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Results for “Photochemical Processes”

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At least 955 records · Page 53Linked to original sources

PubMed2003

Singlet oxygen-induced signaling effects in mammalian cells.

Singlet oxygen, an electronically excited form of molecular oxygen, may be generated photochemically or in dark reactions in vivo. Singlet oxygen is not only toxic to cells and impairs signaling events but is also capable of eliciting a cellular stress response. The signaling processes initiated in this response include the activation of mitogen-activated protein kinases. Two possible activation mechanisms of signaling pathways by singlet oxygen are the generation of positive regulators as well as the inactivation of negative regulators.

Animals↗

PubMed1982

Mechanism of photoactivation of electron transport in intact bryopsis chloroplasts.

The mechanism of photoactivation of photosystem I electron transport was studied in intact Bryopsis corticulans chloroplasts. The evidence from chemical and photochemical studies suggests that photoactivation is a consequence of a reduction of an electron transport component, presumably ferredoxin-NADP(+) reductase. O(2) does not act as a mediator of the process but rather acts as an electron acceptor after photoactivation has occurred. We suggest that the initial function of the chloroplasts in a transition from dark to light is to initiate pseudocyclic electron flow.

Journal Article↗

PubMed2005

Ozone foliar symptoms in woody plant species assessed with ultrastructural and fluorescence analysis.

This paper compares the responses to ozone in five woody species: Fagus sylvatica (FS), Acer pseudoplatanus (AP), Fraxinus excelsior (FE), Viburnum lantana (VL) and Ailanthus altissima (AA). The hypothesis being tested was that the strategies that plants adopt to resist oxidative pressure are species-specific. The study was carried out on field grown plants in an area in Northern Italy characterized by elevated levels of ozone pollution. The observations were made both at ultrastructural (using light and electronic microscopy) and physiological (using chlorophyll a transient fluorescence and microspectral fluorometry) level. Common responses were: the hypersensitive response (i.e. the death of palisade mesophyll cells) and the formation of callose layers separating injured from healthy cells. FS and AP were capable of thickening the palisade mesophyll cell walls. This thickening process involved changes in cell wall chemical structure, evidenced by the accumulation of yellow autofluorescence compounds. Species-specific behaviours were observed with the fluorescence analysis, with special reference to the photochemical de-excitation constant (Kp). This value increased in FE and AP, and decreased in AA. The observed responses are interpreted as adaptative strategies against the ozone stress. The increase of Kp indicates that the reaction centres were working as more effective quenchers.

Chlorophyll↗

PubMed2001

Focal ischemia in the cerebral cortex has an effect on the neurohypophysis. II. Angiogenesis in the neurohypophysis is a consequence of the focal ischemia in the cerebral cortex.

OBJECTIVES: Focal ischemia in the cerebral cortex has an effect on neurohypophysis. The morphological changes of microvessels of neurohypophysis were evaluated in a model of the cerebral infarction initiated by a photochemical reaction in the cerebral cortex. After photochemically induced platelet aggregation, we observed the morphological features of angiogenesis. METHOD: The model of photochemically-induced cerebral ischemia was used. Seven days after intravenous injection of rose bengal and irradiation from a halogen lamp source through an intact cranium, the sampled material from neurohypophysis is processed for transmission electron microscopy using standard procedures. RESULTS: We observed morphological features of the new vessel formation: the alterations in the endothelium and extracellular matrix during separation of the endothelial cell from each other in a "mother" vessel, the migration of the endothelial cells in the extracellular matrix, the communication of the lumen of the new and the "mother" vessels. CONCLUSION: We observed development of the angiogenic phenotype in the neurohypophysis after focal ischemia in the cerebral cortex. The endothelium, basement membrane and extracellular matrix undergo morphological alterations which lead to new blood vessel formation.

Animals↗

PubMed1997

[Photostability of antifungal agents. 2. Photostability of polyene antibiotics].

The polyene antibiotics amphotericin B, natamycin and nystatin are rapidly degraded under the influence of light. Amphotericin B as a heptaene has a markedly higher photostability than the tetraenes natamycin and nystatin. With a new HPLC method for the separation of the different polyene components we could perform a quantitative analysis of the photodegradation process. Initially degradation products with unchanged spectrophotometric absorption spectra are formed from each of the three polyene antibiotics. Longer light exposure leads to the subsequent degradation of the polyene structures. This photochemical instability was also detected in various drug products with polyene antibiotics as active ingredient. On direct sun exposure of topically applied polyene antibiotics a pronounced photodeactivation of the drug substances is expected.

Amphotericin B↗

PubMed1997

Photochemical and photobiological studies with acridine and phenanthridine hydroperoxides in cell-free DNA.

The acridine and phenanthridine hydroperoxides 3 and 7 were synthesized as photochemical hydroxyl radical sources for oxidative DNA damage studies. The generation of hydroxyl radicals upon UVA irradiation (lambda = 350 nm) was verified by trapping experiments with 5,5-dimethyl-1-pyrroline N-oxide and benzene. The enzymatic assays of the damage in cell-free DNA from bacteriophage PM2 caused by the acridine and phenanthridine hydroperoxides 3 and 7 under near-UVA irradiation revealed a wide range of DNA modifications. Particularly, extensive single-strand break formation and DNA base modifications sensitive to formamidopyrimidine DNA glycosylase (Fpg protein) were observed. In the photooxidation of calf thymus DNA, up to 0.69 +/- 0.03% 8-oxo-7,8-dihydroguanine was formed by the hydroperoxides 3 and 7 on irradiation, whose yield was reduced up to 40% in the presence of the hydroxyl radical scavengers mannitol and tert-butanol. The acridine and phenanthridine hydroperoxides 3 and 7 also induce DNA damage through the type I photooxidation process, for which photoinduced electron transfer from 2'-deoxyguanosine to the singlet states of 3 and 7 was estimated by the Rehm-Weller equation to possess a negative Gibb's free energy of ca -5 kcal/ mol. Control experiments with the sensitizers acridine 1 and the acridine alcohol 4 in calf thymus and PM2 DNA confirmed the photosensitizing propensity of the UVA-absorbing chromophores. The present study emphasizes that for the development of selective and efficient photochemical hydroxyl radical sources, chromophores with low photosensitizing ability must be chosen to avoid type I and type II photooxidation processes.

Cell-Free System↗

PubMed1997

Toxic oxidant species and their impact on the pulmonary surfactant system.

In this review the effects of oxidant inhalation on the pulmonary surfactant system of laboratory animals are discussed. Oxidant lung injury is a complex phenomenon with many aspects. Inhaled oxidants interact primarily with the epithelial lining fluid (ELF), a thin layer covering the epithelial cells of the lung which contains surfactant and antioxidants. In the upper airways this layer is thick and contains high levels of antioxidants. Therefore oxidant injury in this area is rare and is more common in the lower airways where the ELF is thin and contains fewer antioxidants. In the ELF oxidants can react with antioxidants or biomolecules, resulting in inactivation of the biomolecules or in the formation of even more reactive agents. Oxidation of extracellular surfactant constituents may impair its function and affect breathing. Oxidized ELF constituents may promote inflammation and edema, which will impair the surfactant system further. Animal species differences in respiratory tract anatomy, ventilatory rate, and antioxidant levels influence susceptibility to oxidants. The oxidant exposure dose dictates injury, subsequent repair processes, and tolerance induction.

Animals↗

PubMed1999

Relative roles of convection and chemical reaction for the disposition of formaldehyde and ozone in nasal mucus.

The mucociliary apparatus is an important respiratory-tract defense system that may provide significant protection of the underlying epithelium from gases and vapors. Limiting-case calculations were performed to determine the significance of convective mucus transport and chemical reaction for formaldehyde (HCHO) and ozone (O(3)) in rat nasal respiratory epithelial mucus. Less than 4.6% of absorbed HCHO can be bound to amino groups (serum albumin) after 20 min of exposure. Thus, at the slowest measured mucus flow rates in rats, approximately 1 mm/min, a fluid element of mucus could travel more than 2 cm before binding 5% of absorbed HCHO, by which time the element would probably leave the nose (the site of toxic responses). In other calculations, HCHO removed by chemical reaction from a volume of mucus exposed for longer times was determined to be less than 0.54% of that removed by mucus flow (convection). Given the solubility of HCHO in mucus (water) and estimates of total mucus flow, however, as much as 22-42% of inhaled HCHO may be removed by total mucus flow. Alternately, O(3) dissolved in mucus would react completely with unsaturated fatty acids in 8.3 x 10(-4) s, in which time the mucus could flow no more than approximately 0.42 microm at the maximum reported flow rate of 30 mm/min. Even if a volume of mucus is flushed by net flow in 1 s, the amount of O(3) removed by flow would only be 0.12% of that removed by chemical reaction. Finally, based on the solubility of ozone, less than 8.0 x 10(-5)% of inhaled material could be removed from the nose by mucus flow. These results indicate which mucociliary processes are significant in site-specific dosimetry modeling.

Algorithms↗

PubMed2001

Synthesis, characterization, and photochemical properties of azobenzene-conjugated Ru(II) and Rh(III) bis(terpyridine) complexes.

We synthesized azobenzene-conjugated bis(terpyridine) Ru(II) and Rh(III) mononuclear and dinuclear complexes and investigated their photochemical properties on excitation of the azo pi-pi band upon 366 nm light irradiation. The Ru mononuclear complex underwent trans-to-cis photoisomerization to reach the photostationary state with only 20% of the cis form, while the Ru dinuclear complex did not isomerize at all photochemically. On the other hand, the mononuclear and dinuclear Rh complexes showed almost complete trans-to-cis photoisomerization behavior. Cis forms of the Rh complexes thermally returned to the trans form at a much slower rate than those of organic azobenzenes, but they did not isomerize photochemically. The reduction potential of the cis forms was 80 mV more negative than that of the trans forms. The photoisomerization quantum yields of the Rh complexes were strongly dependent on the polarity, viscosity, and donor site of the solvents as well as the size of the counterions. We investigated the photoisomerization process of these complexes using femtosecond absorption spectroscopy. For the Rh complexes, we observed S(n) <-- S(2) and S(n) <-- S(1) absorption bands similar to those of organic azobenzenes. For the Ru complexes, we observed very fast bleaching of the MLCT band of the Ru complex, which indicated that the energy transfer pathway to the MLCT was the primary cause of the depressed photoisomerization. The electronic structures, which were estimated from ZINDO molecular orbital calculation, supported the different photochemical reaction behavior between the Ru and Rh complexes.

Journal Article↗

PubMed1989

Partitioning of Noncyclic Photosynthetic Electron Transport to O(2)-Dependent Dissipative Processes as Probed by Fluorescence and CO(2) Exchange.

The partitioning of noncyclic photosynthetic electron transport between net fixation of CO(2) and collective O(2)-dependent, dissipative processes such as photorespiration has been examined in intact leaf tissue from Nicotiana tabacum. The method involves simultaneous application of CO(2) exchange and pulse modulated fluorescence measurements. As either irradiance or CO(2) concentration is varied at 1% O(2) (i.e. absence of significant O(2)-dependent electron flow), the quantum efficiency of PSII electron transport (phi(se)) with CO(2) as the terminal acceptor is a linear function of the ratio of photochemical:nonphotochemical fluorescence quenching coefficients (i.e. q(Q):q(NP)). When the ambient O(2) concentration is raised to 20.5% or 42% the q(Q):q(NP) is assumed to predict the quantum efficiency of total noncyclic electron transport (phi'(se)). A factor which represents the proportion of electron flow diverted to the aforementioned dissipative processes is calculated as (phi'(se) - phi(se))/phi'(se) where phi(se) is now the observed quantum efficiency of electron transport in support of net fixation of CO(2). Examination of changes in electron allocation with CO(2) and O(2) concentration and irradiance at 25 degrees C provides a test of the applicability of the Rubisco model to photosynthesis in vivo.

Journal Article↗

PubMed2006

Process development for the removal and recovery of hazardous dye erythrosine from wastewater by waste materials-Bottom Ash and De-Oiled Soya as adsorbents.

Erythrosine is a water-soluble xanthene class of dye. It is widely used as colorant in foods, textiles, drugs and cosmetics. It is highly toxic, causes various types of allergies, thyroid activities, carcinogenicity, DNA damage behaviour, neurotoxicity and xenoestrogen nature in the humans and animals. The photochemical and biochemical degradation of the erythrosine is not recommended due to formation of toxic by-products. The present paper is an attempt to remove erythrosine from wastewater using adsorption over Bottom Ash-a power plant waste and De-Oiled Soya-an agricultural waste. Under the batch studies, effect of concentration of dye, temperature, pH of the solution, dosage of adsorbents, sieve size of adsorbents, etc., have been studied for the uptake of the dye over both adsorbents. The adsorption process verifies Langmuir and Freundlich adsorption isotherms in both the cases and based on the data different thermodynamic parameters have been evaluated. Batch studies also include kinetic measurements, rate constant study, mass transfer behaviour and establishment of mechanistic pathway for both the cases. For the bulk removal of the dye column operations have been carried out and breakthrough capacities of the Bottom Ash and De-Oiled Soya columns have been calculated. Attempts have also been made for the recovery of the adsorbed dye from exhausted columns by eluting dilute NaOH and more than 90% of the dye was recovered.

Adsorption↗

PubMed1999

Regulation of the photosynthetic electron transport chain.

The regulation of electron transport between photosystems II and I was investigated in the plant Silene dioica L. by means of measurement of the kinetics of reduction of P(700) following a light-to-dark transition. It was found that, in this species, the rate constant for P(700) reduction is sensitive to light intensity and to the availability of CO(2). The results indicated that at 25 degrees C the rate of electron transport is down-regulated by approximately 40-50% relative to the maximum rate achievable in saturating CO(2) and that this down-regulation can be explained by regulation of the electron transport chain itself. Measurements of the temperature sensitivity of this rate constant indicated that there is a switch in the rate-limiting step that controls electron transport at around 20 degrees C: at higher temperatures, CO(2) availability is limiting; at lower temperatures some other process regulates electron transport, possibly a diffusion step within the electron transport chain itself. Regulation of electron transport also occurred in response to drought stress and sucrose feeding. Measurements of non-photochemical quenching of chlorophyll fluorescence did not support the idea that electron transport is regulated by the pH gradient across the thylakoid membrane, and the possibility is discussed that the redox potential of a stromal component may regulate electron transport.Keywords: DeltapH. Electron transport. Photosynthesis. Photosynthetic control. Redox regulation. Silene (photosynthesis)

Journal Article↗

PubMed2005

Kinetic isotope effects in the photochemical reaction cycle of ion transporting retinal proteins.

The kinetics of the photochemical reaction cycle of the bacteriorhodopsin, pharaonis halorhodopsin and proteorhodopsin were determined in H2O and D2O at low and high pH, to get insight in the proton dependent steps of the transport process. While all the steps of the bacteriorhodopsin photocycle at normal pH exhibited a strong isotope effect, the proton uptake step of the photocycle, measured at high pH, became independent of deuterium exchange, making plausible that this step, at low proton concentration, becomes concentration dependent, not mobility dependent. The proton transporting photocycle of the proteorhodopsin at its normal pH (9.5) shows a marked deuterium effect, while at high pH (12.2) this effect almost totally disappears. It was shown earlier that the proton uptake step of the proteorhodopsin is at the rise of the N form. As the proton concentration decreases with rising pH this step becomes the rate limiting, proton concentration dependent step, hiding all the other isotope dependent components. In the case of halorhodopsin in all the chloride, nitrate and proton transporting conditions the photocycle was not strongly affected by the deuterium exchange. While in the cases of the first two ions this seems normal, the absence of the deuterium effect in the case of the proton transporting photocycle was a puzzle. The only plausible explanation is that in the presence of azide the halorhodopsin transports not the proton, but a negatively charged ion the OH-, the mass and mobility of which is only slightly influenced by the deuterium exchange.

Bacteriorhodopsins↗

PubMed2002

H2O2/TiO2 photocatalytic oxidation of metol. Identification of intermediates and reaction pathways.

The applicability of H2O2 to increase the efficiency of TiO2 photocatalytic degradations was investigated. The photographic developer metol [N-methyl-p-aminophenol] that does not adsorb on the surface of TiO2 particulates was used as a model for this purpose. It was proved that metol was mineralised under oxidation with H2O2/TiO2/UV through different thermal and photochemical reactions. Identification of intermediates by both HPLC-electron impact-MS and HPLC-electrospray ionisation-MS helped to elucidate the role of H2O2 and TiO2 in the degradation process and to establish degradation pathways. Intermediates yielded were partially oxygenated aromatic species and dimers, which were amenable to oxidation. The optimal degradation conditions found for mineralisation were 0.4 M H2O2, 5 mg/ml TiO2, pH 9 and irradiation centred at 360 nm (4.9 mW/cm2). The use of oxidants opens an interesting medium to the treatment of effluents containing a diversity of organics since they increase substantially the efficiency of TiO2 photocatalytic degradations.

Aminophenols↗

PubMed2002

The mechanism of Zn-phthalocyanine photosensitized lysis of human erythrocytes.

The phthalocyanines have recently been suggested as one of most effective possible sensitizers for photodynamic therapy and the blood viral inactivation. The further characterisation of the mechanism of human red blood cell lysis and membrane alterations upon photodynamic treatment in the presence of Zn-phthalocyanine was the aim of this study. It was found that there were (2.7+/-0.4).10(7) dye binding sites per red blood cell with the association constant equal to (1.4+/-0.3).10(4) M(-1). Two types of the photosensitized haemolysis: haemolysis during irradiation ("light" haemolysis) and post-irradiation haemolysis ("dark" haemolysis) were studied. The erythrocyte membrane hyperpolarisation, membrane fluidisation and cell swelling preceded the "light" haemolysis. The modification of the erythrocyte membrane band 3 protein by DIDS (an inhibitor of anion exchange) increased the rate of the "light" haemolysis. The rate of "dark" haemolysis was higher and that of "light" haemolysis was lower in potassium media in comparison to sodium ones. The rates of photohaemolysis depended on the erythrocyte membrane potential: a decrease of membrane potential inhibited both types of haemolysis. The cell shrinkage in the presence of sucrose (up to 15 mM) inhibited the "dark" haemolysis but significantly increased the "light" haemolysis. Oxidation of intracellular oxyHb to metHb by nitrite, which drastically decreases intracellular oxygen concentration, as well as GSH concentration, inhibited the rate of the "light" haemolysis. The results allow for the conclusion that the mechanism of photochemical ("light") haemolysis is not of a colloid-osmotical type, in contrast to the post-irradiation ("dark") haemolysis. The photochemical oxidation or denaturation of band 3 protein plays a significant role in the formation of haemolytic holes. The membrane lipid peroxidation, as well as glutathione oxidation, does not participate in the process of photosensitized haemolysis. From the inhibition of "dark" haemolysis by sucrose the apparent pore radius was estimated to be about 1.1 nm. The pores appear to be transient short-lived ones, the average pore number per cell was 0.02.

Erythrocytes↗

PubMed2006

Automotive fuels and internal combustion engines: a chemical perspective.

Commercial transportation fuels are complex mixtures containing hundreds or thousands of chemical components, whose composition has evolved considerably during the past 100 years. In conjunction with concurrent engine advancements, automotive fuel composition has been fine-tuned to balance efficiency and power demands while minimizing emissions. Pollutant emissions from internal combustion engines (ICE), which arise from non-ideal combustion, have been dramatically reduced in the past four decades. Emissions depend both on the engine operating parameters (e.g. engine temperature, speed, load, A/F ratio, and spark timing) and the fuel. These emissions result from complex processes involving interactions between the fuel and engine parameters. Vehicle emissions are comprised of volatile organic compounds (VOCs), CO, nitrogen oxides (NO(x)), and particulate matter (PM). VOCs and NO(x) form photochemical smog in urban atmospheres, and CO and PM may have adverse health impacts. Engine hardware and operating conditions, after-treatment catalysts, and fuel composition all affect the amount and composition of emissions leaving the vehicle tailpipe. While engine and after-treatment effects are generally larger than fuel effects, engine and after-treatment hardware can require specific fuel properties. Consequently, the best prospects for achieving the highest efficiency and lowest emissions lie with optimizing the entire fuel-engine-after-treatment system. This review provides a chemical perspective on the production, combustion, and environmental aspects of automotive fuels. We hope this review will be of interest to workers in the fields of chemical kinetics, fluid dynamics of reacting flows, atmospheric chemistry, automotive catalysts, fuel science, and governmental regulations.

Journal Article↗

PubMedDate not supplied

Effect of membrane fluidity on photoinhibition of isolated thylakoids membranes at room and low temperature.

The relationship between thylakoid membrane fluidity and the process of photoinhibition at room and low (4 degrees C) temperature was investigated. Two different membrane perturbing agents--cholesterol and benzylalcohol were applied to manipulate the fluidity of isolated pea thylakoids. The photochemical activity of photosystem I (PSI) and photosystem II (PSII), polarographically determined, were measured at high light intensity for different time of illumination at both temperatures. The exposure of cholesterol- and benzylalcohol-treated thylakoid membranes to high light intensities resulted in inhibition of both studied photochemical activities, being more pronounced for PSII compared to PSI. Time dependencies of inhibition of PSI and PSII electron transport rates for untreated and membranes with altered fluidity were determined at 20 degrees C and 4 degrees C. The effect is more pronounced for PSII activity during low-temperature photoinhibition. The data are discussed in terms of the determining role of physico-chemical properties of thylakoid membranes for the response of photosynthetic apparatus to light stress.

Benzyl Alcohol↗

PubMed1997

[Singlet oxygen therapy].

Physico-chemical concept of the singlet oxygen therapy application using photochemically sensibilized air or drinking a water, already barbaturized by an activated singlet oxygen of air, is adduced. Therapy technology and examples of its clinical application in the treatment of several pathologic processes for the free-radicals correction are depicted.

Antioxidants↗