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PubMed1975

Quenching of chlorophyll fluorescence and primary photochemistry in chloroplasts by dibromothymoquinone.

The quenching action of dibromothymoquinone on fluorescence and on primary photochemistry was examined in chloroplasts at minus 196 degrees C. Both the initial (F0) and final (FM) levels of fluorescence as well as the fluorescence of variable yield (FV equals FM minus FO) were quenched at minus 196 degrees C to a degree which depended on the concentration of dibromothymoquinone added prior to freezing. The initial rate of photoreduction of C-550 at minus 196 degrees C, which was assumed to be proportional to maximum yield for primary photochemistry, phipo, was also decreased in the presence of dibromothymoquinone. Simple theory predicts that the ratio FV/FM should equal phipo. Excellent agreement was found in a comparison of relative values of phipo with relative values of FV/FM at various degrees of quenching by dibromothymoquinone. These results are taken to indicate that FO and FV are the same type of fluorescence, both emanating from the bulk chlorophyll of Photosystem II. Dibromothymoquinone appears to create quenching centers in the bulk chlorophyll of Photosystem II which compete with the reaction centers for excitation energy. The rate constant for the quenching of excitation energy by dibromothymoquinone is directly proportional to the concentration of the quencher. Rate constants for the de-excitation of excited chlorophyll molecules by fluorescence, kF, by nonradiative decay processes, kD, by photochemistry, kP, and by the specific quenching of dibromothymoquinone, kQ, were calculated assuming the absolute yield of fluorescence at FO to be either 0.02 or 0.05.

Bromine↗
PubMed2003

Photochemistry of 4-chlorophenol on cellulose and silica.

The photochemistry of 4-chlorophenol (4-CP) was studied on silica and cellulose, using time-resolved diffuse reflectance techniques and product degradation analysis. The results have shown that the photochemistry of 4-CP depends on the support, on the concentration, and also on the sample preparation method. Transient absorption and photoproduct results can be understood by assuming the formation of the carbene 4-oxocyclohexa-2,5-dienylidene in both supports. On cellulose, at concentrations lower than 10 micromol g(-1), the carbene leads to the unsubstituted phenoxyl radical, and phenol is the main degradation product. At higher concentrations a new transient resulting from phenoxyl radicals coupling was also observed, and dihydroxybiphenyls are also formed. The reaction of the carbene with ground-state 4-CP was also detected through the formation of 5-chloro-2,4'-dihydroxybiphenyl. 4-Chlorophenoxyl radical and degradations products resulting from its coupling were also detected. Oxygen has little effect on the photochemistry of 4-CP on cellulose. On silica the transient benzoquinone O-oxide was formed in the presence of oxygen. Benzoquinone and hydroquinone are the main degradation products. In well-dried samples the formation of hydroquinone is reduced. At higher concentrations the same products as detected on cellulose were observed. 4-CP undergoes slow photochemical decomposition under solar radiation in both supports. The same main degradation products were observed in these conditions.

Adsorption↗
PubMed2005

From primary photochemistry to biological function in the blue-light photoreceptors PYP and AppA.

To properly respond to changes in fluency conditions, Nature has developed a variety of photosensors that modulate gene expression, enzyme activity and/or motility. Dedicated types have evolved, which can be classified in six families: rhodopsins, phytochromes, xanthopsins, cryptochromes, phototropins and BLUF-proteins. The photochemistry of the first three families is based on cis/trans isomerization of an ethylene bond. Surprisingly, the latter three all use flavin as their chromophore, but each with very different photochemistry. In this contribution we will discuss the molecular basis of signal generation in a xanthopsin (Photoactive Yellow Protein (PYP) from Halorhodospira halophila), a photoreceptor for negative phototaxis, and in a BLUF protein (AppA from Rhodobacter sphaeroides), a transcriptional anti-repressor. PYP is activated through trans/cis isomerization of the 7,8-vinyl bond of its 4-hydroxycinnamic acid chromophore. This initiates a photocycle with multiple intermediates, like pB, which is formed after intramolecular proton transfer. The negative charge thus formed in the interior of the protein triggers formation of a partially unfolded signaling state. For AppA much less is known about the underlying photochemistry. Available evidence suggests that it is based on a light-induced change in the hydrogen-bonding of its flavin chromophore and/or a change in hydrophobic stacking between the flavin and/or nearby aromatic amino acids like Y 21. A signaling state is formed within microseconds, which recovers with a rate of approximately 10(-3) s(-1). The change in conformation between receptor- and signaling-state in AppA, however, appear to be minute as compared to those in PYP. Here we review the underlying chemistry in the various steps of the photocycle of these two photoreceptor proteins and provide new data on their mechanism and function.

Amino Acid Sequence↗
PubMed2005

Photochemistry of cyclohexane on Cu(111).

The photochemistry of cyclohexane on Cu(111) and its excitation mechanism have been studied by temperature-programmed desorption, ultraviolet and X-ray photoelectron spectroscopy. Cyclohexane weakly adsorbed on Cu(111) has been known to show a broadened and redshifted CH stretching band, i.e., CH vibrational mode softening. Although no dehydrogenation takes place thermally on this surface and by the irradiation of photons at 5.0 eV, adsorbed cyclohexane is dissociated to cyclohexyl and hydrogen by the irradiation of photons at 6.4 eV. This is a marked contrast to cyclohexane in the gas phase where the onset of absorption is located at 7 eV. When the surface irradiated by 6.4-eV photons is further annealed, cyclohexyl is dehydrogenated to form cylcohexene that desorbs at 230 K. The systematic measurements of photochemical cross sections at 6.4 eV with linearly polarized light as a function of incident angle indicate that the electronic transition from the highest occupied band of cyclohexane to a partially occupied hybridized band near the Fermi level is responsible for the photochemistry. The hybridized band is formed by the interactions between the electronic states of cyclohexane and the metal substrate. The role of the hybridized band in the photochemistry and the CH vibrational mode softening is discussed.

Copper↗
PubMed2004

Characterization of PSII photochemistry and thermostability in salt-treated Rumex leaves.

A study was conducted, using chlorophyll fluorescence, rapid fluorescence induction kinetics, and polyphasic fluorescence transients, to determine the effect of salt treatment and heat stress on PSII photochemistry in Rumex leaves. Salt treatment was accomplished by adding NaCl solutions of different concentrations ranging from 50 to 200 mmol/L. Heat stress was induced by exposing the plant leaves to temperatures ranging from 29 to 47 degrees C. The control plants were grown without NaCl treatment. The data acquired in this study showed that NaCl treatment alone had no effect on the maximal photochemistry of PSH or the polyphasic rise of chlorophyll fluorescence. However, the NaCl treatment modified heat stress on PSII photochemistry in Rumex leaves, which was manifested by a lesser heat-induced decrease in photochemical quenching (qP), efficiency of excitation energy capture by open PSII reaction centers (Fv'/Fm'), and quantum yield of PSII electron transport (phiPSII). The data also showed that NaCl treatment compromised the impact of heat stress on the capacity of transferring electrons from Q(A)- to Q(B). Furthermore, the NaCl treatment promoted heat resistance of O2-evolving complex (OEC). In summary, NaCl treatment enhanced the thermostability of PSII.

Chlorophyll↗
PubMed2002

UV photochemistry of DNA in vitro and in Bacillus subtilis spores at earth-ambient and low atmospheric pressure: implications for spore survival on other planets or moons in the solar system.

Two major parameters influencing the survival of Bacillus subtilis spores in space and on bodies within the Solar System are UV radiation and vacuum, both of which induce inactivating damage to DNA. To date, however, spore survival and DNA photochemistry have been explored only at the extremes of Earth-normal atmospheric pressure (101.3 kPa) and at simulated space vacuum (10(-3)-10(-6) Pa). In this study, wild-type spores, mutant spores lacking alpha/beta-type small, acid-soluble spore proteins (SASP), naked DNA, and complexes between SASP SspC and DNA were exposed simultaneously to UV (254 nm) at intermediate pressure (1-2 Pa), and the UV photoproducts cis,syn-thymine-thymine cyclobutane dimer (c,sTT), trans,syn-thymine-thymine cyclobutane dimer (t,sTT), and "spore photoproduct" (SP) were quantified. At 101.3 kPa, UV-treated wild-type spores accumulated only SP, but spores treated with UV radiation at 1-2 Pa exhibited a spectrum of DNA damage similar to that of spores treated at 10(-6) Pa, with accumulation of SP, c,sTT, and t,sTT. The presence or absence of alpha/beta-type SASP in spores was partly responsible for the shift observed between levels of SP and c,sTT, but not t,sTT. The changes observed in spore DNA photochemistry at 1-2 Pa in vivo were not reproduced by irradiation of naked DNA or SspC:DNA complexes in vitro, suggesting that factors other than SASP are involved in spore DNA photochemistry at low pressure.

Atmospheric Pressure↗
PubMed2000

Role of light in the response of PSII photochemistry to salt stress in the cyanobacterium Spirulina platensis.

The role of light in the effect of salt stress on PSII photochemistry in the cyanobacterium Spirulina platensis grown at 50 micromol m(-2) s(-1) was investigated. The time-course of changes in PSII photochemistry in response to high salinity (0.8 M NaCl) incubated in the dark and at 30, 50 and 100 micromol m(-2) s(-1) was composed of two phases. The first phase, which was independent of light, was characterized by a rapid decrease (20-50%) in the maximal efficiency of PSII photochemistry (F:(v)/F:(m)), the efficiency of excitation energy capture by open PSII reaction centres (F(1)(v)/F(1)(m)), photochemical quenching (q(P)), and the quantum yield of PSII electron transport (Phi(PSII)) in the first 15 min, followed by a recovery of up to about 86-92% of their initial levels after 4 h of incubation. The second phase took place after 4 h, in which a further decline in the above parameters occurred only in the light but not in the dark, reaching levels as low as 32-56% of their initial levels after 12 h. Moreover, the higher incubation light intensity, the greater the decrease in the above parameters. At the same time, Q(B)-non-reducing PSII reaction centres increased significantly in the first 15 min and then recovered to the initial level during the first phase, but increased again in the light in the second phase. Photosynthetic oxygen evolution activity decreased sharply by 70% in the first 5 min, and then kept largely constant until 12 h. The changes in oxygen evolution activity were independent of light intensity during both phases.

Cyanobacteria↗
PubMed2003

Solution and crystal lattice effects on the photochemistry of 6-substituted cyclohexenones(1)(,)(2).

The photochemistry of 13 4,4-diphenylcyclohexenones, substituted at carbon-6, was investigated in solution and in the crystalline state. The stereoselectivity was of particular interest. In the solution photochemistry of C-6 monosubstituted enones in benzene, there was a unique preference for migration of the cis-phenyl group with formation of bicyclo[3.1.0]hexanone photoproducts, with the original 6-substituent having an endo configuration at carbon-3 of the product. In methanol the reaction was diverted to afford 3,4-diphenylcyclohex-2-enes understood as arising from a hydrogen-bonded zwitterionic intermediate. The solid-state photochemistry was also investigated. There was a dramatic absence of the 3,4-diphenylcyclohex-2-ene products in accord with the absence of the hydrogen bonding encountered in methanol. Further, the solid-state reactivity correlated with a vector analysis using X-ray atomic coordinates. This established that the migrating phenyl group required an orientation facing the enone beta-carbon. While the interesting preference for the cis-endo migration was not intuitively predicted, ab initio computations on the alternative phenyl-bridged triplet intermediates did lead to an understanding of the selectivity.

Journal Article↗
PubMed2000

Selective quenching of the fluorescence of core chlorophyll-protein complexes by photochemistry indicates that Photosystem II is partly diffusion limited.

The spectral characteristics of fluorescence quenching by open reaction centres in isolated Photosystem II membranes were determined with very high resolution and analysed. Quenching due to photochemistry is maximal near 687 nm, minimal in the chlorophyll b emission interval and displays a distinctive structure around 670 nm. The amplitude of this 'quenching hole' is about 0.03 for normalised spectra. On the basis of the absorption spectra of isolated chlorophyll-protein complexes, it is shown that these quenching structures can be exactly described by assuming that photochemistry lowers the fluorescence yield of the reaction centre complex (D1/D2/cytb (559)) plus CP47, with quenching of the former complex being approximately double that of the latter complex. These data, which qualitatively indicate that there are kinetically limiting processes for primary photochemistry in the antenna, have been analysed by means of several different kinetic models. These models are derived from present structural knowledge of the arrangement of the chlorophyll-protein complexes in Photosystem II and incorporate the reversible charge separation characteristic of the exciton/radical pair equilibration model. In this way it is shown that Photosystem II cannot be considered to be purely trap limited and that exciton migration in the antenna imposes a diffusion limitation of about 30%, irrespective of the structural model assumed.

Journal Article↗
PubMed2000

Excitation mechanisms and photochemistry of adsorbates with spherical symmetry.

By comparing the photo-stimulated desorption of Xe from an oxidized Si(100) surface with the photochemistry of methane on metal surfaces, we try to deduce a common concept in the excited state and the excitation mechanism responsible for the photo-induced processes. Xe atoms are desorbed from the oxidized Si(100) surface by the irradiation of photons in the range 1.16-6.43 eV. Two velocity components with average kinetic energy 0.85 and 0.25 eV are observed in the time-of-flight distributions. The fast component appears only if the photon energy exceeds approximately 3 eV, but the slow component is present over the entire photon energy range. By analogy with the photochemistry of methane on metal surfaces, the excitation mechanism responsible for the fast component is postulated to be a transition from the 5p state of Xe to the excited state originating from strong hybridization between the 6s state of Xe and the dangling bond at a surface silicon atom bonded to oxygen inserted in the dimer bond. In this scheme an excited electron is transferred from the adsorbate to the substrate, which is the reverse direction to the substrate-mediated excitation frequently assumed in surface photochemistry.

Journal Article↗
PubMed2003

From metal complexes to fullerene arrays: exploring the exciting world of supramolecular photochemistry fifteen years after its birth.

After over 15 years of extensive research work in many laboratories worldwide, supramolecular photochemistry is a well-established and highly recognized branch of science. A brief retrospective view on the birth and infancy of this research area is given and some of the latest developments are discussed. In supramolecular photochemistry Ru(II) and Cu(I) diimmine complexes and C60 fullerenes are some of the most widely investigated chromophores and over the years big efforts have been made to implement and tune their photophysical and excited state properties, which are briefly reviewed. Thanks to a huge amount of synthetic and analytical research work, it has been possible to insert or combine these organic and inorganic subunits in a variety of fascinating supramolecular architectures. Some results concerned with photoinduced processes occurring in dyads, triads, catenanes, rotaxanes, dendrimers, and protonated self-assembled architectures are briefly illustrated. The overall picture stemming form the current state of the art in supramolecular photochemistry is that of a discipline gaining an increasing degree of multidisciplinarity. Interconnections with biology, physics and information technology are being established at a very fast pace, suggesting a bright future for this still young research field.

Journal Article↗
PubMed2004

Photosynthetic pigment composition and photosystem II photochemistry of wheat ears.

The characteristics of pigment composition and photosystem II (PSII) photochemistry in the flag leaf and ear parts of wheat (Triticum aestivum L.) grown in the field was compared. At the early stage of flowering, awns and the flag leaf showed the highest values in the maximal efficiency of PSII photochemistry (Fv/Fm), actual PSII efficiency (phi(PSII)), photochemical quenching (qP), and the efficiency of excitation capture by open PSII centres (Fv/F'm), followed by glumes, lemmas, and paleae, respectively except that no differences in F'v/F'm were observed among glumes, leamms, and paleae. With progressing grain filling, there was a change in the photosynthetic pigment stoichiometry. In the ear parts, neoxanthin and antheraxanthin decreased equally with chlorophyll levels. Lutein and zeaxanthin decreased less than chlorophyll levels while beta-carotene and violaxanthin decreased faster than chlorophyll levels. No big differences in pigment composition were observed among different ear parts. For the flag leaf, neoxanthin and beta-carotene decreased concomitantly with chlorophyll, whereas lutein and xanthophyll cycle pigment were less affected, leading to increases in lutein/chlorophyll and xanthophyll cycle pigment/chlorophyll ratios. Fv/Fm, phi(PSII), qP, and F'v/F'm decreased gradually in the flag leaf and ear parts but to different extents. The largest changes were observed in awns, followed by the lemmas of floret 2, the lemmas of floret 1, glumes, and the flag leaf, respectively. The results suggest that during grain filling, a down-regulation of PSII associated with an increase of the de-epoxidation state of the xanthophyll cycle carotenoids occurred in the flag leaf but not in the ear parts.

Chlorophyll↗
PubMedDate not supplied

Recent aspects of the photochemistry of nucleic acids and related model compounds.

This survey focuses on recent developments in the far ultraviolet photochemistry of nucleic acids and related model compounds. The photoproducts discussed are the cyclobutidipyrimidines, the pyrimidine-pyrimidone adducts, the purine-pyrimidine adducts and the addition products of amino acids to pyrimidine bases. The specific aspects of the high-intensity laser photochemistry of nucleic acid components are also briefly reviewed.

Amino Acids↗
PubMed2000

Laser photochemistry of molecular oxygen.

Molecular oxygen poses very difficult challenges in molecular photochemistry due to the extreme weakness of its ultraviolet absorption spectrum. In the past few years a new technique called velocity map imaging has been able to overcome many of the experimental obstacles to the study of this all-important atmospheric species. In this Account several aspects of laser photochemistry of O(2) will be described, including the unraveling of the Herzberg and Schumann-Runge continua using velocity map imaging.

Lasers↗
PubMed1990

Reaction center photochemistry of Heliobacterium chlorum.

Reaction center photochemistry in Heliobacterium chlorum has been investigated by using EPR and flash absorption spectroscopy at low temperatures. The following results were obtained. At 5 K, in the presence of ascorbate, continuous illumination resulted in the formation of P798+ and a reduced iron-sulfur center designated FB (gz = 2.07, gy = 1.93, gx = 1.89). This state was stable at low temperatures, but the yield for this reaction was low, and it was estimated that it occurred only in about 3% of the centers upon the first flash. After continuous illumination of a dilute sample for 10 min, still only half of the centers attained this state. In most centers, flash excitation at 5 K produced a state which recombined with time constants of 2.5 ms (congruent to 80%) and 850 microseconds (congruent to 20%). These two phases were differently influenced by the redox state of the reaction center, indicating that two different acceptors were involved in the recombination reactions. When continuous illumination was given at 200 K, a second center, designated FA, was additionally reduced (gz = 2.05, gy = 1.95, gx = 1.90). High concentrations of dithionite resulted in the chemical reduction of FB and of most of FA; illumination at 200 K resulted in the further reduction of FA. Two triplet states were identified by EPR and optical spectroscopy. The amplitude of the narrower triplet (magnitude of D = 226 x 10(-4) cm-1) varied with the redox state of the iron-sulfur centers and was influenced by a component thought to be a quinone undergoing double reduction. It correlated with a triplet state observed by flash absorption spectroscopy showing a bleaching at 798 nm and is attributed to a triplet state formed by charge recombination in the reaction center. Its narrowness is taken as an indication of its origin on a pair of bacteriochlorophylls, and its orientation indicates an orientation of the chlorophyll ring plane perpendicular to the membrane plane. The second triplet had a wider splitting (magnitude of D = 242 x 10(-4) cm-1), did not vary systematically with redox conditions, corresponds to an optical spectrum with a maximum at 812 nm, and is not ordered in the membrane. It was thus attributed to a triplet located on a BChl g monomer in the antenna. The reaction center photochemistry in H. chlorum is comparable in many respects to that of photosystem I and green sulfur bacteria. Earlier contrasting conclusions are discussed and rationalized in light of the present results.

Bacteria↗
PubMed2004

Surface photochemistry of pesticides: an approach using diffuse reflectance and chromatography techniques.

The photochemistry of pesticides triadimenol and triadimefon was studied on cellulose and beta-cyclodextrin (beta-CD) in controlled and natural conditions, using diffuse reflectance techniques and chromatographic analysis. The photochemistry of triadimenol occurs from the chlorophenoxyl moiety, while the photodegradation of triadimefon also involves the carbonyl group. The formation of 4-chlorophenoxyl radical is one of the major reaction pathways for both pesticides and leads to 4-chlorophenol. Triadimenol also undergoes photooxidation and dechlorination, leading to triadimefon and dechlorinated triadimenol, respectively. The other main reaction process of triadimefon involves alpha-cleavage from the carbonyl group, leading to decarbonylated compounds. Triadimenol undergoes photodegradation at 254 nm but was found to be stable at 313 nm, while triadimefon degradates in both conditions. Both pesticides undergo photochemical decomposition under solar radiation, being the initial degradation of rate per unit area of triadimefon 1 order of magnitude higher than the observed for triadimenol in both supports. The degradation rates of the pesticides were somewhat lower in beta-CD than on cellulose. Photoproduct distribution of triadimenol and triadimefon is similar for the different irradiation conditions, indicating an intramolecular energy transfer from the chlorophenoxyl moiety to the carbonyl group in the latter pesticide.

Adsorption↗
PubMed2004

Photochemistry of A1E, a retinoid with a conjugated pyridinium moiety: competition between pericyclic photooxygenation and pericyclization.

The photochemistry of the retinoid analogue A1E shows an oxygen and solvent dependence. Irradiation of A1E with visible light (lambda(irr) = 425 nm) in methanol solutions resulted in pericyclization to form pyridinium terpenoids. Although the quantum yield for this cyclization is low (approximately 10(-4)), nevertheless the photochemical transformation occurs with quantitative chemical yield with remarkable chemoselectivity and diastereoselectivity. Conversely, irradiation of A1E under the same irradiation conditions in air-saturated carbon tetrachloride or deuterated chloroform produced a cyclic 5,8-peroxide as the major product. Deuterium solvent effects, experiments utilizing endoperoxide, phosphorescence, and chemiluminescence quenching studies strongly support the involvement of singlet oxygen in the endoperoxide formation. It is proposed that, upon irradiation, in the presence of oxygen, A1E acts as a sensitizer for generation of singlet oxygen from triplet oxygen present in the solution; the singlet oxygen produced reacts with A1E to produce cyclic peroxide. Thus, the photochemistry of A1E is characterized by two competing reactions, cyclization and peroxide formation. The dominant reaction is determined by the concentration of oxygen, the concentration of A1E, and the lifetime of singlet oxygen in the solvent employed. If the lifetime of singlet oxygen in a given solvent is long enough, then oxidation (peroxide formation) is the major reaction. If the singlet oxygen produced is quenched by the protonated solvent molecules faster than singlet oxygen reacts with A1E, then cyclization dominates.

Cyclization↗
PubMed2003

General patterns in the photochemistry of pregna-1,4-dien-3,20-diones.

The photochemistry of six pregna-1,4-dien-3,20-diones has been compared and found to involve both the cyclohexadienone moiety in ring A and the isolated ketone at C-20. The two reactions take place proportionally to the fraction of light absorbed by each chromophore. The cross-conjugated ketone absorbs predominantly or exclusively at both 254 and 366 nm and undergoes the "lumi" rearrangement to bicyclo[3.1.0]hex-3-en-2-one. The quantum yield of the reaction diminished somewhat with increasing lambda(exc), e.g., for prednisolone Phi(254) (nm) = 0.42, Phi(366) (nm) = 0.3. A much stronger lowering is caused by halogen substitution in position 9 (by a factor of 3 for F, >50 for Cl), apparently due to a shortened triplet lifetime caused by heavy atom effect. At 310 nm, both chromophores absorb to a comparable degree and both may react. The reaction at C(20) ketone involves either quite efficient alpha-cleavage (C(17)-C(20)) for compounds bearing an acetal or hydroxyl function at C(17) or less effective (by a factor of ca. 10) hydrogen abstraction from the 18-methyl group in the other cases (finally resulting in Norrish II fragmentation or Yang cyclization). The results allow generalizing how the substitution pattern surrounding each chromophore affects the photoreactivity at that site and the competition between the two modes, allowing predicting the photochemistry of this family of antiinflammatory drugs.

Anti-Inflammatory Agents↗