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At least 19 recordsLinked to original sources
PubMedDate not supplied

[Spectral transformations in purple membranes of Halobacterium halobium: effect of 560-570 transition and blue light on photochemical processes].

Effect of reversible photodependent spectral transition 560 in equilibrium 570 of the basic absorption band in purpur membranes (PM) in water on the accumulation of short-living forms 412 and 650 of bacteriorhodopsin (BR) photochemical cycle was studied. It has been shown that photochemical transformations in BR proceed as a minimum by two independent cycles, whose initial spectral forms were represented by BR with retinene in the form 13 cis and BR with retinene in the form of trans. In the course of the transition 560-- greater than 570 a long-living spectral form 370--410--430 sensitive to the blue light is formed. On the basis of our own and literature data a new scheme of photochemical processes in PM was suggested and connection between some stages of these processes and ejection and absorption of PM protones proposed.

Bacteriorhodopsins↗
PubMed1978

Spectral transitions in purple membranes from Halobacterium halobium. I. Effect of preliminary illumination on photochemical processes.

The effect of preliminary illumination of purple membranes by yellow light on the difference spectra of short-lived intermediates has been studied. It has been found that changes of the optical density of two of these intermediates, which have the maxima of the difference spectrum at 412 nm and 650 nm, coincides well with the kinetics off the known reversible transitions of the main band of the purple membrane absorption (560 570), i.e., 13-cis-trans transitions. The changes at 412 nm and 650 nm are proportional to the concentration changes of the all-trans retinal and 13-cis retinal, respectively. It was concluded that the formation of the short-lived 412 and 650 intermediates occurs in different photochemical cycles. The pH is found to affect the formation of the 650 intermediate.

Bacteriorhodopsins↗
PubMed1977

Primary photophysical and photochemical processes in visual excitation.

The color of visual pigments is experimentally shown to be controlled by excited state effects. These effects which define the primary absorption of light by rhodopsin are considered together with results obtained from emission and picosecond spectroscopy. In addition, the molecular changes induced in rhodopsin when a photon is absorbed are analyzed using resonance Raman spectroscopy. The molecular changes observed are compared in bacterial and photoreceptor rhodopsins. This comparison yields a unique explanation for the biological role of the cis-trans isomerization in visual transduction.

Bacteriorhodopsins↗
PubMed1978

Primary photochemical processes in isolated reaction centers of Rhodopseudomonas viridis.

Picosecond and nanosecond spectroscopic techniques have been used to study the primary electron transfer processes in reaction centers isolated from the photosynthetic bacterium Rhodopseudomonas viridis. Following flash excitation, the first excited singlet state (P*) of the bacteriochlorophyll complex (P) transfers an electron to an intermediate acceptor (I) in less than 20 ps. The radical pair state P+I-) subsequently transfers an electron to another acceptor (X) in about 230 ps. There is an additional step of unknown significance exhibiting 35 ps kinetics. P+ subsequently extracts an electron from a cytochrome, with a time constant of about 270 ns. At low redox potential (X reduced before the flash), the state P+I- (or PF) lives approx. 15 ns. It decays, in part, into a longer lived state (PR), which appears to be a triplet state. State PR decays with an exponential time of approx. 55 microseconds. After continuous illumination at low redox potential (I and X both reduced), excitation with an 8-ps flash produces absorption changes reflecting the formation of the first excited singlet state, P*. Most of P* then decays with a time constant of 20 ps. The spectra of the absorbance changes associated with the conversion of P to P* or P+ support the view that P involves two or more interacting bacteriochlorophylls. The absorbance changes associated with the reduction of I to I- suggest that I is a bacteriopheophytin interacting strongly with one or more bacteriochlorophylls in the reaction center.

Bacteriochlorophylls↗
PubMed1975

Inhibition of oxygen evolution following illumination of Chlorella cells with far-red light.

The effect of brief far-red illumination on Chlorella pyrenoidosa cells has been investigated. An inhibition of oxygen evolution occurs 20 to 30 s after the end of the far-red illumination. This inhibition occurs in a step following the initial charge separation process by the System II centers. It is reversible in the light through a purely photochemical process.

Chlorella↗
PubMed1985

The two-photon induced fluorescence of the tumor localizing photosensitizer hematoporphyrin derivative via 1064 nm photons from a 20 ns Q-switched Nd-YAG laser.

We demonstrate the direct 1064 nm two-photon excitation of hematoporphyrin derivative (HPD), a complex mixture of photosensitizing porphyrins which is selectively retained in tumor tissue and used in cancer photochemotherapy. Although 1064 nm is outside of the one-photon HPD absorption spectrum, two-photon induced fluorescence from HPD was observed following excitation by the 20 ns output of an amplified, Q-switched Nd-YAG laser at peak power levels of 0.1 to 3 GW/cm2. Evidence for the successful two-photon excitation to vibrational levels of the S1 state consists of the observation of the known HPD fluorescence spectrum exhibiting peaks at approximately 615 and 675 nm, with the observed two-photon induced fluorescence intensity exhibiting a quadratic dependence on the excitation laser intensity as required for a direct two-photon process. More generally, these results suggest the possibility for the achievement of photosensitized oxidations utilizing photons of lower energy than that required for single photon excitation, offering the potential for both greater selectivity and a reduction in competing photochemical processes.

Antineoplastic Agents↗
PubMed1992

Detection of free radicals in UV-irradiated lens by spin trapping ESR.

We have made an ESR study on the UV-photolysis of lens and identified the origins of free radicals involved in the initial photochemical process by spin trapping technique. Two spin adducts were detected on irradiation of canine lens in the presence of a spin trapping reagent (DMPO); a spin adduct of sulfur centered radical derived from glutathione and the protonated adduct of hydrated electron. A free radical mechanism of initial photochemical injury in UV-irradiated lens was discussed, comparing with a photolysis of tryptophan plus cysteine solution.

Animals↗
PubMedDate not supplied

[Collective exposure to secondary pollutants. Origin, chemistry, environmental cycles, impact on urban and rural atmospheric contamination, criteria for prevention].

Consequently to a bibliographic review, the Authors confirm that aliphatic aldehydes are ubiquitous constituents of rural and urban atmospheres and that they are certainly important by-products in many kind of antropogenic combustions (primary sources) as well as important intermediates in a wide variety of tropospheric chemical processes (secondary sources). The results of the monitoring of atmospheric aliphatic aldehydes in Turin city (north western Italy), during one year, are reported. The dynamic presence and concentration of aldehydes (mainly formaldehyde and acetaldehyde) is always limited below 100 ppb, however variable in the time. The results of our work point out the main role of photochemical processes during the summer, since the summer data well correlate with solar irradiation, ground temperature and relative humidity. During the winterthough it has not been possible to compare our data with other atmospheric pollutant (CO; NMHC), it is presumable a major role of primary sources in generating the atmospheric aldehydes. Without forgetting the irritant, mutagenic and cancerogenic properties of some aldehydes, we can assume that the next qualitative gasoline modification in 1991, according to the E.E.C. directive (8/5210/E.E.C. 20/3/1985), could cause an atmospheric aldehyde increase followed by a human risk increase.

Air Pollutants↗
PubMed1988

In situ polymerization of a microencapsulating medium round living cells.

A new process for microencapsulating living cells is described. Pancreatic islet cells in solution in agarose were extruded from a syringe into a paraffin-oil-containing medium. A rigid interface around each bead was produced when the agarose gelled. A polymerizable mixture of monomers containing 30% of acrylamide and 1.5% of bisacrylamide was then added to these beads. Polymerization was initiated by a photochemical process using riboflavin as a photosensitizer, which enabled use of oxygenated solutions. As other polymerizations initiated by an oxido-reduction reaction, this method did not require heating, and could thus be carried out at ambient temperature. The oil phase near the agarose beads effectively prevented any rise in temperature, and also led to formation of an acrylamide concentration gradient in the aqueous phase, which reduced the potential cytotoxicity of the reaction. Radioimmunoassay of insulin liberated from encapsulated islets of Langerhans demonstrated a good viability of the cells.

Acrylamides↗
PubMed1988

Low-temperature magnetic circular dichroism studies of the photoreaction of horseradish peroxidase compound I.

Horseradish peroxidase (HRP) compound I is photolabile at all temperatures between room temperature and 4 K. The photoredox reaction has been studied in frozen glassy solutions by using optical absorption and magnetic circular dichroism spectra following photolysis of HRP compound I with visible-wavelength light at 4.2 and 77 K. The photochemical process is characterized as a concerted two-electron transfer reaction which results in the conversion of the Fe(IV) heme pi-cation radical species of HRP compound I into a low-spin Fe(III) heme species. This reaction occurs even when photolysis is carried out at 4.2 K. Spectra recorded between 4.2 and 80 K for the low-spin ferric hydroxide complex of HRP closely resemble the data measured for the photochemical product. The proposed mechanism for the photoreaction is (formula; see text) No evidence is found for the formation of an Fe(II) heme at these temperatures.

Circular Dichroism↗
PubMedDate not supplied

[Isolation and characterization of photochemical properties of the photosynthetic reaction centers from Rhodopseudomonas shperoides, strain 1760-1].

Photosynthetic reaction centres were isolated from the cells of Rhodopseudomonas spheroides, strain 1760-1, using sodium dodecyl sulphate. The preparations purified by precipitation with ammonium sulphate showed absorbance ratios of A280 : A800=2.1. and A765 : A800 : A870=1 : 2 : 1; about 75% of the bacteriochlorophyll absorbing at 870 nm (P870) were photochemically active. Both absolute and difference "light minus dark" absorption spectra were obtained for the reaction centre suspensions and vacuum-dried films at room and low temperatures. Shift to the longer wavelength of the 870 nm absorption band resulting from temperature lowering suggests the existence of temperature-determined conformations of the bacteriochlorophyll-protein complex of the reaction centres. Characteristic time of an electron transfer from the photoexcited P870 to the primary intermediate of photochemical process as evaluated from the data of pulsed laser fluorometry of the reaction centres was found to be (21--15)+/-8 picoseconds. The oxidized P870 dark reduction kinetics dependence on the actinic light intensity gives evidence for the functioning of heterogeneous pool of the secondary electron acceptors in the reaction centre preparations. Filling in of this pool with electrons is decreased under temperature lowering or vacuum drying and its electron capacity is limited under isooctane treatment resulting in ubiquinon extraction. The ability of the reaction centre preparations to catalyze the photochemical oxidation of iminoxyl aromatic radical was demonstrated.

Bacteriochlorophylls↗
PubMed1987

Photolytic degradation of benorylate: effects of the photoproducts on cultured hepatocytes.

The photodegradation of benorylate [4'-(acetamido)phenyl-2-acetoxybenzoate], a drug frequently used in rheumatoid arthritis therapy, has been examined under different sets of experimental conditions. Several photoproducts have been isolated and identified on the basis of their IR, NMR, and MS spectra. The most significant photochemical process is the photo-Fries rearrangement of benorylate, leading to 5-acetamido-2'-acetoxy-2-hydroxybenzophenone (1). This compound undergoes a rapid transacylation to the isomeric 5'-acetamido-2'-acetoxy-2-hydroxybenzophenone (2). A primary culture of rat hepatocytes has been used to evaluate the possible toxicity of these two benzophenones, keeping in mind the following criteria: leakage of cytosolic enzymes, attachment index to culture plates, gluconeogenesis from lactate and fructose, glycogen balance, and albumin synthesis. At the concentrations assayed, neither of the two major photoproducts of benorylate (benzophenones 1 and 2) had significant toxic effects on liver cells in culture.

Albumins↗
PubMedDate not supplied

Photochemical synthesis of simple organic free radicals on simulated planteary surfaces-an ESR study.

Electron spin resonance spectroscopy provided evidence for formation of hydroxyl radicals during ultraviolet photolysis (254 nm) at -170C of H2O adsorbed on silica gel or of silica gel alone. The carboxyl radical was observed when CO or CO2 or a mixture of CO and CO2 absorbed on silica gel at -170C was irradiated. The ESR signals of these radicals slowly disappeared when the irradiated samples were warmed to room temperature. However, re-irradiation of CO or CO2, or the mixture CO and CO2 on silica gel at room temperature then produced a new species, the carbon dioxide anion radical, which slowly decayed and which was identical with that produced by direct photolysis of formic acid adsorbed on silica gel. The primary photochemical process may involve formation of hydrogen and hydroxyl radicals by means of (1) photodissociation of H2O physically adsorbed on the silica gel, or (2) absorption of the excitation energy by the silica gel surface with subsequent cleavage of the silanol bonds, or (3) dissociation of H2O molecules throug photosensitization by the surfaces or a combination of (1) to (3). Subsequent reactions of these radicals with adsorbed CO or CO2 or both yield carboxyl radicals, CO2H, the precsursors of formic acid. Our results confirm the formation of formic acid under simulated Martian conditions and provide a mechanistic basis for gauging the potential importance of gas-solid photochemistry for chemical evolution on other extraterrestrial bodies, on the primitive earth and on dust grains in the interstellar medium.

Adsorption↗