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PubMed2006

Photooxidative degradation of dye pollutants accumulated in self-assembled natural polyelectrolyte microshells under visible radiation.

A novel route to facilitate the degradation of dye pollutants, a class of well-known recalcitrant organic pollutants, is reported. This new approach is based on a natural polyelectrolyte microshell that was preformed by the alternate adsorption of the anionic alginate sodium (ALG) and the cationic chitosan (CHI) onto weakly cross-linked melamine formaldehyde (MF) colloidal particles, and the subsequent sacrifice of MF templates in 0.1 M HCl. The as-prepared microshells could accumulate rhodamine B (RhB) and fluorescein (Flu) efficiently in water under ordinary conditions by means of a simple mixing process. The photodegradation of the accumulated RhB and Flu was examined in the presence of Fe3+ and H2O2 under visible radiation. The accumulated RhB and Flu are rapidly degraded and the assembled shells maintain their intact spherical shape throughout the photoreaction process. Results of recycling degradation experiments and the photochemical behavior of the shells, as demonstrated by confocal laser scanning microscopy (CLSM), UV-visible spectroscopy, and scanning force microscopy (SFM), further suggest that the constructed shells may be used as environmentally friendly microcontainers for the elimination of dyes in wastewater.

Journal Article↗
PubMed2004

Destruction of organic pollutants in reusable wastewater using advanced oxidation technology.

This work studied the destruction of various M-EDTA complexes and trace organic pollutants in treated reusable wastewater under advanced oxidation using UV irradiation and ozonation. Effect of dosage of hydrogen peroxide and acidity of reaction matrices on oxidation efficiencies were investigated. The rate constant of mineralization presents a decreasing trend as Fe(III)-EDTA > Fe(II)-EDTA > Al(III)-EDTA > Pb(II)-EDTA > Na(I)-EDTA > Zn(II)-EDTA > Cu(II)-EDTA. The mineralization efficiencies using ozone alone are 15%, 40% and 15% for the water samples after reverse osmosis (RO), microfiltration (MF) and superfiltration (SF) processes, respectively. The presence of hydrogen peroxide in photochemical reaction matrixes can effectively enhance the mineralization of organic carbon species. When 150 mg l(-1) of H2O2 was added in the effluents, the mineralization markedly increased to 80%, 92% and 89%, respectively.

Edetic Acid↗
PubMed2001

Photochemical instability of CdSe nanocrystals coated by hydrophilic thiols.

The photochemical instability of CdSe nanocrystals coated by hydrophilic thiols was studied nondestructively and systematically in water. The results revealed that the photochemical instability of the nanocrystals actually included three distinguishable processes, namely the photocatalytic oxidation of the thiol ligands on the surface of nanocrystals, the photooxidation of the nanocrystals, and the precipitation of the nanocrystals. At first, the thiol ligands on the surface of a nanocrystal were gradually photocatalytically oxidized using the CdSe nanocrystal core as the photocatalyst. This photocatalytic oxidation process was observed as a zero-order reaction in terms of the concentration of the free thiols in the solution. The photogenerated holes in a nanocrystal were trapped onto the thiol ligands bound on the surface of the nanocrystal, which initiated the photooxidation of the ligands and protected the nanocrystal from any photooxidation. After nearly all of the thiol ligands on the surface of the nanocrystals were converted into disulfides, the system underwent several different pathways. If the disulfides were soluble in water, then all of the disulfides fell into the solution at the end of this initial process, and the nanocrystals precipitated out of the solution without much variation over their size and size distribution. When the disulfides were insoluble in water, they likely formed a micelle-like structure around the nanocrystal core and kept it soluble in the solution. In this case, the nanocrystals only precipitated after severe oxidation, which took a long period of time. If the system contained excess free thiol ligands, they replaced the photochemically generated disulfides and maintained the stability and solubility of the nanocrystals. The initiation stage of the photooxidation of CdSe nanocrystals themselves increased as the thickness and packing density of the ligand shell increased. This was explained by considering the ligand shell on the surface of a nanocrystal as the diffusion barrier of the oxygen species from the bulk solution into the interface between the nanocrystal and the surface ligands. Experimental results clearly indicated that the initiation stage of the photooxidation was not caused by the chemical oxidation of the system kept in air under dark conditions or the hydrolysis of the cadmium-thiol bonds on the surface of the nanocrystals, both of which were magnitudes slower than the photocatalytic oxidation of the surface ligands if they occurred at all. The results described in this contribution have already been applied for designing new types of thiol ligands which dramatically improved the photochemical stability of CdSe nanocrystals with a ligand shell that is as thin as approximately 1 nm.

Journal Article↗
PubMed2003

An experimental and theoretical study of the type C enone rearrangement: mechanistic and exploratory organic photochemistry.

We recently described a new photochemical rearrangement which we termed a Type C process. The reaction involves a delta to alpha aryl migration in 5-disubstituted cyclohexenones also having bulky C-3 substituents. In contrast to most cyclohexenone rearrangements, the reaction occurs via a twisted pi-pi excited triplet rather than the usual n-pi state. The electronic nature of the rearrangement was assessed using migration selectivity with p-anisyl and p-cyanophenyl groups. A synthesis of the reactants was elaborated, and the product structures were established by X-ray and NMR analysis. The reaction mechanism was established further with DFT and CASSCF computations. In the latter, localized NBO basis orbitals permitted proper selection of the active space. The nature of the diradical intermediates as well as the transition states was established computationally. Sensitization experiments with regioselectivities the same as those in direct irradiation confirmed the triplet multiplicity of the process.

Journal Article↗
PubMed2006

Charge-transfer transitions in protein circular dichroism calculations.

Charge-transfer transitions in proteins play a key role in many biophysical processes, from the behavior of redox proteins to photochemical reactions. We present ab initio calculations on a model dipeptide and more approximate calculations of the electronic excited states of proteins which, taken together, provide the most definitive assignment and characterization of charge-transfer transitions in proteins to date. We have calculated from first principles the electronic circular dichroism (CD) spectra of 31 proteins on the basis of their structures. Compared to previous studies, we achieve more accurate calculated CD spectra between 170 and 190 nm, owing mainly to the importance in alpha-helices of a charge-transfer transition from the lone pair on one peptide group to the pi* orbital on the next peptide group.

Acetamides↗
PubMed2005

Increase in tropospheric nitrogen dioxide over China observed from space.

Emissions from fossil fuel combustion and biomass burning reduce local air quality and affect global tropospheric chemistry. Nitrogen oxides are emitted by all combustion processes and play a key part in the photochemically induced catalytic production of ozone, which results in summer smog and has increased levels of tropospheric ozone globally. Release of nitrogen oxide also results in nitric acid deposition, and--at least locally--increases radiative forcing effects due to the absorption of downward propagating visible light. Nitrogen oxide concentrations in many industrialized countries are expected to decrease, but rapid economic development has the potential to increase significantly the emissions of nitrogen oxides in parts of Asia. Here we present the tropospheric column amounts of nitrogen dioxide retrieved from two satellite instruments GOME and SCIAMACHY over the years 1996-2004. We find substantial reductions in nitrogen dioxide concentrations over some areas of Europe and the USA, but a highly significant increase of about 50 per cent-with an accelerating trend in annual growth rate-over the industrial areas of China, more than recent bottom-up inventories suggest.

Air Pollutants↗
PubMed2001

Fast coordination changes in cytochrome c do not necessarily imply folding.

Folding of globular proteins occurs with rates that range from microseconds to minutes; consequently, it has been necessary to develop new strategies to follow the faster processes that exceed stopped-flow capabilities. Rapid photochemical methods have been employed to study the rate of folding of reduced cytochrome c. In this protein, the iron of the covalently bound heme binds a His and a Met, proximal and distal. Unfolding by guanidine or urea weakens the Fe-Met bond, and the reduced unfolded cytochrome c easily binds CO and other heme ligands, which would react slowly or not at all with the native protein. Therefore in the presence of CO, reduced cytochrome c unfolds at lower denaturant concentrations than in the absence of this ligand, and rapid photochemical removal of CO from unfolded cytochrome c, is expected to trigger at least an incomplete refolding. This approach is complicated by the breakage of the proximal His-Fe bond that may occur as a consequence of CO photodissociation in the unfolded cytochrome c because of the so-called base elimination mechanism. Rebinding of CO to the four-coordinate heme yields kinetic intermediates unrelated to folding. Our hypothesis is supported by parallel observations carried out with protoheme and microperoxidase.

Animals↗
PubMed2005

Photodecolorization of azo dyes by extracellular metabolites under fluorescent light and influence of operational parameters.

The contribution of extracellular metabolites (EM) to the decolorant activity of newly isolated photosynthetic bacteria was observed. The decolorization process was considered to occur by two paths: photochemical decolorization by EM and photobiological decolorization by photosynthetic bacteria. In addition, the decolorization of several azo dyes by EM under black light and fluorescent light irradiation was investigated. It was found that EM were capable of decolorizing the azo dyes directly under visible light irradiation, and the overall dye decolorization followed first-order decay kinetics. Moreover, the decolorization reaction is a nonenzymatic reaction and the unknown metabolite with the decolorizing ability had an apparent molecular weight lower than 3 kDa as determined by ultrafiltration. In addition, its decolorization activity was stable even after heating sterilization at 121 degrees C for 10 min. Furthermore, the decolorization rate increased with increasing optical intensity, temperature and EM concentration, and decreased with increasing initial dye concentration. Decolorization of dye was best at pH 8.

Azo Compounds↗
PubMedDate not supplied

Indirect evidence for a very fast recovery kinetics of chlorophyll-aII in spinach chloroplasts.

The 690 nm absorption change reflecting the turnover of the system-II-reaction center chlorophyll, Chl-aII (often referred to as P 680), has been investigated under different experimental conditions in spinach chloroplasts. A comparison was made with oxygen evolution and with absorption changes of Chl-aI measured at 703 nm, both indicating the number of electrons produced by system II. It was found: 1. The dependency on actinic flash intensity of the initial amplitudes of the measured 690 nm absorption change, deltaalpha0(Chl-aII) in Tris-washed chloroplasts is similar to that for the total amplutude of the 703 nm absorption change, deltaalpha0(Chl-aI) in normal chloroplasts, and can be described by an exponential function. On the other hand, deltaalphao(chl-aII) in normal chloroplasts exhibits a more complex biphasic dependency and much higher flash intensities are required for saturation. 3. Unver repetitive flash group excitation and in the presence of an ADRY(= acceleration of the deactivation reactions of the water-splitting enzyme system Y)-reagent the initial amplitude of the 690 nm absorption change oscillates in the same characteristic pattern as the oxygen evolution. 4. The initial amplitude of the 690 nm absorption change, deltaalpha0(Chl-aII), IN Tris-washed chloroplasts becomes significantly smaller (more than 50%) by the addition of system-II-electron donors (benzidine, p-phenylendiamine, tetraphenylboron), whereas the total amplitude of the 703 nm absorption change, detalalpha0)Chl-aI) increases 3-4-fold. In order to explain these results, the existance of a very fast reduction kinetics of Chl-aII+ is postulated, which is not detectable by our measuring equipment. The half time of this reaction is less than or equal to mus. Reaction centers with the very gast "undetected" Chl-aII+-reduction are photochemically transformed into slower one by double hit processes with a comparatively low quantum yield. Furthermore, it is inferred, that the dark recovery kinetics of Chl-aII is dependent on be charge accumulation state of the watersplitting enzyme system Y. This phenomenon is shown to explain also the oscillation pattern of delayed fluorescence. On the basis of the present results two alternative reaction schemes for the functional organization of the electron transport on the donor side of system II are discussed.

Chlorophyll↗
PubMedDate not supplied

[Mechanism of photosynthetic water oxidation in the dimeric oxygen-evolving complex of chloroplast photosystem II].

Based on the analysis of the molecular organization and properties of an isolated oxygen-evolving complex of photosystem II of plant chloroplasts, a mechanism of water oxidation and oxygen release during photosynthesis was proposed. It is suggested that the photolysis of water occurs in a dimeric oxygen-evolving complex consisting of two core complexes. In the region of contact of these complexes, a hydrophobic "boiler" is formed where the conditions for screening and stabilization of Z-linanded manganese cations accumulating positive charges for the oxidation of water molecules are created. A prerequisite to the photolysis of water is the formation of a binuclear [Mn(3+)-OH ... HO-Mn3+] hydroxyl-manganese associate, which appears in the dimeric oxygen-evolving complex after the first two light flashes as a result of photohydrolysis of photochemically oxidized Z-liganded manganese cations. The process is accompanied by the release of the first water protons to the medium. The photosynthetic oxidation of water hydroxyls occurs at the next stage and is considered as synchronous detachment of four electrons from two bound OH-groups of the associate upon photooxidation of Mn3+ cations to Mn4+ cations after two subsequent light flashes. This process is accompanied by the disproportionation of electron density and the formation of a bond between oxygen atoms of hydroxyls followed by the evolution of molecular oxygen and protons, and regeneration of two starting Mn2+ cations and the primary state of the system.

Chloroplasts↗
PubMed1996

A direct interaction between a DNA-tracking protein and a promoter recognition protein: implications for searching DNA sequence.

Bacteriophage T4 gene 45 protein, gp45, serves as the sliding clamp of viral DNA replication and as the activator of T4 late gene transcription. In the latter context, DNA tracking is an essential feature of the unique mechanism of action. T4 late promoters, which consist of a simple TATA box, TATAAATA, are recognized by the small sigma-family gene 55 protein, gp55, which binds to Escherichia coli RNA polymerase core. A direct and RNA polymerase-independent interaction of gp45 with gp55 has been demonstrated in two ways. (i) gp45 tracks along DNA; co-tracking of gp55 requires the previously documented DNA-loading process of gp45, and can be detected by photochemical crosslinking. (ii) The dynamics of DNA tracking by gp45 can be followed by footprinting; the catenated DNA-tracking state of gp45 is short-lived, but is stabilized by gp55. The ability of this topologically linked DNA-tracking transcriptional activator to interact directly with a promoter recognition protein suggests the existence of multiple pathways of promoter location, which are discussed.

Bacterial Proteins↗
PubMed2005

Efficient decomposition of environmentally persistent perfluorocarboxylic acids by use of persulfate as a photochemical oxidant.

Photochemical decomposition of persistent perfluorocarboxylic acids (PFCAs) in water by use of persulfate ion (S2O8(2-)) was examined to develop a technique to neutralize stationary sources of PFCAs. Photolysis of S2O8(2-) produced highly oxidative sulfate radical anions (SO4-), which efficiently decomposed perfluorooctanoic acid (PFOA) and other PFCAs bearing C4-C8 perfluoroalkyl groups. The major products were F- and CO2; also, small amounts of PFCAs with shorter than initial chain lengths were detected in the reaction solution. PFOA at a concentration of 1.35 mM (typical of that in untreated wastewater after an emulsifying process in fluoropolymer manufacture) was completely decomposed by a photochemical system with 50 mM S2O8(2-) and 4 h of irradiation from a 200-W xenon-mercury lamp. The initial PFOA decomposition rate was 11 times higherthan with photolysis alone. All sulfur-containing species in the reaction solution were eventually transformed to sulfate ions by this method. This method was successfully applied to the decomposition of perfluorononanoic acid contained in a floor wax solution.

Carboxylic Acids↗
PubMed2004

Analysis of protein solvent accessible surfaces by photochemical oxidation and mass spectrometry.

Protein surfaces are important in most biological processes, including protein-protein interactions, enzymatic catalysis, and protein-ligand binding. We report a method in which hydroxyl radicals generated by a rapid-UV irradiation of a 15% hydrogen peroxide solution were utilized to oxidize specific amino acid side chains of two model proteins (lysozyme, beta-lactoglobulin A), according to the residues' chemical reactivities and the solvent accessibility of the reactive carbons and sulfurs in the residue. Oxidized peptides generated by tryptic digestion were identified by electrospray-Fourier transform mass spectrometry. The specific sites of the stable modification were then identified by reverse-phase liquid chromatography coupled to quadropole ion trap tandem mass spectrometry. The solvent accessibility of the residue was shown to directly affect the rate of oxidation by this method (with the exception of methionine), supporting its use as a rapid measure of the solvent accessibility of specific residues, and in some cases, individual atoms.

Amino Acid Sequence↗
PubMed2003

Combinatorial discovery of two-photon photoremovable protecting groups.

A design principle for a two-photon photochemically removable protecting group based on sequential one-photon processes has been established. The expected performance of such groups in spatially directed photoactivation/photodeprotection has been shown by a kinetic analysis. One particular molecular class fitting into this design, the nitrobenzyl ethers of o-hydroxycinnamates, has been presented. An initial demonstration of two-photon deprotection of one such group prompted further optimization with respect to photochemical deprotection rate. This was accomplished by the preparation and screening of a 135-member indexed combinatorial library. Optimum performance for lambda >350 nm deprotection in organic solvent was found with 4,5-dialkoxy and -cyano substitution in the nitrobenzyl group and 4-methoxy substitution in the cinnamate.

Cinnamates↗
PubMed2003

Rearrangement of the cyclohexadiene derivatives of C60 to bis(fulleroid) and bis(methano)fullerene: structure, stability, and mechanism.

The cyclohexadiene derivative of C(60) rearranges photochemically to bis(fulleroid) (two [6,5] open structure) and bis(methano)fullerene (two [6,6] closed structure). During this process, a [6,5] open/[6,6] closed intermediate is observed. The isolated intermediate undergoes photochemical rearrangement to bis(fulleroid) and bis(methano)fullerene. On the other side, it undergoes retrorearrangement to the starting material in the dark. The structure and energetics of these C(60) derivatives have been studied at the AM1, PM3, RHF, and B3LYP levels of theory. It is found that bis(fulleroid) bearing four tert-butoxycarbonyl substituents is 5.8 kcal/mol (B3LYP) more stable than the corresponding bis(methano)fullerene. The isolated intermediate having the [6,5] open/[6,6] closed structure is 6.7 kcal/mol more favorable than the previously proposed two [6,5] closed intermediate, and the formation of this compound is well explained by the di-pi-methane rearrangement. (13)C NMR calculation at the B3LYP level reproduced the experimental chemical shifts with very good accuracy for each molecular system. Theoretical studies mainly at the unrestricted B3LYP level on singlet and triplet state potential energy surfaces on fullerene derivatives support the di-pi-methane rearrangement mechanism. The previously proposed symmetrical [4+4]/[2+2+2] and the novel proposed unsymmetrical di-pi-methane pathways may coexist during the reaction.

Journal Article↗
PubMed2006

Unusual photochemical C-N bond cleavage in the novel methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate.

The structure, preferred conformers, vibrational spectrum, and photochemical behavior of the novel azirine, methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate (MCMAC) were investigated in low-temperature matrixes and in the neat solid amorphous state by infrared spectroscopy and quantum-chemical calculations. Two conformers of the compound were observed in argon, krypton, and xenon matrixes, in agreement with the DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) theoretical calculations. Both conformers were found to exhibit the carboxylic ester group in the cis conformation, differing in the arrangement defined by the O=C-C-Cl dihedral angle (cis and trans, for Ct and Cc forms, respectively). The Ct conformer was found to be the most stable conformer in the gaseous phase as well as in both argon and krypton matrixes, whereas the more polar Cc conformer became the most stable form in the xenon matrix and in the neat solid amorphous phase. In situ broadband UV (lambda > 235 nm) excitation of matrix-isolated MCMAC led to azirine ring C-C and C-N bond cleavages, the latter process corresponding to the most efficient reaction channel. The photochemical cleavage of the C-N bond had never been previously observed in the case of aliphatic 2H-azirines. Two electron withdrawing substituents (methoxycarbonyl group and chlorine atom) are connected to the azirine ring in the novel MCMAC azirine. The simultaneous presence of these two groups accelerates intersystem crossing toward the triplet state where cleavage of the C-N bond takes place. The primary photoproducts resulting from the C-N and C-C ring-opening reactions were also found to undergo further photochemical decarbonylation or decarboxylation reactions.

Argon↗
PubMed2003

Electrically enhanced photodegradation of an azodye (acid orange II) using a Pt/TiO2 film electrode irradiating with an UV lamp.

A photoelectrochemical process in the degradation of an azodye (Acid Orange II) on a Pt/TiO2 film electrode was investigated. By using the glass device and the voltage stabilized source of direct current, decolorization ratios higher than 78% were observed during aperiod of 5 h. Comparing this value with the sum of the decolorization ratios obtained by a sole application of electrochemical (lower than 3%) and photochemical (about 23%) procedures, a significant synergic effect between both processes was observed. The effects of adscititious voltage and pH value on the decolorization ratios were obvious while the effect of the amount of aeration was minor.

Azo Compounds↗