THE REDUCTION AND QUENCHING OF PHOTOEXCITED FLAVINS BY EDTA

Riboflavin was irradiated anaerobically in aqueous EDTA solutions over the pH range 2.5–10. In other dye systems (Bonneau and Pereyre, 1975), only the trivalent anion of EDTA was found to have significant reactivity for photoreduction. For riboflavin, the reactivity begins with monoanionic EDTA, and the reactivity is markedly increased as the charge increases. This suggests that the charge on the reductant is more important to the electron transfer process for riboflavin than the formation of a nonhydrogen bonded nitrogen site on EDTA. At high concentrations of EDTA in the pH range 4–8, quenching of the photoreduction occurs, which can be explained by an energy transfer between the excited singlet state of riboflavin and trianionic EDTA, possibly as an association complex. The rate constants for the photoreduction of riboflavin by the monovalent, divalent, and trivalent anions of EDTA are 1.0 times 10⁷M^-1 s^-l, 4.8 times 10′M^-1 s^-l, and 2.0 times 10⁸M^-1s^-1, respectively. The rate constant for the singlet state quenching by trianionic EDTA is 3 times 10⁹M^-l s^-1, and the limiting quantum yield for intersystem crossing for riboflavin in aqueous solution is 0.50 ± 0.05.     

THREE LIGHT REACTIONS AND THE TWO PHOTOSYSTEMS OF PLANT PHOTOSYNTHESIS*

Abstract— Recent work in our laboratory yielded new evidence that noncyclic electron transport in chloroplasts from water to ferredoxin (Fd) and N ADP is carried out solely by System II which, unexpectedly, was found to include not one but two photoreactions (IIa and IIb). The evidence suggests that these operate in series, being joined together by a ‘dark’ chain of electron carriers that includes (but is not limited to) cytochrome b₅₅₉ and plastocyanin (PC): H₂O → IIb_hv→ C550 → Cyt b₅₅₉ rarr;PC→IIa_hv→ Fd → NADP. Photoreaction IIb involves an electron transfer from water to C550, a new chloroplast component distinct from cytochromes, whose photoreduction is observed as a decrease in absorb-ance with a maximum at 550 nm. The photoreduction of CSSO proceeds effectively only in short-wavelength System II light, is insensitive to low temperature (at least down to — 189°C). does not require plastocyanin, and is the first known System II photoreaction which is resistant to inhibition by DCMU or o-phenanthroline. Photoreaction IIa involves an electron transfer from cytochrome b₅₅₉ to ferredoxin-NADP and also proceeds effectively only in System II light. The photooxidation of cytochrome b₅₅₉ requires plastocyanin. Cytochrome b₅₅₉ is reduced by C550 in a reaction that is readily inhibited by DCMU or o-phenanthroline. Thus, the site of DCMU (and o-phenanthroline) inhibition of System II appears to lie between C550 and cytochrome b₅₅₉. System I, comprising a single long-wavelength light reaction and a cyclic electron transport chain that includes cytochromes b₆ and f, is viewed as operating in parallel to System II. The photoreduction of NADP by artificial electron donors via System I involves a portion of the cyclic electron transport chain and appears to be independent of plastocyanin. Chloroplast fragments have been prepared which either (a) exhibit System II activity (water → NADP) and lack functional cytochrome f and P700 or (b) exhibit System I activity and lack plastocyanin. The present concept is consistent with the following: (i) No enhancement effect was found for NADP reduction by water where only System II is thought to be involved, but a large enhancement effect was observed in chloroplasts engaged in complete photosynthesis where both cyclic (System I) and noncyclic photophosphorylation (System II) are needed for CO₂ assimilation. (ii) The transfer of one electron from water to ferredoxin via System II requires optimally two quanta, but the transfer of one electron from reduced dye to ferredoxin via System I requires optimally only one quantum of light.     

MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH-PHOTOLYSIS-I

Abstract— The photoreduction of thiazine dyes by ethylene diamine tetraacetic acid (EDTA) was investigated by Rash photolysis. This reaction was found to occur according to a three-step mechanism. the first being the formation of the dye triplet state followed, in weakly acid solutions, by protonation. During the second step, the triplet state of the dye disappears through two competing processes: spontaneous deactivation and reaction with EDTA, which leads to the semireduced dye. The third step leads to the leucodye. It is shown that the overall quantum yield of photoreduction is governed by the second step and can be calculated from the ratio of the rate constants of the two elementary processes involved in this step. This ratio was measured over a wide pH range.     

TiO2/EDTA-rich carbon composites: Synthesis,characterization and visible-light-driven photocatalytic reduction of Cr(VI)

TiO₂/EDTA-rich carbon composites exhibits excellent photoreduction of Cr(VI) activity via ligand-to-metal charge transfer process.     

Determination of paraquat in biological and environmental samples by a photokinetic method

A fast and sensitive photokinetic method for the determination of paraquat (MV²⁺) (1?27×10^?5M) is described, based on the rate of photoreduction of MV²⁺ by EDTA, sensitized by acridine yellow in the absence of oxygen. The rate of photoreduction, which is a linear function of the concentration of MV²⁺ is monitored polarographically by recording the limiting current of p- benzoquinone, which is reduced by the radical monocation MV^+· generated in the photochemical reaction. The results obtained by the application of the fixed-time, fixed-concentration change and initial-rate kinetic methods are evaluated. An alternative method for monitoring the rate of the process is by measuring the time necessary for the total reduction of p-benzoquinone. The end-point is detected with two platinum electrodes at an applied voltage of 100 mV. The procedure has been successfully applied to the determination of paraquat in commercial herbicides, waters, and flowers and in spiked soils and blood sera.     

Analytical applications of photochemical reduction of Azur B by EDTA-iodide determination

A kinetic study of the photochemical reaction of Azur B and EDTA (in the absence of oxygen) has been made in connection with development of a new kinetic method for iodide. The reaction is first-order with respect to the dye, EDTA and absorbed light-intensity. The rate of photoreduction is strongly pH-dependent, and maximal at about pH 6.8. The photoreduction involves a long-lived excited state of the dye and is dramatically retarded by small amounts of iodide. A tentative mechanism is proposed, and the experimental conditions have been optimized. The variable time method appear to be the most suitable. A detection limit of 1.0 mug/ml and a coefficient of variation of about 3% can be achieved. Chloride and bromide do not interfere at levels below 100-fold mole ratio to iodide. Metal ions do not interfere if enough excess of EDTA is used. Coloured species may interfere at high concentration (filter effect).     

VECTORIAL ELECTRON TRANSPORT ACROSS LIPID VESICLE MEMBRANES DRIVE BY TWO PHOTOREACTIONS. II. PHOTOCHEMICAL REGENERATION OF REDUCED CYTOCHROME c BY FLAVIN TRIPLET STATE AND ETHYLENEDIAMINETETRAACETIC ACID IN THE INNER COMPARTMENT AND REDUCTION OF FERREDOXIN BY CHLOROPHYLL TRIPLET STATE IN THE OUTER COMPARTMENT

We have attempted to mimic natural photosynthesis with regard to the photogeneration of a powerful reductant, using a negatively charged lipid bilayer vesicle system incorporating two photoreactions sensitized by a flavin analog (flavin mononucleotide [FMN]) and chlorophyll (chl) in their respective triplet states. Ethylenediamine-tetraacetic acid (EDTA) in the inner aqueous compartment was used as a sacrificial electron donor to the FMN triplet, and ferredoxin in the outer aqueous compartment served as the final electron acceptor (mediated via triplet electron transfer chain in this multicomponent system to be elucidated. By itself, EDTA does not function as an effective donor to membran-bound oxidized chl (chl^+.), which is formed by electron transfer from triplet chl to the viologen follwed by transbilayer electron migration. This is a consequence of electrostatic repulsive interactions with the negatively charged membrane. This limitation is avoided when FMN is used as a photomediator between EDTA and chl^+.. The overall reaction is dramatically increased in rate by enclosing cytochrom c together with EDTA and FMN in the inner compartment. The rate constant of the key step in the reaction, i.e. elctron transfer from reduced cytochrome c, generated via photoreduction by the FMN/EDTA system, to chl^+. is increased 20-fold over that obtained with cytochrome c alone as the elctron donor. One of the important constraints that limited the net electron transfer across the bilayer to 50% of the added cytochrome, i.e. inhibition by oxidized cytochrome c formed in the inner compartment, is avoided by the inclusion of the second photoreaction in this system, thus allowing photoreduction of all of the added ferredoxin to be achieved. This system provides a model for a photochemical energy storage process that utilizes two photorections operating in series resulting in electron flow across a lipid bilayer membrane.     

The photoreduction of 8-substituted 5-deazaflavins and the 5-deazaflavin catalyzed photoreduction of methyl viologen

Prolonged illumination of 8-X-5-deazaflavins (X = C1, N(CH₃)₂, NH₂, p-NH₂-C₆H₄) in the presence of an electron donor leads to the formation of a 5,5′-dimer and/or a 6,7-dihydro compound. The course and rate of these photoreductions were established and discussed in terms of electronic and steric effects, exerted by the substituent at position 8 and the electron donor. Pseudo first-order kinetics were found to apply to the photoreduction of 8-X-5-deazaflavins (X = Cl, NH₂, p-NH₂-C₆H₄) while the rate of the photoreduction of 8-X-5-deazaflavin (X = N(CH₃)₂) appeared to contain an autocatalytic element. The catalytic effect of 8-X-5-deazaflavins in the photoreduction of methyl viologen by EDTA was investigated. The substituent effect on the rate of the 8-X-5-deazaflavin mediated photoreduction of methyl viologen by EDTA was found to be comparable with that on the photoreduction rate of 8-X-5-deazaflavin in the presence of EDTA with the exception of 8-X-5-deazaflavin (X = N(CH₃)₂), which showed a remarkable relative enhancement of the reactivity towards methyl viologen photoreduction.     

Nicotinamide Adenine Dinucleotides Arrest Photoreduction of Class II DNA Photolyases in FADH˙ State

All light‐sensitive members of the photolyase/cryptochrome family rely on FAD as catalytic cofactor. Its activity is regulated by photoreduction, a light‐triggered electron transfer process from a conserved tryptophan triad to the flavin. The stability of the reduced flavin depends on available external electron donors and oxygen. In this study, we show for the class II photolyase of Methanosarcina mazei , Mm CPDII , that it utilizes physiologically relevant redox cofactors NADH and NADPH for the formation of the semiquinoid FAD in a light‐dependent reaction. Using redox‐inert variants Mm CPDII /W388F and Mm CPDII /W360F, we demonstrate that photoreduction by NADH and NADPH requires the class II ‐specific tryptophan cascade of Mm CPDII . Finally, we confirmed that mutations in the tryptophan cascade can be introduced without any substantial structural disturbances by analyzing crystal structures of Mm CPDII /W388F, Mm CPDII /W360F and Mm CPDII /Y345F.     

Polarographic study of xylenol orange

In neutral medium the ring carbonyl group of xylenol orange is reduced in two one-electronsteps at a dropping mercury electrode. Step I is nearly reversible, but step II strongly irreversible. The half-wave potentials of both steps depend on pH, for the products of reduction, the dye radical formed in step I and the OH group produced in step II, behave as acids. From the E_1/2 vs. pH dependence, the pK of the dye radical is found to be 9.5, that of the hydroxy compound 5.6. On progressive acidification the height of step II continuously decreases, while that of step I increases. In solutions with pH<2, only a single (according to logarithmic analysis) one-electron, but overall (according to coulometry) two-electron step results. The ratio i^I/i^II for the two steps increases when larger amounts of alkali metal ions are added. The exaltation of the first wave at the expense of wave II is explained by an acid-dependent dismutation of the dye radical formed during the first one-electron step.     

CHARACTERIZATION OF THE RED LIGHT INDUCED REDUCTION OF A PARTICLE ASSOCIATED b-TYPE CYTOCHROME FROM CORN IN THE PRESENCE OF METHYLENE BLUE*

Methylene blue transfers electrons to a membrane-associated b-type cytochrome in particulate fractions from corn coleoptiles. The K_m for methylene blue is less than 1 µM under optimal conditions. This reaction is destroyed by boiling, but not by 7 M urea. Kinetic analyses of the influence of light intensity on cytochrome reduction suggest that a first order photochemical reaction is limiting. Free EDTA may serve as an electron donor in this system at least at high methylene blue and protein concentrations. The photoactivity does not coincide either with mitochondrial or endoplasmic reticulum markers, and may be localized in plasma membrane. There is an estimated 5 times 10^-10 mol photoreducible cytochrome per g coleoptile tissue. Studies on the effect of pH on the reaction in the presence of methylene blue or thionine indicate that dye photoreduction itself is not rate-limiting. Wavelength dependence studies suggest that it is methylene blue monomer and not dimer which mediates the reaction. Although oxygen is apparently required for the reaction, neither superoxide nor excited singlet oxygen appear to be involved. The reaction mechanism is still unknown.     

Construction of visible-light-responsive metal–organic framework with pillared structure for dye degradation and Cr(VI) reduction

A water-stable mixed-linker metal–organic framework (MOF) was rationally synthesized using a controllable pillared-layer method. The prepared Co(II)–MOF shows wide-range absorption in the visible light region due to the incorporation of highly conjugated anthracene-based bipyridine ligand. Experiments suggest that the MOF is highly efficient for the photoreduction of toxic Cr(VI) ions in water under visible light. Important issues affecting photocatalytic performance, such as the influence of pH and the control of electron–hole separation by scavenger, were carefully examined. Beyond Cr(VI) ions, we also explored the photocatalytic degradation performance of the MOF using a persistent azo dye as a model substrate, where H₂O₂-involved advanced oxidation process was applied. Control experiments suggest that the introduction of environmentally benign H₂O₂ significantly enhances the degradation performance due to the generation of reactive hydroxyl radicals. The study not only demonstrates the great feasibility of the preparation of a new MOF photocatalyst through a controllable pillared-layer method, but also reveals that rational functionalization of ligand in the MOF is convenient for achieving desirable applications.     

Synthesis and Study of Plasmon‐Induced Carrier Behavior at Ag/TiO2 Nanowires

Nanocomposites of Ag/TiO₂ nanowires with enhanced photoelectrochemical performance have been prepared by a facile solvothermal synthesis of TiO₂ nanowires and subsequent photoreduction of Ag⁺ ions to Ag nanoparticles (AgNPs) on the TiO₂ nanowires. The as‐prepared nanocomposites exhibited significantly improved cathodic photocurrent responses under visible‐light illumination, which is attributed to the local electric field enhancement of plasmon resonance effect near the TiO₂ surface rather than by the direct transfer of charge between the two materials. The visible‐light‐driven photocatalytic performance of these nanocomposites in the degradation of methylene blue dye was also studied, and the observed improvement in photocatalytic activity is associated with the extended light absorption range and efficient charge separation due to surface plasmon resonance effect of AgNPs.     

Reduction of Methylene Green by EDTA:Kinetic and Thermodynamic Aspects

The reduction of methylene green (MG) into protonated leuco dye with ethylenediamine tetraacetic acid (EDTA) in aqueous alkaline medium was studied spectrophotometrically at λ_max 660 nm. EDTA behaved as an effective electron donor during the reduction of MG in an aerobic condition. Consumption of EDTA in the reduction of MG means that it is oxidized. This is an unexpected result since EDTA does not normally function as a reducing agent. The nitrogen-containing chelating agents with secondary or tertiary nitrogen behaved as an electron donor in photochemical reaction of dye. The rate of reduction depends upon pH in the same way as the base titration of EDTA. Effects of salt and temperature have been investigated for the reduction process. The salting agent KNO₃ has been found to uniquely enhance the rate of reduction of MG by EDTA in the aerobic condition. Detailed kinetic and thermodynamic aspects have been discussed to realize the interaction between MG and EDTA. Kinetic studies revealed that reaction was sensitive and regeneration of oxidized form of the dye was observed. Reversible first order reaction kinetics with respect to EDTA, MG and NaOH was found.     

Reaction Mechanism on the Synergistic Photodegradation of Dye X3B and Photoreduction of Dichromate in the Presence of Polyoxometalate

Photodegradation of a textile dye X3B and photoreduction of dichromate (Cr(VI)) in an acidic aqueous solution were studied under 320 nm cut-off UV light irradiation in the presence of two polyoxometalates (POM), H₃PW₁₂O₄₀ (PW), and H₄SiW₁₂O₄₀ (SiW). The reactions in POM-X3B-Cr(VI) system were faster than those in POM-X3B, POM-Cr(VI), and X3B-Cr(VI) systems. For all reactions, PW was more photoactive than SiW. The reaction rates were proportional to the initial concentration of each component. The effects of N₂, O₂, and air were small but regular, indicating Cr(VI) photoreduction by a reduced POM. Quenching experiments with H₂O₂ and ethanol revealed that X3B photodegradation mainly occurred through hydroxyl radical (OH). It was proposed that the production of OH and a reduced POM⁻ by the reaction between H₂O and excited POM^* was the rate determining step, with which all evidence could be well interpreted. Different effects of POM concentration in a two- or three-component system on the reaction rates suggested that the reaction between H₂O and excited POM^* was reversible.     

Ultrathin zinc selenide nanosheet-based intercalation hybrid coupled with CdSe quantum dots showing enhanced photocatalytic CO_2 reduction

Fabrication of well-designed heterojunctions is an extraordinarily attractive pathway for boosting the photocatalytic activity toward CO_2 photoreduction.Herein,a novel kind of na nosheet-based intercalation hybrid coupled with CdSe quantum dots(QDs) was successfully fabricated by a facile solvothermal method and served as photocatalyst for full-spectrum-light-driven CO_2 reduction.Ultra-small CdSe QDs were rationally in-situ introduced and coupled with lamellar ZnSe-intercalation hybrid nanosheet,resulting in the formation of CdSe Q.Ds/ZnSe hybrid heterojunction.Significantly,the concentration of Cd~(2+) could change directly the crystallinity and micromorphology of ZnSe intercalation hybrid,which in turn would impact on the photocatalysis activity.The optimized CdSe QDs/ZnSe hybrid-5 composite demonstrated a considerable CO yield rate of the 25.6 μmol g~(-1) h~(-1) without any additional cocatalysts or sacrificial agents assisting,making it one of the best reported performance toward CO_2 photoreduction under full-spectrum light.The elevated CO_2 photoreduction activity could be attributed to the special surface heterojunction,leading to improving the ability of light absorption and promoting the separation/transfer of photogenerated carriers.This present study developed a new strategy for designing inorganic-organic heterojunctions with enhanced photocatalyst for CO_2 photoreduction and provided an available way to simultaneously mitigate the greenhouse effect and alleviate energy shortage pressure.     

Efficient Photoredox Cycles to Control Perylenediimide Self-Assembly**

Photoreduction of perylenediimide (PDI) derivatives has been widely studied for use in photocatalysis, hydrogen evolution, photo-responsive gels, and organic semiconductors. Upon light irradiation, the radical anion (PDI⋅⁻) can readily be obtained, whereas further reduction to the dianion (PDI²⁻) is rare. Here we show that full 2-electron photoreduction can be achieved using UVC light: 1) in anaerobic conditions by ‘direct photoreduction’ of PDI aggregates, or 2) by ‘indirect photoreduction’ in aerobic conditions due to acetone ketyl radicals. The latter strategy is also efficient for other dyes, such as naphthalenediimide (NDI) and methylviologen (MV²⁺). Efficient photoreduction on the minute time-scale using simple LED light in aerobic conditions is attractive for use in dissipative light-driven systems and materials.     

X-ray photoelectron spectra of dinitrogen chelating ligands with some transition metals

The complexes of two chelating agents, CDTA (trans-1,2-cyclohexanediaminetetraacetic acid) and EDTA (ethylenediaminetetraacetic acid), with Ni(II), Co(II) and Cu(II) are studied by means of X-ray photoelectron spectroscopy (XPS). The N1s spectrum of CDTA has two components at 398.5 (±0.2 eV) and 400.9 (±0.2 eV) while only one signal is found after coordination at 400.0±0.2 eV. EDTA shows a N1s spectrum with one component at 400.9±0.2eV. A negative shift of about -1.3eV is observed in its complexes. The results are compared with the literature data on the complexes in the solid state.     

One-electron reduction of excited NMN and NADP in the presence of amino acids

Excitation of oxidized forms of nicotinamide coenzymes (NADP⁺, NMN⁺) at 254 nm under anaerobic conditions in the presence of EDTA, lysine, serine, glycine leads, in the initial stage of irradiation, to photoreduction of coenzyme. Formation of the photoreduction products was observed by polarographic, spectrophotometric and enzymatic methods. Quantum yields for NMN⁺ and NADP⁺ photoreduction have been calculated and a mechanism proposed. No photoreduction was observed with histidine. Long-term irradiation leads to further reduction of the nicotinamide ring to tetrahydroderivatives absorbing at 280–290 nm. The photochemically generated dimers undergo phototransformation to the parent monomers on irradiation at 365 nm either in the presence or absence of oxygen. The biological significance of the results is discussed.     

Nitro group photoreduction of 4-(2-nitrophenyl)- and 4-(3-nitrophenyl)-1,4-dihydropyridines

The photoprocesses of nifedipine, a 4-(2-nitrophenyl)-1,4-dihydropyridine, and nimodipine and nitrendipine, two 3-nitrophenyl Hantzsch-type analogues, were studied by steady-state and time-resolved methods. The intramolecular photoreduction of nifedipine into its nitrosophenyl product takes place within a few ns. The quantum yield of conversion is Φ_red = 0.3 and does not depend significantly on the oxygen concentration and solvent properties. Formation of the fully reduced 4-(2-aminophenyl)-1,4-dihydropyridine as minor product is indicated by fluorescence spectroscopy. The photoreduction of nimodipine and nitrendipine is inefficient, Φ_red = 0.002 in acetonitrile, but markedly enhanced in the presence of donors such as triethylamine (TEA) and 2-propanol, e.g. for TEA Φ_red is up to 0.03. The triplet states of nimodipine and nitrendipine were characterized. They react intermolecularly with TEA and 2-propanol, forming radicals as intermediates and eventually several reduction products.