) and Fe(2+) concentration (0.01-1 mM). Experimental results demonstrated that the photo-Fenton process is technically feasible for the treatment of wastewaters containing diesel oil constituents, with total mineralization. A combination of factorial experimental design and gradient descent techniques was employed to optimize the amount of the Fenton reagents, resulting in Fe(2+) (0.1 mM) and H(2)O(2) (50 mM). These optimized levels did not exceed the limit for disposal of ferrous ions (0.27 mM) proposed at the local environmental legislation.

Gasoline↗

PubMed1995

The photoreceptor sensory rhodopsin I as a two-photon-driven proton pump.

Proton translocation experiments with intact cells of Halobacterium salinarium overproducing sensory rhodopsin I (SRI) revealed transport activity of SRI in a two-photon process. The vectoriality of proton translocation depends on pH, being outwardly directed above, and inwardly directed below, pH 5.7. Activation of the transport cycle requires excitation of the initial dark state of SRI, SRI590, to form the intermediate SRI380. Action spectra identify the photocycle intermediates SRI380 and SRI520 as the two photochemically reactive species in the outwardly directed transport process. As shown by flash photolysis experiments, SRI520 undergoes a so-far unknown photochemical reaction to SRI380 with a half-time of <200 micros. Mutation of SRI residue Asp-76, the residue which is equivalent to the proton acceptor Asp-85 in bacteriorhodopsin, to asparagine leads to inactivation of proton translocation. This demonstrates that the underlying mechanisms of proton transport in both retinal proteins share similar features. However, SRI is to our knowledge the first case where photochemical reactions between two thermally unstable photoproducts of a retinal protein constitute a catalytic ion transport cycle.

Journal Article↗

PubMed1950

Delay of cleavage of the Arbacia egg by ultraviolet radiation.

While our data do not permit us to state the exact locus or mode of action of ultraviolet radiation in the Arbacia egg, certain general conclusions may be reached. The amount of delay of cleavage of these eggs is determined by two principal factors: (1) The extent of an effect, resulting from photochemical action induced by ultraviolet radiation, which is reversible in a biological sense, the reversibility not being directly dependent upon the process of cell division. (2) The sensitivity of the cell division process to the effects of the ultraviolet-induced photochemical reaction. This factor varies with the stage of cell division, the cell being insensitive during a period corresponding to most of mitosis. It seems likely that these findings may apply to cell division in general, but, since the quantitative relationships observed must, in this case, reflect the integration of two semi-independent factors, the over-all picture may appear quite different for different kinds of cells.

Animals↗

PubMed1978

[Nature of changes in the fluorescence process of P700-enriched chloroplast fragments].

Spectral and photochemical properties of P700-enriched chloroplast fragments, obtained by ether treatment of liophylized digitonin fragments, were studied. It was shown that time course of fluorescence changes of isolated fragments (in contrast to digitonin fragments) at 20 degrees does not correspond to time course of absorption changes at 700 nm. Differences in low temperature fluorescence spectra of fragments, initially distinguished by redox states of photosystem 1 reaction centers were found. However, the fragments under study were incapable of light-induced changes of fluorescence yield at--196 degrees, independently of spectral region of measured fluorescence (lambda greater than 660 nm or lambda greater than 710 nm), though these fragments reveal phototransformation of P700. Thus changes in the low temperature fluorescence spectra cannot be accounted for by redox changes of P700. The fragments, isolated by ether treatment at 20 degrees as well at--196 degrees do not reveal light-induced fluorescence changes caused by redox changes of P700. The fluorescence changes observed may be due to accessory photoprocesses of chlorophyllprotein complex.

Cell Fractionation↗

PubMed2004

Detection of free nickel monocarbonyl, NiCO: rotational spectrum and structure.

Unsaturated transition metal carbonyls are important in processes such as organometallic synthesis, homogeneous catalysis, and photochemical decomposition of organometallics. In particular, a metal monocarbonyl offers a zeroth-order model for interpreting the chemisorption of a CO molecule on a metal surface in catalytic activation processes. Quite large numbers of theoretical papers have appeared which predict spectroscopic and structural properties of transition metal carbonyls. The nickel monocarbonyl NiCO has been one of the metal carbonyls most extensively studied by the theoretical calculations. At least 50 theoretical studies have been published on this simplest transition metal carbonyl up to the present time. However, experimental evidence of NiCO is much more sparse than theoretical predictions, and the actual structure of NiCO has never been determined by any experimental methods. This Communication reports the first preparation of free nickel monocarbonyl and observation of its rotational transitions. The NiCO molecule was generated by the sputtering reaction of a Ni cathode in the presence of CO. The accurate bond lengths of Ni-C and C-O were experimentally determined from isotopic data and were compared with the theoretical predictions for the first time.

Journal Article↗

PubMed2001

Competition between vibrational relaxation and photochemistry: relevance of vibronic quantum effects.

Two molecules showing photochemistry but no fluorescence have been investigated at 80 K in a rigid matrix regarding the behavior of the quantum yield for bond fragmentation as a function of the vibrational/vibronic level and electronic excited state. A new equation was developed to determine the photochemical quantum yield under ambient conditions (80 K). The levels/bands involved were those within a given vibrational progression, in different progressions as well as in combination. The yield was low (phi = 0.1) with excitation into the n = 0 level of S1 but very rapidly increased with excitation into higher levels whether they were harmonics or combination levels. A parallel result was observed upon excitation into S2. Vibrational relaxation/deactivation occurs only between levels of the same vibrational progression. Deactivation from the 0 level of S2 does not occur via levels of S1. The photochemically active modes correspond to the vibrational modes present in the region of the molecule where bond breakage occurs. These results add further proof of the complex nature and number of processes that can occur within excited states of photochemically active molecules.

Journal Article↗

PubMed1973

Adaptation in the ventral eye of Limulus is functionally independent of the photochemical cycle, membrane potential, and membrane resistance.

The early receptor potential (ERP), membrane potential, membrane resistance, and sensitivity were measured during light and/or dark adaptation in the ventral eye of Limulus. After a bright flash, the ERP amplitude recovered with a time constant of 100 ms, whereas the sensitivity recovered with an initial time constant of 20 s. When a strong adapting light was turned off, the recovery of membrane potential and of membrane resistance had time-courses similar to each other, and both recovered more rapidly than the sensitivity. The receptor depolarization was compared during dark adaptation after strong illumination and during light adaptation with weaker illumination; at equal sensitivities the cell was more depolarized during light adaptation than during dark adaptation. Finally, the waveforms of responses to flashes were compared during dark adaptation after strong illumination and during light adaptation with weaker illumination. At equal sensitivities (equal amplitude responses for identical flashes), the responses during light adaptation had faster time-courses than the responses during dark adaptation. Thus neither the photochemical cycle nor the membrane potential nor the membrane resistance is related to sensitivity changes during dark adaptation in the photoreceptors of the ventral eye. By elimination, these results imply that there are (unknown) intermediate process(es) responsible for adaptation interposed between the photochemical cycle and the electrical properties of the photoreceptor.

Action Potentials↗

PubMed2001

One- vs two-photon processes in the photochemistry of 1,n-dihaloalkanes.

1,n-Dihaloalkanes can be photochemical precursors of n-haloalkyl and -allyl radicals and cations, which are generated via one-photon processes. Time-resolved techniques have provided considerable information on the structures and reactivity of these intermediates. Low-temperature matrix isolation, two-laser two-color, laser-drop, and laser-jet photolysis techniques are powerful tools to photolyze haloalkyl radicals and to generate carbenes or biradicals via two-photon processes.

Journal Article↗

PubMed1999

The photochemical attachment of the O-glucoside of 3-hydroxykynurenine to alpha-crystallin: a model for lenticular aging.

The young human lens contains a small metabolite from tryptophan called the O-glucoside of 3-hydroxykynurenine (3-HKG). Its function is to absorb most radiation between 295 and 400 nm, preventing it from reaching the retina. With age the concentration of this component decreases while the lens crystallins acquire covalently attached chromophores. This study investigates the photochemical attachment of 3-HKG to lens alpha-crystallin. Initial studies showed that alpha-crystallin photolyzed in the presence of 3-HKG developed a fluorescence (emission, 440 nm) and UV-visible spectrum similar to that found in aged human lens proteins. Extensive studies were then performed on the tryptic HPLC maps as monitored by photodiode array and fluorescent detection. Numerous photoproducts with either blue (emission, > 400 nm) or green (emission, > 500 nm) fluorescence were formed in addition to nonfluorescent compounds with absorption maxima above 300 nm. Comparisons were made between these model photoproducts and peptide maps from alpha-crystallin isolated from old human lenses. In terms of retention time and UV-visible spectra at least two of the peptides that appear in the model system are also present in the human samples. It is concluded that one of the aging processes in the human lens is the photochemically induced attachment of 3-HKG to lens proteins.

Aging↗

PubMed2003

Photochemical degradation and mineralization of 4-chlorophenol.

INTENTION, GOAL, SCOPE, BACKGROUND: Since the intermediate products of some compounds can be more toxic and/or refractory than the original compund itself, the development of innovative oxidation technologies which are capable of transforming such compounds into harmless end products, is gaining more importance every day. Advanced oxidation processes are one of these technologies. However, it is necessary to optimize the reaction conditions for these technologies in order to be cost-effective. OBJECTIVE: The main objectives of this study were to see if complete mineralization of 4-chlorophenol with AOPs was possible using low pressure mercury vapour lamps, to make a comparison of different AOPs, to observe the effect of the existence of other ions on degradation efficiency and to optimize reaction conditions. METHODS: In this study, photochemical advanced oxidation processes (AOPs) utilizing the combinations of UV, UV/H2O2 and UV/H2O2/Fe2+ (photo-Fenton process) were investigated in labscale experiments for the degradation and mineralization of 4-chlorophenol. Evaluations were based on the reduction of 4-chlorophenol and total organic carbon. The major parameters investigated were the initial 4-chlorophenol concentration, pH, hydrogen peroxide and iron doses and the effect of the presence of radical scavengers. RESULTS AND DISCUSSION: It was observed that the 4-chlorophenol degradation efficiency decreased with increasing concentration and was independent of the initial solution pH in the UV process. 4-chlorophenol oxidation efficiency for an initial concentration of 100 mgl(-1) was around 89% after 300 min of irradiation in the UV process and no mineralization was achieved. The efficiency increased to > 99% with the UV/H2O2 process in 60 min of irradiation, although mineralization efficiency was still around 75% after 300 min of reaction time. Although the H2O2/4-CP molar ratio was kept constant, increasing initial 4-chlorophenol concentration decreased the treatment efficiency. It was observed that basic pHs were favourable in the UV/H2O2 process. The results showed that the photo-Fenton process was the most effective treatment process under acidic conditions. Complete disappearance of 100 mgl(-1) of 4-chlorophenol was achieved in 2.5 min and almost complete mineralization (96%) was also possible after only 45 min of irradiation. The efficiency was negatively affected from H2O2 in the UV/H2O2 process and Fe2+ in the photo-Fenton process over a certain concentration. The highest negative effect was observed with solutions containing PO4 triple ions. Required reaction times for complete disappearance of 100 mgl(-1) 4-chlorophenol increased from 2.5 min for an ion-free solution to 30 min for solutions containing 100 mgl(-1) PO4 triple ion and from 45 min to more than 240 min for complete mineralization. The photodegradation of 4-chlorophenol was found to follow the first-order law. CONCLUSION: The results of this study showed that UV irradiation alone can degrade 4-CP, although at very slow rates, but cannot mineralize the compound. The addition of hydrogen peroxide to the system, the so-called UV/H2O2 process, significantly enhances the 4-CP degradation rate, but still requires relatively long reaction periods for complete mineralization. The photo-Fenton process, the combination of homogeneous systems of UV/H2O2/Fe2+ compounds, produces the highest photochemical elimination rate of 4-CP and complete mineralization is possible to achieve in quite shorter reaction periods when compared with the UV/H2O2 process. RECOMMENDATIONS AND OUTLOOK: It is more cost effective to use these processes for only purposes such as toxicity reduction, enhancement of biodegradability, decolorization and micropollutant removal. However the most important point is the optimization of the reaction conditions for the process of concern. In such a case, AOPs can be used in combination with a biological treatment systems as a pre- or post treatment unit providing the cheapest treatment option. The AOP applied, for instance, can be used for toxicity reduction and the biological unit for chemical oxygen demand (COD) removal.

Chlorophenols↗

PubMed2003

Photochemical oxidation and autoxidation of chlorophyll phytyl side chain in senescent phytoplanktonic cells: potential sources of several acyclic isoprenoid compounds in the marine environment.

Visible light-induced degradation of the chlorophyll phytyl side chain was studied in senescent cells of two phytoplanktonic strains (Skeletonema costatum and Thalassiosira weissflogii). Particular attention was paid to the induction of autoxidation processes on the phytyl chain and its photoproducts by photochemically produced hydroperoxides. The combination of photochemical oxidation and autoxidation reactions resulted in the production of several acyclic isoprenoid compounds that have been unambiguously identified by comparison of their retention times and mass spectra with those of appropriate standards. Various mechanisms are proposed to explain the formation of these oxidation products. These processes appear to be potential sources of numerous oxidized acyclic isoprenoids that previously have been detected in lacustrine and marine environments. Some oxidation products newly described or whose presence in natural samples was never reported in the literature were then sought in particulate matter, sediment, and microbial mat samples. The results obtained supported the significance of photochemical oxidation and autoxidation of phytoplanktonic chlorophyll phytyl side chain in the marine environment.

Cellular Senescence↗

PubMed2001

Photochemical mutagenesis: examples and toxicological relevance.

Induction of DNA damage as a consequence of exposure to UV light has been established as the major cause of skin cancer. DNA molecules absorb photon energy directly for wavelengths <320 nm, and lead to well-characterized mutagenic DNA damage. Alternatively, endogenous or exogenous chemicals (sensitizers) may absorb light with the potential of subsequent energy or electron transfer, and lead indirectly to DNA damage. A few light-absorbing pharmaceuticals have long been known to cause photo(geno)toxic effects. Notably, psoralen and chlorpromazine derivatives have been established as photomutagens and the reaction mechanisms have been identified; the fluoroquinolone antibiotics have more recently been recognized as being photomutagenic. The type of DNA damage and the modulation by antioxidants indicate the involvement of reactive oxygen species (ROS), but other mechanisms are also reported for, at least, some derivatives. In routine genotoxicity studies, we observed the photomutagenic activity of a compound (Ro 19-8022) under development as an anxiolytic agent in the Ames tester strain TA102 under normal laboratory illumination conditions. Further investigations showed strong photogenotoxic activity in tests for gene mutations and chromosomal aberrations in mammalian cells. The finding led to the termination of drug development. Another example of a pharmaceutical for which photogenotoxic properties were observed during development is Ro 47-7737, a bisquinoline derivative of the antimalaria compound chloroquine. Also in this case, the photochemical reactivity contributed to the termination of the development process. The risk/benefit assessment for the described compounds has to take into account the human exposure situation, for example, the ability to avoid light exposure during treatment. Consideration of photochemical mutagenesis is specifically important for sunscreen ingredients. The active components of sunscreen lotions are efficient UV absorbers. Consequently, they reduce the genotoxicity of UV light and thus may be considered antimutagens. However, photodegradation to reactive molecules or energy transfer to DNA is possible, in principle, as has been reported for para-aminobenzoic acid (PABA).

Antioxidants↗