SEARCH · PubMed Health

Results for “Photochemical Processes”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 631 records · Page 35Linked to original sources

PubMed1995

Self-association processes involving anthracene labeled phosphatidylcholines in model membrane.

When studying lipid-lipid or lipid-protein interaction in membranes, the correct interpretation of data obtained when using fluorescent phospholipid probes requires the best possible knowledge of probe behaviour in phospholipid membranes. Analysis of the translational dynamics and photochemical properties of the anthracene-labeled phosphatidylcholine (EAPC) shows that a self-association process occurs with this probe in the membrane at the ground state. This anthracene self-association is characterized and leads to a hypochromic effect which has been studied by means of ultraviolet absorption spectroscopy in unilamellar egg-yolk phosphatidylcholine (EggPC) vesicles. A model with indefinite linear self-association, in which each step has the same equilibrium constant, best describes the data. The equilibrium constant was found to be in the 300-500 M(-1) range and the complex lateral distribution pattern of EAPC in model membranes, which results from this self-association process, is characterized and seems to be mainly controlled by the amount of EAPC incorporated into the lipid bilayer.

Journal Article↗

PubMed2002

Transfer hydrogenation processes to mu(3)-alkylidyne groups on the organotitanium oxide [Ti(3)Cp*O(3)].

The photochemical treatment of mu(3)-alkylidyne complexes [[TiCp*(mu-O)](3)(mu(3)-CR)] (R=H (1), Me (2), Cp*=eta(5)-C(5)Me(5)) with the amines (2,6-Me(2)C(6)H(3))NH(2), Et(2)NH, and Ph(2)NH and the imine Ph(2)C=NH leads to the partial hydrogenation of the alkylidyne moiety that is supported on the organometallic oxide, [Ti(3)Cp*O(3)], and the formation of new oxoderivatives [[TiCp*(3)(mu-CHR)(R'NR")] (R"=2,6-Me(2)C(6)H(3), R'=H, R=H (3), Me (4); R'=R"=Et, R=H (5), Me (6); R'=R"=Ph, R=H (7), Me (8)) and [[TiCp*(mu-O)](3)(mu-CHR)(N=CPh(2))] (R=H (9), R=Me (10)), respectively. A sequential transfer hydrogenation process occurs when complex 1 is treated with tBuNH(2), which initially gives the mu-methylene [[TiCp*(mu-O)](3)(mu-CH(2))(HNtBu)] (11) complex and finally, the alkyl derivative [[TiCp*(mu-O)](3)(mu-NtBu)Me] (12). Furthermore, irradiation of solutions of the mu(3)-alkylidyne complexes 1 or 2 in the presence of diamines o-C(6)H(4)(NH(2))(2) and H(2)NCH(2)CH(2)NH(2) (en) affords [[TiCp*(mu-O)](3)(mu(3)-eta(2)-NC(6)H(4)NH)] (13) and [[TiCp*(mu-O)](3)(mu(3)-eta(2)-NC(2)H(4)NH)] (14) by either methane or ethane elimination, respectively. In the reaction of 1 with en, an intermediate complex [[TiCp*(mu-O)](3)(mu-CH(2))(NHCH(2)CH(2)NH(2))] (15) is detected by (1)H NMR spectroscopy. Thermal treatment of the complexes 4-10 quantitatively regenerates the starting mu(3)-alkylidyne compounds and the amine R'(2)NH or the imine Ph(2)C=NH; however, heating of solutions of 3 or 4 in [D(6)]benzene or a equimolecular mixture of both at 170 degrees C produces methane, ethane, or both, and the complex [[TiCp*(mu-O)](3)[mu(3)-eta(2)-NC(6)H(3)(Me)CH(2)]] (16). The molecular structure of 8 has been established by single-crystal X-ray analysis.

Journal Article↗

PubMed2007

The role of the beta-ionone ring in the photochemical reaction of rhodopsin.

Time-dependent density functional theory (TDDFT) calculations on the photoabsorption process of the 11-cis retinal protonated Schiff base (PSB) chromophore show that the Franck-Condon relaxation of the first excited state of the chromophore involves a torsional twist motion of the beta-ionone ring relative to the conjugated retinyl chain. For the ground state, the beta-ionone ring and the retinyl chain of the free retinal PSB chromophore form a -40 degrees dihedral angle as compared to -94 degrees for the first excited state. The double bonds of the retinal are shorter for the fully optimized structure of the excited state than for the ground state suggesting a higher cis-trans isomerization barrier for the excited state than for the ground state. According to the present TDDFT calculations, the excitation of the retinal PSB chromophore does not primarily lead to a reaction along the cis-trans torsional coordinate at the C11-C12 bond. The activation of the isomerization center seems to occur at a later stage of the photo reaction. The results obtained at the TDDFT level are supported by second-order Møller-Plesset (MP2) and approximate singles and doubles-coupled cluster (CC2) calculations on retinal chromophore models; the MP2 and CC2 calculations yield for them qualitatively the same ground state and excited-state structures as obtained in the density functional theory and TDDFT calculations.

Bacteriorhodopsins↗

PubMed1994

Interest of photochemical methods for induction of lipid peroxidation.

Lipid peroxidation, which plays a part in a wide variety of biological processes, is an integral process in the oxidation of unsaturated fatty acids via a radical chain reaction. Among the various species which may induce this reaction in vivo, reactive forms of oxygen such as peroxide anion, the hydroxyl radical and singlet oxygen are of cardinal importance. These species may be generated enzymatically, chemically or by various radiochemical and photochemical reactions. We present here the advantages of photochemical induction of peroxidation, and we describe the principles of the reactions, the photosensitizers that can be employed to generate the various reactive species of oxygen, and the techniques, direct (ESR) or indirect (specific traps), used to detect the reactive species. Photosensitization can induce the formation of a whole gamut of products of lipid peroxidation: conjugated dienes, aldehydes, hydroperoxides, etc. The relative proportions of the various hydroperoxides of fatty acids or cholesterol depend on the nature of the reactive species involved. Utilization of photochemical reactions is an effective and clean way of inducing peroxidation, allowing fine control of both initiation and orientation.

Lipid Peroxidation↗

PubMed1987

New, man-made N2-fixing systems.

The major inputs of fixed N into the global nitrogen cycle are assessed and compared as indicators of both the need for and the likely basis of new, complementary, man-made N2-fixing processes. The development, since 1964, of the purely chemical, highly reactive systems for the reduction of N2, including those driven electro- and photochemically, is traced, along with the parallel efforts to synthesize metal-N2 complexes (the first step in any likely fixation process) and subsequently protonate them to produce hydrazine or ammonia. These experimental approaches are convergent. Successful cycling or catalysing of some of these N2-binding systems has been achieved. The advantages and limitations of the more successful systems are noted. Approaches to this problem via direct modelling of the nitrogenase active site are outlined, as is the one successful use of such complexes in achieving N2 reduction. This wealth of effort on the reductive approaches contrasts vividly with the almost complete absence of research on N2 oxidation. Currently, only a re-evaluation of the arc discharge process is continuing. Finally, the author's studies of the extruded molybdenum-containing prosthetic group of nitrogenase, the enzymic N2-reducing site, are described in relation to future N2-fixing systems.

Agriculture↗

PubMed2005

Photochemical preparation of poly(N-vinyl-2-pyrrolidone)-stabilized platinum colloids and their deposition on titanium dioxide.

Preparation processes for Pt-deposited TiO(2) (Pt/TiO(2)) by the synthesis of Pt nanoparticles and their deposition were pursued by transmission electron microscopy, extended X-ray absorption fine structure, UV-vis spectroscopy, and Fourier transform infrared spectroscopic studies. Colloidal dispersions of Pt particles stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) were photochemically synthesized in aqueous ethanol solution. The average diameter of Pt particles was estimated to be 2.0 +/- 0.5 nm, which was almost unchanged by changing the reducing agent from ethanol to methanol and 2-propanol. The PVP-stabilized Pt particles were distributed over a TiO(2) surface only by mixing the Pt colloidal dispersions and TiO(2). CO was chemically coordinated on the Pt particles on a TiO(2) surface after heat treatment was carried out in an O(2) flow at 673 K to completely remove the residual PVP on Pt/TiO(2). Hydrogen reduction at 473 K did not increase the amount of CO adsorbed on Pt sites. The Pt/TiO(2) catalyst after the oxidation treatment showed higher activity for CO photooxidation than that obtained for pure TiO(2) catalyst. The CO photooxidation rate was not unchanged by the H(2) reduction.

Journal Article↗

PubMed2003

Multi-colour organic light-emitting displays by solution processing.

Organic light-emitting diodes (OLEDs) show promise for applications as high-quality self-emissive displays for portable devices such as cellular phones and personal organizers. Although monochrome operation is sufficient for some applications, the extension to multi-colour devices--such as RGB (red, green, blue) matrix displays--could greatly enhance their technological impact. Multi-colour OLEDs have been successfully fabricated by vacuum deposition of small electroluminescent molecules, but solution processing of larger molecules (electroluminescent polymers) would result in a cheaper and simpler manufacturing process. However, it has proved difficult to combine the solution processing approach with the high-resolution patterning techniques required to produce a pixelated display. Recent attempts have focused on the modification of standard printing techniques, such as screen printing and ink jetting, but those still have technical drawbacks. Here we report a class of electroluminescent polymers that can be patterned in a way similar to standard photoresist materials--soluble polymers with oxetane sidegroups that can be crosslinked photochemically to produce insoluble polymer networks in desired areas. The resolution of the process is sufficient to fabricate pixelated matrix displays. Consecutive deposition of polymers that are luminescent in each of the three RGB colours yielded a device with efficiencies comparable to state-of-the-art OLEDs and even slightly reduced onset voltages.

Journal Article↗

PubMed2004

An operational assessment of the application of the relative reduction factors in the demonstration of attainment of the 8-hr ozone National Ambient Air Quality Standard.

The U.S. Environmental Protection Agency in 1997 revised the 1-hr ozone (O3) National Ambient Air Quality Standard (NAAQS) to one based on an 8-hr average, resulting in potential nonattainment status for substantial portions of the eastern United States. The regulatory process provides for the development of a state implementation plan that includes a demonstration that the projected future O3 concentrations will be at or below the NAAQS based on photochemical modeling and analytical techniques. In this study, four photochemical modeling systems, based on two photochemical models, Community Model for Air Quality and the Comprehensive Air Quality Model with extensions, and two emissions processing models, Sparse Matrix Optimization Kernel for Emissions and Emissions Modeling System, were applied to the eastern United States, with emphasis on the northeastern Ozone Transport Region in terms of their response to oxides of nitrogen and volatile organic carbon-focused controls on the estimated design values. With the 8-hr O3 NAAQS set as a bright-line test, it was found that a given area could be termed as being in or out of attainment of the NAAQS depending upon the modeling system. This suggests the need to provide an estimate of model-to-model uncertainty in the relative reduction factor (RRF) for a better understanding of the uncertainty in projecting the status of an area's attainment. Results indicate that the model-to-model differences considered in this study introduce

Air Pollutants↗

PubMed2003

Anodic oxidation with doped diamond electrodes: a new advanced oxidation process.

Boron-doped diamond anodes allow to directly produce OH* radicals from water electrolysis with very high current efficiencies. This has been explained by the very high overvoltage for oxygen production and many other anodic electrode processes on diamond anodes. Additionally, the boron-doped diamond electrodes exhibit a high mechanical and chemical stability. Anodic oxidation with diamond anodes is a new advanced oxidation process (AOP) with many advantages compared to other known chemical and photochemical AOPs. The present work reports on the use of diamond anodes for the chemical oxygen demand (COD) removal from several industrial wastewaters and from two synthetic wastewaters with malic acid and ethylenediaminetetraacetic (EDTA) acid. Current efficiencies for the COD removal between 85 and 100% have been found. The formation and subsequent removal of by-products of the COD oxidation has been investigated for the first time. Economical considerations of this new AOP are included.

Boron↗

PubMed2004

Toward a photochemical and thermal molecular machine: reversible ligand dissociation and binding in a ruthenium(II)-2,2'-bipyridine complex with tris(2-pyridylmethyl)amine.

A Ru(II) complex having tris(2-pyridylmethl)amine (TPA) and 2,2'-bipyridine (bpy), [Ru(TPA)(bpy)]X(2) (X = ClO(4), PF(6)), exhibited a severe distortion of the coordination of the axial pyridine moiety of TPA due to steric hindrance. The complex showed interesting dissociation-binding behavior of the axial pyridine arm to form a solvent adduct with TPA ligation in a unique meridional tridentate fashion. The complex undergoes thermal dissociation to form solvent-coordinated species via an S(N)2-like mechanism with activation energy of 117 kJ/mol. In contrast, the complex showed reversible photochemical dissociation and rebinding via an S(N)1-like mechanism by MLCT irradiation. The photochemical dissociation was accelerated approximately 200-fold faster than the thermal process. The dissociation process involves selective binding behavior toward external ligands (solvents) with pi-acceptor character, which is indispensable, and no sigma-donating molecules could bind to the Ru(II) center. The guest molecule can be released upon photoirradiation after its thermal binding.

Journal Article↗

PubMed2006

Electron-acceptor-induced isomerization of aryl [6,5] open fulleroids to [6,6] closed methanofullerenes and the electrochemical evaluation of their free energy difference.

The thermal and photochemical rearrangements of a series of aryl substituted [6,5] open fulleroids to [6,6] closed methanofullerenes are accelerated in the presence in of electron acceptors. These [6,5] open fulleroids, facilitated by electron acceptors, rearrange thermally by a zwitteronic-type intermediate, while the photochemical reactions proceed via an excited-state electron-transfer process. The oxidation potentials of these [6,5] open fulleroids and their corresponding [6,6] closed methanofullerenes isomers have been evaluated. The free energy difference between the [6,5] open fulleroids and their corresponding [6,6] closed isomers have been estimated from the difference in their oxidation potentials.

Journal Article↗

PubMed2004

Photoremovable protecting groups based on electron transfer chemistry.

Photoremovable protecting groups (also known as photolabile protecting groups, phototriggers, or caged molecules) are functional groups that are attached to a molecule in such a way as to render the latter inactive. Exposure to light releases the protecting group, restoring functionality to the molecule. The use of photoremovable protecting groups (PRPGs) allows for precise spatial and temporal control of chemical reactions. Such groups have found use in many diverse applications, ranging from time resolved studies of physiological processes, to fabrication of spatially resolved combinatorial libraries of DNA. Recent research efforts have focused on designing protecting groups that are removed through photoinduced electron transfer (PET), rather than by direct photolysis. The PET strategy allows the light absorption step to be decoupled from the bond breaking step, thus permitting more control over the wavelengths of light used in the release process. The application of these types of protecting groups to the photochemical release of amines, alcohols, ketones, and carboxylic acids is described.

Journal Article↗

PubMed1986

2-Nitrofluoren-9-one: a unique mutagen formed in the photo-oxidation of 2-aminofluorene.

Exposure of solutions of 2-aminofluorene (2-AF, dissolved in dimethylsulfoxide) to near ultraviolet light (u.v.a. wavelengths of 320-400 nm) results in the formation of a variety of photo-products, several of which are direct-acting mutagens in the Ames/Salmonella standard-plate assay. Previously published results from our laboratory have described the chemical identification and kinetics of formation of two of these photo-induced mutagens, 2-nitrosofluorene and 2-nitrofluorene. In this report we present recent data concerning the isolation and chemical identification of another mutagenic photoproduct of u.v.a.-irradiated 2-AF, 2-nitrofluoren-9-one (2-NO2F-9-one). Data are also presented concerning the kinetics of phototransformation of 2-aminofluoren-9-one, an early-appearing and predominant photoproduct in u.v.a.-irradiated solutions of 2-AF, into 2-NO2F-9-one. It is well established that N-oxidation is a critical step in the biotransformations (i.e. enzymatic metabolism) of primary aromatic amines into proximate mutagens/carcinogens. In addition to u.v.a.-mediated N-oxidation of aromatic amines, selective ring photo-oxidation can also occur, resulting in, for example, the production of a carbonyl group at the 9-position of the fluorene molecule. The formation of mutagenic 2-NO2F-9-one in the photochemical oxidation of 2-AF appears to be unique to this process.

Chromatography, High Pressure Liquid↗

PubMed2001

Photochemistry of methoxyhydroquinone and methoxy-p-benzoquinone in solution related to the photoyellowing of the lignocellulosics.

The photoreactivity of methoxy-p-benzoquinone (MQ) and methoxyhydroquinone (MHQ) in dilute solution (10(-4)-10(-3) M) was studied using continuous irradiation and laser flash photolysis (LFP). The quinone irradiated in degassed tetrahydrofuran (THF) gives MHQ and an adduct with the solvent. Only the formation of hydroquinone is observed in ethanol, and hydroxylation is evidenced in water, whereas the compound is stable in CCl4. The bis-quinone, 4,4'-dimethoxybiphenyl-2,5,2',5'-bisquinone, and the dibenzofurane-quinone, 8-hydroxy-3,7-dimethoxydibenzofuran-1,4-quinone, are formed in the presence of MHQ, whereas the reactivity is low with ethylconiferyl alcohol. When MHQ is irradiated selectively in degassed THF, the formation of MQ and of the bis-hydroquinone, 4,4'-dimethoxy-2,5,2',5'-tetrahydroxy-biphenyl, are observed. The dimer is oxidized photochemically or thermally into the mono- or bis-quinones, the process being accelerated in alkaline medium. The formation of the dimers is strongly favored by the contiguous presence of quinone and hydroquinone. When MHQ is selectively irradiated in the presence of transethylconiferaldehyde (EtC), quinone formation and isomerization of EtC are observed. LFP experiments, performed with a selective excitation of MQ, indicate that the triplet state of the quinone is strongly quenched by MHQ to conduce to a semiquinone radical. The interaction between 3MQ* and MQ is mainly driven by an electron transfer process according to the similar value of the quenching rate constant found with another electron donor compound such as 1,4-dimethoxybenzene. By contrast, no strong quenching of 3EtC* by MHQ was observed. It is proposed that the photochemistry of the couple MQ/MHQ is governed by the formation of encounter complex between either 3MQ* and MHQ or 3MHQ* and MQ. Consequently, the fast part of the photoyellowing of lignocellulosics does not appear to involve the couple MHQ/MQ or MHQ/etherified coniferaldehyde, but more likely a combination of oxidation of the hydroquinone by ground-state oxygen and photohydration of the formed quinone from its triplet state, giving inter-alia more colored o-quinonoid type molecules.

Benzoquinones↗

PubMed1987

[Argyria. A clinical, chemical analytic and micromorphologic study].

A 62-year-old male patient developed generalized argyria following the intake of silver-proteinacetyltannate (Targesin; approx. 60 g in 10 years) as treatment for gastric discomfort. On histological and ultrastructural examination of the skin, silver particles were found not only in the usual locations but also in the Schwann cell, the mast cell, and in smooth muscle cells. This corresponded to chemical analysis, proving the presence of this metal in the skin. In the blood, a level of 0.26 +/- 0.04 ppm silver was found. By means of an equation, attempts were made to demonstrate the reaction process involved in the formation of Ag2S as subjected to the photochemical effect of sunlight.

Argyria↗

PubMed2001

Excitation energy transfer in Photosystem I from oxygenic organisms.

This Review discusses energy transfer pathways in Photosystem I (PS I) from oxygenic organisms. In the trimeric PS I core from cyanobacteria, the efficiency of solar energy conversion is largely determined by ultrafast excitation transfer processes in the core chlorophyll a (Chl a) antenna network and efficient photochemical trapping in the reaction center (RC). The role of clusters of Chl a in energy equilibration and photochemical trapping in the PS I core is discussed. Dimers of the longest-wavelength absorbing (red) pigments with strongest excitonic interactions localize the excitation in the PS I core antenna. Those dimers that are located closer to the RC participate in a fast energy equilibration with coupled pigments of the RC. This suggests that the function of the red pigments is to concentrate the excitation near the RC. In the PS I holocomplex from algae and higher plants, in addition to the red pigments of the core antenna, spectrally distinct red pigments are bound to the peripheral Chl a/b-binding light-harvesting antenna (LHC I), specifically to the Lhca4 subunit of the LHC I-730 complex. Intramonomeric energy equilibration between pools of Chl b and Chl a in Lhca1 and Lhca4 monomers of the LHC I-730 heterodimer are as fast as the energy equilibration processes within the PS I core. In contrast to the structural stability of the PS I core, the flexible subunit structure of the LHC I would probably determine the observed slow excitation energy equilibration processes in the range of tens of picoseconds. The red pigments in the LHC I are suggested to function largely as photoprotective excitation sinks in the peripheral antenna of PS I.

Journal Article↗

PubMed1990

Waste treatment technologies from the Genesearch Group of companies.

Genesearch is a 10 year old research and development company of Australian scientists, specializing in microbiology and genetics. This research expertise has formed the basis of a number of microbial processes for waste treatment. In addition, a novel type of high yield fermenter for aerobic bacteria allows the economical production of high-potency bacterial preparations for waste treatment processes. A novel approach to the rapid biodegradation of polychlorinated biphenyls has been developed. Photochemical pretreatment partially dechlorinates the molecules, rendering them susceptible to complete and rapid digestion by wild-type soil bacteria. In the area of non-toxic waste, Genesearch has developed products for on-site treatment of, for example, grease-trap wastes and waste oil in ship bilges; and a large scale process for conversion of municipal grease wastes into protein-rich biomass. The prospects for novel biological waste treatment are improving, as public pressure grows, and as increasing government monitoring and penalties make inadequate waste disposal uneconomic.

Bacteria, Aerobic↗

PubMed1989

Photoactivation and photocontrolled release of bioactive materials.

Photoresponsive systems are widespread in nature, and life processes such as photosynthesis, vision, phototropism, and phototaxis are linked with structural changes of molecules caused by sunlight. Similar photochemical transformations can be employed to render synthetic agrochemicals and drugs active or to govern their bioavailability at the site of action. Related fundamental physical and chemical processes are discussed as part of the introductory paragraph. In agrochemical applications mainly three different ways have been reported as to how light can be utilized to optimize pesticidal effects in crop protection. The hypersensitization of certain pests to sunlight by chemical compounds represents an effective approach and exists also as a natural mechanism of defense. Photo-removable protective groups and various photoreactive structural moieties have successfully been applied for the rational design of light-activated propesticides. Finally, daylight can function as a triggering and governing factor in controlled-release systems based on photoresponsive polymers. Among the major advantages of light-regulated pesticides, the target-directed activation and the inherent detoxification predominate. In biomedical applications, light has also been used to activate compounds to time their action. Although radiation of various types (microwave, X-ray, and gamma radiation) has been used extensively for both treatment and diagnostics, the limited access of UV and visible light to most tissues of the body prevents its general use as the activating trigger. We review here three areas only: cutaneous photobiology-related applications; photoaffinity labeling; and porphyrins as therapeutic agents.

Animals↗