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PubMed1991

Differences in the photobleaching process between 7-cis- and 11-cis-rhodopsins: a unique interaction change between the chromophore and the protein during the lumi-meta I transition.

The photochemical and subsequent thermal reactions of 7-cis-rhodopsin prepared from cattle opsin and 7-cis-retinal were investigated by low-temperature spectrophotometry and laser photolysis, and compared with those of 11-cis-rhodopsin prepared from cattle opsin and 11-cis-retinal. Low-temperature experiments revealed that the absorption maxima of batho and lumi intermediates from 7-cis-rhodopsin were at slightly shorter wavelengths than those of 11-cis-rhodopsin while the meta I intermediates of both rhodopsin isomers showed the same absorption maxima. Kinetic experiments of the photobleaching process of 7-cis-rhodopsin using picosecond and nanosecond laser pulses revealed the formation of intermediates corresponding to the batho, lumi, meta I, and meta II intermediates from 11-cis-rhodopsin. An intermediate of 7-cis-rhodopsin corresponding to photorhodopsin (a precursor of bathorhodopsin), however, was not detected. Batho and lumi intermediates from 7-cis-rhodopsin had shorter lifetimes (approximately 40 ns and 300 microseconds) than those of 11-cis-rhodopsin (250 ns and 800 microseconds), but the lifetime of the meta I intermediate from 7-cis-rhodopsin was identical with that from 11-cis-rhodopsin (12 ms). These results indicate that the difference in configuration of the original chromophore between 7-cis- and 11-cis-rhodopsins is a cause of different chromophore-opsin interactions in the batho and lumi stages, while in the meta I stage the difference has disappeared by the relaxation of the protein near the chromophores. A possible interaction change between the 9-methyl group of the chromophore and its neighboring protein during the lumi-meta I transition will be discussed.

Animals↗

PubMed1996

The light intensity dependence of protochlorophyllide photoconversion and its significance to the catalytic mechanism of protochlorophyllide reductase.

The light-dependent step in chlorophyll synthesis by higher plants involves hydrogen transfer from NADPH+ to the porphyrin protochlorophyllide catalysed by the photoenzyme protochlorophyllide reductase. The light intensity dependence of the process has been studied in vitro using wheat etioplast membranes. The results suggest that a single photochemical event is involved in the photoconversion. In support of this conclusion we also demonstrate that illumination of these membranes with a train of ultrashort (150 fs) flashes resulted in chlorophyll accumulation. The significance of the findings in terms of possible mechanisms for the reductase are discussed.

Catalysis↗

PubMed2002

Photocatalytic degradation of dyes in aqueous solution operating in a fluidised bed reactor.

This work reports a preliminary design of a new photochemical reactor and its application to photochemical degradation of two dyes, Crystal Violet and Azure B, operating in both batch and continuous processes. A novel kind of photocatalyst, consisting of ZnO immobilised in alginate gel beads, which is able to photodegrade organic dyes effectively, has been employed in the present study. When this photocatalyst, at a concentration of 1 g of ZnO per litre of alginate gel at 3%, was employed in batch process, almost total decolourisation of Crystal Violet in reaction times lower than 120 min was observed. Operating in continuous process at different residence times, it was possible to achieve a total decolourisation of both Crystal Violet and Azure B. Moreover, the total organic carbon content (TOC) was reduced to 90% in the former and to 52% in the latter. These results indicated that the photoreactor developed in the present work was able to degrade effectively dyes of different structures, revealing the non-specificity of the system.

Alginates↗

PubMed2005

A highly emissive Cu2N2 diamond core complex supported by a [PNP]- ligand.

A Cu2N2 diamond core structure, {(PNP)CuI}2 (2), supported by a [PNP]- ligand (1) ([PNP]- = bis(2-(diisobutylphosphino)phenyl)amide) has been prepared. 2 is highly emissive at ambient temperature in both the solid and solution states and is characterized by a relatively long-lived excited state (tau > 10 mus) and an unusually high quantum yield (phi > 0.65). These observations are consistent with a low degree of structural reorganization between the ground state of 2 and its excited state *2, and also with a high degree of steric protection of the two copper centers of 2 afforded by the bulky [PNP]- ligand. An estimate for the excited-state reduction potential of *2 (ca. -3.2 V vs Fc+/Fc), and the availability of two well-separated and reversible ground-state redox processes, suggests that bimetallic copper systems of these types may be interesting candidates to consider for photochemically driving multielectron redox transformations.

Journal Article↗

PubMed2004

Role of red carotenoids in photoprotection during winter acclimation in Buxus sempervirens leaves.

The red leaf coloration of several plant species during autumn and winter is due to the synthesis of phenolic compounds such as anthocyanins or red carotenoids. The latter occur very rarely and are non-ubiquitous and taxonomically restricted compounds. The present study shows that the leaves of common box ( Buxus sempervirens L.) accumulate red carotenoids (eschscholtzxanthin, monoanhydroeschscholtzxanthin, anhydroeschscholtzxanthin) as a response to photoinhibitory conditions during winter acclimation. These compounds are produced in a coordinated manner with the operation of other photoprotective systems: accumulation and sustained deepoxidation of VAZ pigments with a concomitant decrease in maximal photochemical efficiency, accumulation of alpha-tocopherol and a gradual decrease on chlorophyll content. All these processes were reversed when the photosynthetic tissues were transferred from photoinhibitory winter conditions to room temperature for 9 days. Buxus leaves showed a large degree of phenotype variation in the degree of reddening, ranging from green to orange. The differences in colour pattern were mainly due to differences in the accumulation of red carotenoids and xanthophyll esters. Red pigments were mainly anhydroeschscholtzxanthin and esters of eschscholtzxanthin. Conversely to fruit or petal chromoplasts, the plastids of red leaves in this species are not the terminal differentiated state but are able to redifferentiate again to chloroplasts. Their photoprotective role during winter as a light screen system or as antioxidants, in a similar way to other red pigments, and their implications on the wide ecological tolerance of this evergreen species are discussed.

Acclimatization↗

PubMed1997

Ozone-induced DNA strand breaks In guinea pig tracheobronchial epithelial cells.

Ozone (O3), the major oxidant of photochemical smog, is thought to be genotoxic and a potential respiratory carcinogen or promoter of carcinogenic processes. Because of oxidative reactions with the mucus in the upper airway, O3 reaction products are able to penetrate into the tracheobronchial epithelial (TE) cells. The carcinogenic effects of O3 on the TE cells are especially of interest since most previous studies have focused on the morphology or permeability changes of tracheas only. Therefore, the objective of this study was to examine the potential O3 genotoxicity in TE cells after an in vivo exposure, using DNA strand breaks as an index. Two-month-old male Dunkin-Hartley guinea pigs, specific pathogen free, 4 in each group, were exposed to 1.0 ppm O3 for 0, 12, 24, 48, 72, or 96 h. Animals exposed to filtered air without O3 exposure were used as controls. After O3 exposure, the trachea with two main bronchi was removed from each animal, and TE cells were isolated and employed for determination of DNA strand breaks by fluorometric analysis of DNA unwinding (FADU). The statistical significance level was set at alpha = .05. Compared with controls, ozone exposure did not alter the TE cell yield or viability, but caused an increase in protein content in tracheal lavage and an increase in DNA strand breaks. The amount of DNA left in the alkali lysate of TE cells found at 72 h exposure was significantly decreased from controls for 3 different alkali incubation times. An increase of the double-stranded DNA left in the alkali lysate of TE cells was observed at 96 h of exposure and approached the value of 24 h of exposure. The same pattern was seen with all 3 different alkali incubation times at 15 degrees C. One Qd unit was estimated to correspond to 100 strand breaks per cell. The Qd was also used as an indicator for O3 damage. Compared to controls, the Qd increases significantly after 1 ppm O3 exposure for 72 h, regardless of the alkali incubation time at 15 degrees C.

Animals↗

PubMed2000

Emission of ozone and organic volatiles from a selection of laser printers and photocopiers.

To estimate the impact of office equipment on the quality of indoor air, the emission of ozone and organic volatiles was measured from one photocopier and four laser printers, three of which operated according to traditional corona discharge technology. The laser printers equipped with traditional technology emitted significant amounts of ozone and formaldehyde. Lesser amounts of other volatile aldehydes were emitted during printing. The photocopier emitted mainly ozone. In a well-ventilated office environment, the amounts encountered here for individual volatiles were within recommended maximum exposure limits for a reasonable density of printers. Because it is not known whether the concentration of irritating volatiles, such as formaldehyde, should be kept lower in an ozone rich environment or not, and because emissions in the immediate vicinity of the printers exceeded recommendations, the authors recommend that laser printers equipped with the traditional corona rods not be placed beside or immediately at the working site of office personnel. This way, ozone concentrations can be kept below recommended maximum exposure limits, provided that the ventilation rate is adequate. Further, it seems that if a reliable quantitative comparison of total organic volatiles prior to and during printing is to be made, the inertness of the sorbent toward ozone should be confirmed.

Air Pollutants, Occupational↗

PubMed2002

Linking drought-resistance mechanisms to drought avoidance in upland rice using a QTL approach: progress and new opportunities to integrate stomatal and mesophyll responses.

The advent of saturated molecular maps promised rapid progress towards the improvement of crops for genetically complex traits like drought resistance via analysis of quantitative trait loci (QTL). Progress with the identification of QTLs for drought resistance-related traits in rice is summarized here with the emphasis on a mapping population of a cross between drought-resistant varieties Azucena and Bala. Data which have used root morphological traits and indicators of drought avoidance in field-grown plants are reviewed, highlighting problems and uncertainties with the QTL approach. The contribution of root-growth QTLs to drought avoidance appears small in the experiments so far conducted, and the limitations of screening methodologies and the involvement of shoot-related mechanisms of drought resistance are studied. When compared to Azucena, Bala has been observed to have highly sensitive stomata, does not roll its leaves readily, has a greater ability to adjust osmotically, slows growth more rapidly when droughted and has a lower water-use efficiency. It is also a semi-dwarf variety and hence has a different canopy structure. There is a need to clarify the contribution of the shoot to drought resistance from the level of the biochemistry of photosynthesis through stomatal behaviour and leaf anatomy to canopy architecture. Recent advances in studying the physical and biochemical processes related to water use and drought stress offer the opportunity to advance a more holistic understanding of drought resistance. These include the potential use of infrared thermal imaging to study energy balance, integrated and online stable isotope analysis to dissect processes involved in carbon dioxide fixation and water evaporation, and leaf fluorescence to monitor photosynthesis and photochemical quenching. Justification and a strategy for this integrated approach is described, which has relevance to the study of drought resistance in most crops.

Adaptation, Physiological↗

PubMed1991

Post-PCR sterilization: development and application to an HIV-1 diagnostic assay.

We have developed an effective post-PCR sterilization process and have applied the procedure to a diagnostic assay for HIV-1. The method, which is based on isopsoralen photochemistry, satisfies both the inactivation and hybridization requirements of a practical sterilization process. The key feature of the technique is the use of isopsoralen compounds which form covalent photochemical adducts with DNA. These covalent adducts prevent subsequent extension of previously amplified sequences (amplicons) by Taq polymerase. Isopsoralens have minimal inhibitory effect on the PCR, are activated by long wavelength ultraviolet light, provide sufficient numbers of covalent adducts to impart effective sterilization, modify the amplified sequence such that it remains single-stranded, and have little effect on subsequent hybridization. The sterilization procedure can be applied to a closed system and is suitable for use with commonly used detection formats. The photochemical sterilization protocol we have devised is an effective and pragmatic method for eliminating the amplicon carryover problem associated with the PCR. While the work described here is limited to HIV-1, proper use of the technique will relieve the concern associated with carryover for a wide variety of amplicons, especially in the clinical setting.

Base Sequence↗

PubMed1976

[EPR and low temperature spectrophotometric study of the phototransformation products of bacteriorhodopsin].

It is shown that BR and intermediate products of its phototransformation P600, P550 and P415 (the maximum at -196 degrees C at 419 nm) are not paramagnetic. Illumination of samples containing P415 (P419) at -- 196 degrees C with light in the region of 360-480 nm results in the formation of paramagnetic centres with a sunglet spectrum deltaH=18 Oe and g=2.002 (R1). In parallel formation of a new photoproduct P421 in the absorption spectrum is observed. During subsequent heating at -140 degrees C formation of an asymmetric signal with deltaH=45 Oe and g=2.006 and g=2.03 was observed. In the absorption spectra a dark transition. P421-P565 was observed under the same conditions. P565 differs from initial BR P570 as to its photochemical properties. R1 is identified as retinal radical, R2 as a peroxide radical of the BR-complex lipids. Paramagnetic, spectral, and photochemical properties of some products of BR transformation are compared. A scheme of oxidative-phosphorylation processes with participation of Mn ions in BR phototransformation.

Bacteriorhodopsins↗

PubMedDate not supplied

[New intermediate reactions in the process of protochlorophyllide photoreduction].

It has been found that formation of non-fluorescine intermediate product with the absorption maximum 690 nm during photochemical hydration of protochlorophyllide in ethiolated plant leaves can also proceed at -196 degrees C. During dark transformation of this product two forms of the pigment with the fluorescence maxima 695 and 685 nm are initiated, the first one preceding. The dependence of the spectral changes discovered on the light spectral composition is interpreted as a consequence of switching on of the parallel reactions of precursors or as a result of photoreversible reaction into chlorophill biosynthesis.

Chlorophyll↗

PubMed2006

Photochemical approach to probing different DNA structures.

The various conformations of DNA--the A, B, and Z forms, the protein-induced DNA kink, and the G-quartet form--are thought to play important biological roles in processes such as DNA replication, gene expression and regulation, and the repair of DNA damage. The investigation of local DNA conformational changes associated with biological events is therefore essential for understanding the function of DNA. In this Minireview, we discuss the use of photochemical dehalogenation of 5-halouracil-containing DNA to probe the structure of DNA. Hydrogen abstraction by the resultant uracil-5-yl radicals is atom-specific and highly dependent on the structure of the DNA, suggesting that this photochemical approach could be applied as a probe of DNA conformations in living cells.

Crystallography, X-Ray↗

PubMed1993

Picosecond energy transfer and trapping kinetics in living cells of the green bacterium Chloroflexus aurantiacus.

The excitation energy transfer and trapping processes in intact cells of Chloroflexus aurantiacus were studied by picosecond time-resolved fluorescence spectroscopy. The fluorescence decay kinetics is investigated over the near infrared emission range between 730 nm and 920 nm using various excitation wavelengths and excitation intensities. The data were analyzed by global decay analysis and are presented as decay-associated spectra (DAS). The specific dependence of the decay kinetics on the excitation wavelength and on the photochemical redox state of the reaction center (RC) allows the identification of the energy transfer and trapping components. The DAS provide evidence for two chlorosomal energy transfer processes. The first one occurs between the chlorosomal bacteriochlorophyll (BChl)-c and the BChl-a792 complex (B792) in the chlorosomal baseplate with an equilibration time constant of 15-16 ps, while the second one occurs from the B792 pigments to the BChl-a806 pigments in the B806-866 complex with a time constant of 35-40 ps. The overall energy trapping process in whole cells is mainly determined by the kinetics of the primary charge separation process in the RCs. With open RCs (QA oxidized) the trapping time constant is 70-90 ps, while the trapping process with closed RCs (QA reduced) takes as long as 180-200 ps. The results on whole cells reported here are interpreted in conjunction with those reported earlier for the various isolated complexes, i.e., two different chlorosome preparations (Holzwarth, A.R., Müller, M.G. and Griebenow, K. (1990) J. Photochem. Photobiol. B 5, 457-465), the B806-866 complex (Griebenow, K., Müller, M.G. and Holzwarth, A.R. (1991) Biochim. Biophys. Acta 1059, 226-232) and isolated reaction centers (Müller, M.G., Griebenow, K. and Holzwarth, A.R. (1991) Biochim. Biophys. Acta 1098, 1-12). Based on these data, a unified and self-consistent scheme for the primary processes in the whole photosynthetic system of C. aurantiacus is presented. The BChl antenna pigment groups are arranged to form a linear energy transfer cascade with four energy transfer steps from shorter-wavelength- to longer-wavelength-absorbing antenna pools. The overall fluorescence decay kinetics of the photosynthetic system of C. aurantiacus turns out to be 'trap-limited' by the reaction center rather than 'diffusion-limited' by the energy transfer processes.

Cyanobacteria↗

PubMed2002

Molecular triads composed of ferrocene, C60, and nitroaromatic entities: electrochemical, computational, and photochemical investigations.

Synthesis and physicochemical characterization of a series of molecular triads composed of ferrocene, C(60), and nitroaromatic entities are reported. Electrochemical studies revealed multiple redox processes involving all three redox active ferrocene, C(60), and nitrobenzene entities. Up to eight redox couples within the accessible potential window of o-dichlorobenzene containing 0.1 M (TBA)ClO(4) are observed. A comparison between the measured redox potentials with those of the starting compounds revealed absence of any significant electronic interactions between the different redox entities. The geometric and electronic structure of the triads are elucidated by using ab initio B3LYP/3-21G methods. In the energy-optimized structures, as predicted by electrochemical studies, the first HOMO orbitals are found to be located on the ferrocene entity, while the first LUMO orbitals are mainly on the C(60) entity. The coefficients of the subsequent LUMO orbitals track the observed site of electrochemical reductions of the triads. The photochemical events of the triads are probed by both steady-state and time-resolved techniques. The steady-state emission intensities of the triads and the starting dyad, 2-(ferrocenyl)fulleropyrrolidine, are found to be completely quenched compared to fulleropyrrolidine bearing no redox active substituents. The subpicosecond and nanosecond transient absorption spectral studies revealed efficient charge separation (and rapid charge recombination) in the triads, and this has been attributed to the close spacing of the redox entities of the triad to one another.

Computational Biology↗

PubMed1990

NMR and photo-CIDNP studies of human proinsulin and prohormone processing intermediates with application to endopeptidase recognition.

The proinsulin-insulin system provides a general model for the proteolytic processing of polypeptide hormones. Two proinsulin-specific endopeptidases have been defined, a type I activity that cleaves the B-chain/C-peptide junction (Arg31-Arg32) and a type II activity that cleaves the C-peptide/A-chain junction (Lys64-Arg65). These endopeptidases are specific for their respective dibasic target sites; not all such dibasic sites are cleaved, however, and studies of mutant proinsulins have demonstrated that additional sequence or structural features are involved in determining substrate specificity. To define structural elements required for endopeptidase recognition, we have undertaken comparative 1H NMR and photochemical dynamic nuclear polarization (photo-CIDNP) studies of human proinsulin, insulin, and split proinsulin analogues as models of prohormone processing intermediates. The overall conformation of proinsulin is observed to be similar to that of insulin, and the connecting peptide is largely unstructured. In the 1H NMR spectrum of proinsulin significant variation is observed in the line widths of insulin-specific amide resonances, reflecting exchange among conformational substates; similar exchange is observed in insulin and is not damped by the connecting peptide. The aromatic 1H NMR resonances of proinsulin are assigned by analogy to the spectrum of insulin, and assignments are verified by chemical modification. Unexpectedly, nonlocal perturbations are observed in the insulin moiety of proinsulin, as monitored by the resonances of internal aromatic groups. Remarkably, these perturbations are reverted by site-specific cleavage of the connecting peptide at the CA junction but not the BC junction. These results suggest that a stable local structure is formed at the CA junction, which influences insulin-specific packing interactions. We propose that this structure (designated the "CA knuckle") provides a recognition element for type II proinsulin endopeptidase.

Amino Acid Sequence↗

PubMed2001

The photochemistry of 2-(1-naphthyl)ethyl benzoates: cycloaddition and intramolecular exciplex formation.

The photochemistry of the 2-(1-naphthyl)ethyl benzoates 6 and 7 was examined in order to compare them to previously studied 2-arylethyl 4-cyanobenzoates that underwent a Norrish Type II fragmentation. The 1-naphthyl group was incorporated to provide a fluorescent chromophore for probing the intramolecular electron transfer proposed previously for the mechanism. The naphthalene fluorescence was quenched for both 6 and 7 although at very different rates. For 6, with the higher thermodynamic driving force (-68.9 kJ/mol), intramolecular electron transfer was fast in all solvents, independent of their polarity (cyclohexane to methanol). For 7, with the lower driving force (-26.5 kJ/mol) the process was fast only in polar solvents. Exciplex emission, observed for 6 (but not for 7), exhibited a large solvatochromic effect possibly indicating a high dipole moment (28 D) in polar solvents (stretched conformation) but a lower one (17 D) in nonpolar solvents (folded conformation). Finally, the 4-cyanobenzoate 6 was very unreactive photochemically. In contrast, benzoate 7 underwent a 2 + 2 cycloaddition of the ester carbonyl to the naphthalene ring to give products 8 and 9, a process for which we have found no precedent.

Journal Article↗

PubMed1991

Plasma discharge in N2 + CH4 at low pressures: experimental results and applications to Titan.

We report the yields of gaseous hydrocarbons and nitriles produced in a continuous flow, low-dose, cold plasma discharge excited in a 10% CH4, 90% N2 atmosphere at 295 K and pressures p of 17 and 0.24 mbar, and use the results to compute expected abundances of minor constituents in Titan's atmosphere. These experiments are, by design, relevant to the atmospheric chemistry induced by cosmic rays in Titan's troposphere and (at the lower pressure) to chemistry initiated by Saturnian magnetospheric electrons and other charged particle sources which excite stratospheric aurorae. At p = 17 mbar, 59 gaseous species including 27 nitriles are detected in overall yield 4.0 (C + N) atoms incorporated into products per 100 eV (heV). At p = 0.24 mbar, 19 species are detected, including six nitriles and three other unidentified N-bearing compounds; the yield is 0.79 (C + N)/heV, a mild decrease with pressure. The types of molecules formed change more markedly, with high degrees of multiple bonding at 0.24 mbar prevailing over more H-saturated molecules at 17 mbar. The molecules and yields at 0.24 mbar bear a striking resemblance to the minor constituents found in Titan's atmosphere, all of which are abundant products in the laboratory experiment. Using the altitude-integrated flux of charged particle energy deposition at Titan, the laboratory yields at p = 0.24 mb, and a simple eddy mixing model, we compute absolute stratospheric column abundances and mole fractions. These are found to be in very good agreement with the Voyager IRIS observations. Except for the primarily photochemical products, C2H6 and C3H8, the match is much better than that obtained by photochemical-kinetic models, demonstrating that properly designed laboratory experiments are directly applicable to modeling radiation-chemical processes in planetary atmospheres. On the basis of this agreement we expect CH3-C triple bond N (ethanenitrile = acetonitrile) CH2=CH-CH=CH2 (1,3-butadiene), CH2=C=CH2 (1,2-propadiene = allene), and CH2=CH-C triple bond CH (1-buten-3-yne) to be present at mol fractions X > 10(-9), and CH2=CH-C triple bond N (propenenitrile), CH3-CH=CH2 (propene), and CH3-CH2-C triple bond N (propanenitrile) at X > 10(-10) in Titan's atmosphere.

Astronomical Phenomena↗

PubMedDate not supplied

Rhodopsin(s) in eubacteria.

While the biochemical basis of photosynthesis by bacteriochlorophyll-based reaction centres in purple phototrophic Eubacteria and retinal-based bacteriorhodopsin in the Archaebacterium Halobacterium salinarium has been elucidated in great detail, much less is known about photosensory signal transduction; this is especially the case for Eubacteria. Recent findings on two different photosensory proteins in two different Eubacteria, which both show clear resemblances to the rhodopsins, will be presented. The photoactive yellow protein (PYP) from the purple phototrophic organism Ectothiorhodospira halophila probably functions as the photoreceptor for a new type of negative phototaxis response and has been studied in some detail with respect to its structural and photochemical characteristics. On basis of crystallographic an photochemical data it has been proposed that PYP contains retinal as a chromophore. However, we have unambiguously demonstrated that the PYP chromophore is different from retinal, in spite of the fact that PYP's photochemical properties show striking similarities with the rhodopsins. The cyanobacterium Calothrix sp. displays complementary chromatic adaptation, a process in which the pigment composition of the phycobilisomes is adjusted to the spectral characteristics of the incident light. In orange light the blueish chromophore phycocyanin is present, in green light the reddish phycoerythrin is synthesized. On the basis of the action spectrum of this adaptation process, we hypothesized that a rhodopsin is the photosensor in this process. In line with this, we found that nicotine, an inhibitor of the biosynthesis of beta-carotene (which is the precursor of retinal), abolishes chromatic adaptation. Direct proof of the involvement of a photosensory rhodopsin was obtained in experiments in which the chromatic adaptation response was restored by the addition of retinal to the cultures. The two photosensory proteins mentioned above represent the first examples of eubacterial photoreceptors that can be studied at a molecular level. Our current knowledge on these two proteins and their status as retinal proteins will be reviewed.

Journal Article↗