Absence Of Electron Donors Research Articles

A long-running in situ experiment in clay: 12 years of the Bitumen–Nitrate–Clay interaction experiment at Mont Terri rock laboratory

Abstract. In various countries such as Belgium and Germany, deep clay formations are investigated as potential host rock to dispose of radioactive waste. Intermediate-level long-lived bituminised waste, produced from spent fuel reprocessing, is currently foreseen to be disposed of in such a deep repository. This type of waste typically contains large amounts of salts embedded in an organic matrix, of which NaNO3 is the most prevalent. NaNO3, along with radionuclides immobilised in the bitumen matrix, will slowly dissolve and leach out into the surrounding host rock. The transport and reactivity of nitrate in a deep clay formation, in the presence and absence of electron donors coming from bituminised waste (i.e. acetate, H2), has been studied extensively inside anoxic and water-saturated chambers in the clay in the Bitumen–Nitrate–Clay interaction (BN) experiment. This in situ experiment in Opalinus clay is located in the Mont Terri rock laboratory (Switzerland). The effect of a nitrate plume on redox-sensitive radionuclides is also studied with stable selenium in selenate form as a proxy for 79Se, an important fission product for the long-term dose to humans. The BN experiment consists of a vertical borehole containing three packed-off intervals, in contact with the surrounding clay through cylindrical sintered stainless steel filter screens. The intervals are saturated with artificial Opalinus clay pore water at a pH between 7.5 and 8, containing all major ions at concentrations specific to the location of the BN experiment but without natural organic matter. Each interval is connected to a water circulation system with a gear pump, a flow meter and water sampling containers. This ensures a chemically well-mixed solution, constant water flow and easy sampling of the interval solutions under anoxic conditions. Water samples are regularly taken to assess the chemical composition and the microbial population in the intervals. An online UV spectrophotometer and pH and redox electrodes are installed in the water circuit of the intervals, to continuously monitor the nitrate and nitrite concentrations, pH and Eh. A given solution can be injected into the intervals to follow up redox reactions affecting nitrate and selenate in situ, in the presence or absence of additional electron donors. From 2011 to 2019, several injections with nitrate were performed in the intervals to investigate the biogeochemical reactivity of nitrate in the borehole and clay. These tests showed microbial reduction of nitrate, using electron donors from Opalinus clay (e.g. dissolved organic matter or pyrite). Nitrite and nitrogenous gases were formed. Pulses of electron donors (acetate or H2) boosted the microbial activity and nitrate reduction rate. From 2019 until now, injection tests with selenate, with and without nitrate, have been performed. These tests showed clearly that the presence of nitrate, a stronger oxidiser, inhibited the microbial reduction of selenate. In the absence of nitrate, the microbial population was able to reduce selenate to selenite and more reduced Se species. The results obtained in this in situ experiment will be summarised in this work, addressing questions concerning the effect of biogeochemical perturbations of the clay on the migration of redox-sensitive radionuclides.

Differentiation of Superoxide Radical Anion and Singlet Oxygen and Their Concurrent Quantifications by Nuclear Magnetic Resonance.

While there have been various techniques, assays, and commercial kits developed to measure reactive oxygen species (ROS) with varying degrees of success, there is a lack of innovative methods to differentiate and quantify them simultaneously. In this work, we demonstrate a 19F nuclear magnetic resonance (NMR)-based method to differentiate two important types of ROS, superoxide radical anion and singlet oxygen, and to quantify them concurrently. By taking advantage of the unique chemical reactivity of two fluorine-containing molecules, 4-fluoro-3-methylphenyl boronic acid and 4-fluoro-3-methylphenol, serving as 19F NMR probes, we are able to differentiate and quantify, for the first time, superoxide radical anion and singlet oxygen generated by photosensitizers (PSs) concurrently. The results reveal that relative amounts of superoxide radical anion and singlet oxygen generated by a PS under light illumination are oftentimes sensitive to the environment, such as the presence or absence of electron donors. This method provides a means to identify the type of mechanism by which a PS functions under a given condition. We envision that this relatively simple, yet robust, method would be beneficial to a broad range of ROS-pertinent studies, such as photodynamic therapy and photoredox reactions.

A Convenient Assay to Detect Protein Oxidation Caused by Redox-Active Quinones.

Redox-active quinones generate reactive oxygen species (ROS) through their redox cycling with electron donors. Hydrogen peroxide (H2O2) causes S-oxidation of proteins and is associated with activation of the redox signaling pathway and/or toxicity (Chem. Res. Toxicol., 30, 2017, Kumagai et al.). In the present study, we developed a convenient assay based on a combination of an enzyme-linked immunosorbent assay and a biotin-PEAC5-maleimide assay and used it to determine protein S-oxidation by ROS during redox cycling of 9,10-phenanthrenequinone (9,10-PQ) and pyrroloquinoline quinone (PQQ). S-Oxidation of proteins in a mouse liver supernatant was detected during reaction of 9,10-PQ or PQQ with electron donors such as dithiothreitol or reduced nicotinamide adenine dinucleotide phosphate (NADPH), whereas cellular protein oxidation was not observed in the absence of electron donors. These results suggest that the developed assay is useful for the detection of S-oxidation of proteins.

Long-term and stable antimicrobial properties of immobilized Ni/TiO2 nanocomposites against Escherichia coli, Legionella thermalis, and MS2 bacteriophage

Nickel has been extensively used as a high work function metal because of its abundance, low cost, relatively non-toxic nature, and environmentally benign characteristics. However, it has rarely been extended in a form of immobilized composite, which is a practical strategy applicable for photocatalytic antimicrobial activities. In this study, a composite of nickel and TiO2 (Ni/TiO2) was prepared using a photodeposition method, and its antibacterial properties were investigated using Escherichia coli (E. coli). To optimize Ni/TiO2 synthesis, the effect of various photodeposition conditions on antibacterial performance were investigated, such as the light irradiation time, metal content, TiO2 crystalline structure, and presence or absence of electron donors (i.e., methanol). The optimized 2 wt% Ni/TiO2 exhibited an antibacterial efficiency of 3.74 log within 7 min, which is more than 10-fold higher than that of pristine TiO2 (2.54 log). Based on this optimized weight ratio, Ni/TiO2 was immobilized on a steel mesh using an electrospray/thermal compression method, and its antibacterial performance was further assessed against E. coli, MS2 bacteriophage virus (MS2 phage), and a common pulmonary pathogen (Legionella thermalis, L. thermalis). Within 70 min, all target microorganisms achieved an inactivation that exceeded 4 log. Furthermore, the long-term stability and sustainable usability of the Ni/TiO2 mesh were confirmed by performing more than 50 antibacterial evaluation cycles using E. coli. The results of this study facilitate the successful utilization of immobilized Ni/TiO2 mesh in water disinfection applications.

Catalytic reactions on magnesium dichloride clusters saturated by titanium tetrachloride

Magnesium dichloride, titanium tetrachloride, and triethyl aluminum are essential components in the Ziegler-Natta catalysts for olefin polymerization. We report a systematic computational investigation (M06-2X/def-TZVP level of theory, thus accounting for dispersion) of the thermodynamics of a sequence of catalytic reactions leading to ethylene polymerization in the absence of electron donors. The reactions are explored for a series of TiCl4-adsorbed magnesium dichloride clusters as a function of the adsorption site, involving (104)- and (110)-type ideal edge sites alongside with corners representing defect sites. The reactions comprise alkylation of TiCl4, reduction from Ti(IV) to Ti(III) and Ti(II), ethylene π-complexation to Ti(III), and chain growth. The polymerization process as a whole is shown to be strongly exergonic for all adsorption sites, with coordination numbers of the site mainly contributing to the reduction and π-complexation energies.

Genotypic, physiological and biochemical features of Desulfovibrio strains in a sulfidogenic microbial community isolated from the soil of ferrosphere

The purpose of this work was the isolation of the predominant representatives of sulfate-reducing bacteria (SRB) of the sulfidogenic microbial community separated from the soil ferrosphere and the examination of their morphological, physiological, biochemical and genotypic peculiarities, the evaluation of some physiological processes under co-culturing with their satellite species Anaerotignum propionicum. During the study two isolates of sulfatereducing bacteria NUChC SRB1 and NUChC SRB2 were obtained from sulfidogenic microbial community isolated from soil ferrosphere on Postgate’s “B” medium and their belonging to different strains (using ISSR-PCR method) was proved. As a result of molecular-genetic analysis of the strains, a 16S rRNA gene fragments of 613 bp and 522 bp were amplified and sequenced. The strains were identified as Desulfovibrio oryzae by the complex of microbiological, physiological and biochemical features and on the basis of 16S rRNA gene sequences (phylogenetic analysis). The 16S rRNA gene sequences were submitted in GenBank as MT102713 (NUChC SRB1) and MT102714 (NUChC SRB2). The co-cultivation of isolated SRB strains with A. propionicum NUChC Sat1 strain (in the absence of electron donors, the presence of sulfates and yeast extract) showed the formation of sulfur-reducing bacteria of hydrogen sulfide, which was not observed during their mono-cultivation. In this case, the phenomenon of syntrophy probably takes place- co-growth on the nutrient substrate, and the electron donor appears due to the use of the yeast extract compounds by the NUChC Sat1 strain. Therefore, in the sulfidogenic community isolated from the soil ferrosphere, there is a mutual growth of the association of bacteria D. oryzae and A. propionicum, which is caused by trophic interaction. Possibly the contribution of these associated bacteria to the corrosion process lies in the utilization of hydrogen (D. oryzae) and the formation of substrate products of SRB metabolism (hydrogen and organic acids), which are both corrosive compounds (A. propionicum). Without a doubt the corrosion process involving this association needs further investigation.

Kinetic studies on the oxidation of semiquinone and hydroquinone forms of Arabidopsis cryptochrome by molecular oxygen

Cryptochromes (crys) are flavoprotein photoreceptors present throughout the biological kingdom that play important roles in plant development and entrainment of the circadian clock in several organisms. Crys non-covalently bind flavin adenine dinucleotide (FAD) which undergoes photoreduction from the oxidised state to a radical form suggested to be active in signalling in vivo. Although the photoreduction reactions have been well characterised by a number of approaches, little is known of the oxidation reactions of crys and their mechanisms. In this work, a stopped-flow kinetics approach is used to investigate the mechanism of cry oxidation in the presence and absence of an external electron donor. This in vitro study extends earlier investigations of the oxidation of Arabidopsis cryptochrome1 by molecular oxygen and demonstrates that, under some conditions, a more complex model for oxidation of the flavin than was previously proposed is required to accommodate the spectral evidence (see P. Müller and M. Ahmad (2011) J. Biol. Chem. 286, 21033–21040 [1]). In the absence of an electron donor, photoreduction leads predominantly to the formation of the radical FADH. Dark recovery most likely forms flavin hydroperoxide (FADHOOH) requiring superoxide. In the presence of reductant (DTT), illumination yields the fully reduced flavin species (FADH−). Reaction of this with dioxygen leads to transient radical (FADH) and simultaneous accumulation of oxidised species (FAD), possibly governed by interplay between different cryptochrome molecules or cooperativity effects within the cry homodimer.

Coupling dark metabolism to electricity generation using photosynthetic cocultures

We investigated the role of green sulfur bacteria inlight-responsive electricity generation in microbial electrochemical cells (MXCs). We operated MXCs containing either monocultures or defined cocultures of previously enriched phototrophic Chlorobium and anode-respiring Geobacter under anaerobic conditions in the absence of electron donor. Monoculture control MXCs containing Geobacter or Chlorobium neither responded to light nor produced current, respectively. Instead, light-responsive current generation occurred only in coculture MXCs. Current increased above background levels only in the dark and declined slowly over 96 h. This pattern suggested that Chlorobium exhausted intracellular glycogen reserves via dark fermentation to supply an electron donor, presumably acetate, to Geobacter. With medium containing sulfide as the sole photosynthetic electron donor, current generation had a similar and reproducible negative light response. To investigate whether this metabolic interaction also occurred without an electrode, we performed coculture experiments in batch serum bottles. In this setup, sulfide served as the sole electron donor, whose oxidation by Chlorobium was required to provide S(0) as the electron acceptor to Geobacter. Copies of Geobacter 16S rDNA increased approximately 14-fold in batch bottle cocultures containing sulfide compared to those lacking sulfide, and did not decline after termination of sulfide feeding. These results suggest that products of both photosynthesis and dark fermentation by Chlorobium were sufficient both to yield an electrochemical response by Geobacter biofilms, and to promote Geobacter growthin batch cocultures. Our work expands upon the fusion of MXCs with coculture techniques and reinforces the utility of microbial electrochemistry for sensitive, real-time monitoring of microbial interactions in which a metabolic intermediate can be converted to electrical current.

Adsorption of TiCl4 and electron donor on defective MgCl2 surfaces and propylene polymerization over Ziegler-Natta catalyst: A DFT study

The formations of defective MgCl2 surfaces, and subsequent adsorption of Ti species and electron donor, as well as propylene polymerization over the Ziegler-Natta catalyst have been investigated using density functional theory (DFT) method. Twelve possible support models of regular and defective MgCl2 (110) and (100) surfaces were built. The individual adsorptions of titanium chlorides as mononuclear or dinuclear, and ethyl benzoate (EB) as electron donor, on these models were evaluated. The analysis of energies presented the cases of EB adsorption were generally more stable than titanium chlorides on both surfaces. Thus, EB as internal electron donor mainly prevented TiCl4 from coordinating on the MgCl2 surfaces where mostly non-stereospecific active sites could be formed. Exceptionally, A5 the site model with terminal Cl-vacancy on the MgCl2 support, presented stronger adsorption of TiCl4 than that of EB on (110) surface. Since the TiCl4 and ethyl benzoate (EB) would compete to adsorb on the support surface, it seems reasonable to assume that TiCl4 might predominately occupy this site, which can act as the most plausible active site for propylene polymerization. The first insertion of propylene monomer into the A5 active site model showed that it exhibited good regioselectivity but poor stereospecificity in the absence of electron donor.

Plasmonic Antenna Effects on Photochemical Reactions

Efficient solar energy conversion has been vigorously pursued since the 1970s, but its large-scale implementation hinges on the availability of high-efficiency modules. For maximum efficiency, it is important to absorb most of the incoming radiation, which necessitates both efficient photoexcitation and minimal electron-hole recombination. To date, researchers have primarily focused on the latter difficulty: finding a strategy to effectively separate photoinduced electrons and holes. Very few reports have been devoted to broadband sunlight absorption and photoexcitation. However, the currently available photovoltaic cells, such as amorphous silicon, and even single-crystal silicon and sensitized solar cells, cannot respond to the wide range of the solar spectrum. The photoelectric conversion characteristics of solar cells generally decrease in the infrared wavelength range. Thus, the fraction of the solar spectrum absorbed is relatively poor. In addition, the large mismatch between the diffraction limit of light and the absorption cross-section makes the probability of interactions between photons and cell materials quite low, which greatly limits photoexcitation efficiency. Therefore, there is a pressing need for research aimed at finding conditions that lead to highly efficient photoexcitation over a wide spectrum of sunlight, particularly in the visible to near-infrared wavelengths. As characterized in the emerging field of plasmonics, metallic nanostructures are endowed with optical antenna effects. These plasmonic antenna effects provide a promising platform for artificially sidestepping the diffraction limit of light and strongly enhancing absorption cross-sections. Moreover, they can efficiently excite photochemical reactions between photons and molecules close to an optical antenna through the local field enhancement. This technology has the potential to induce highly efficient photoexcitation between photons and molecules over a wide spectrum of sunlight, from visible to near-infrared wavelengths. In this Account, we describe our recent work in using metallic nanostructures to assist photochemical reactions for augmenting photoexcitation efficiency. These studies investigate the optical antenna effects of coupled plasmonic gold nanoblocks, which were fabricated with electron-beam lithography and a lift-off technique to afford high resolution and nanometric accuracy. The two-photon photoluminescence of gold and the resulting nonlinear photopolymerization on gold nanoblocks substantiate the existence of enhanced optical field domains. Local two-photon photochemical reactions due to weak incoherent light sources were identified. The optical antenna effects support the unprecedented realization of (i) direct photocarrier injection from the gold nanorods into TiO(2) and (ii) efficient and stable photocurrent generation in the absence of electron donors from visible (450 nm) to near-infrared (1300 nm) wavelengths.

Inhibition of Cyclooxygenase Activity of Prostaglandin-H-Synthase by Excess Substrate (Molecular Oxygen)

For the cyclooxygenase reaction of prostaglandin-H-synthase isolated from ram vesicular glands, dependences of the initial reaction rate, the maximal yield of the product, and the rate constant of enzyme inactivation in the course of reaction on oxygen concentration were studied in the absence and in the presence of electron donor in the reaction medium. It is shown that in the absence of electron donor the cyclooxygenase reaction is strictly governed by Michaelis-Menten kinetics over a wide range of oxygen concentrations (5-800 µM). In the presence of electron donor in the reaction medium it was found that cyclooxygenase reaction is inhibited by an excess of dissolved oxygen: the maximal values of the initial reaction rate and yield of the product are attained at oxygen concentration 50 µM, and its increase to 500 µM causes twofold decrease in the initial rate and maximal yield. The rate constant of enzyme inactivation in the course of reaction increases on increase in oxygen concentration both in the presence and in the absence of electron donor.

Effect of Electron Donors on the Radical Polymerization of Vinyl Acetate Mediated by [Co(acac)2]: Degenerative Transfer versus Reversible Homolytic Cleavage of an Organocobalt(III) Complex

The molecular structure of bis(acetylacetonate)cobalt(II) ([Co(acac)2]) in solution and in the presence of the electron donors (ED) pyridine (py), NEt3, and vinyl acetate (VOAc) was investigated using 1H NMR spectroscopy in C6D6. The extent of formation of ligand adducts, [Co(acac)2(ED)x], varies in the order py>NEt3>VOAc (no interaction). Density functional theory (DFT) calculations on a model system agree with Co--ED bond strengths decreasing in the same order. The effect of electron donors on the [Co(acac)2]-mediated radical polymerization of VOAc was examined at 30 degrees C by the addition of excess py or NEt3 to the complex in the molar ratio [VOAc]0/[Co]0/[V-70]0/[py or NEt3]0=500:1:1:30 (V-70=2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile)). As previously reported by R. Jerome et al., the polymerization showed long induction periods in the absence of ED. However, a controlled polymerization without an induction period took place in the presence of ED, though the level of control was poorer. The effective polymerization rate decreased in the order py>NEt3. A similar behavior was found when these electron donors were added to an ongoing [Co(acac)2]-mediated radical polymerization of VOAc. On the basis of the NMR and DFT studies, it is proposed that the polymerization is controlled by the reversible homolytic cleavage of an organocobalt(III) dormant species in the presence of ED. Conversely, the faster polymerization after the induction period in the absence of ED is due to a degenerative transfer process with the radicals produced by the continuous decomposition of the excess initiator. Complementary experiments provide additional results in agreement with this interpretation.

Photodamage to Pigment in the Photosystem II Reaction Center D1/D2/Cytochrome b559 Complex

AbstractStrong light (800 μmol photons/m2 per s)‐induced bleaching of the pigment in the isolated photosystem II reaction center (PSII RC) under aerobic conditions (in the absence of electron donors or acceptors) was studied using high‐pressure liquid chromatography (HPLC), absorption spectra, 77K fluorescence spectra and resonance Raman spectra. Changes in pigment composition of the PSII RC as determined by HPLC after light treatment were as follows: with increasing illumination time chlorophyll (Chl) a and β‐carotene (β‐car) content decreased. However, decreases in pheophytin (Pheo) could not be observed because of the mixture of the Pheo formed by degraded chlorophyll possibly. On the basis of absorption spectra, it was determined that, with a short time of illumination, the initial bleaching occurred maximally at 680 nm but that with increasing illumination time there was a blue shift to 678 nm. It was suggested that P680 was destroyed initially, followed by the accessory chlorophyll. The activity of P680 was almost lost after 10 min light treatment. Moreover, the bleaching of Pheo and β‐car was observed at the beginning of illumination. After illumination, the fluorescence emission intensity changed and the fluorescence maximum blue shifted, showing that energy transfer was disturbed. Resonance Raman spectra of the PSII RC excited at 488.0 and 514.5 nm showed four main bands, peaking at 1 527 cm‐1 (v1), 1 159 cm‐1 (v2), 1 006 cm‐1 (v3), 966 cm‐1 (v4) for 488.0 nm excitation and 1 525 cm‐1 (v1), 1 159 cm‐1 (v2), 1 007 cm‐1 (v3), 968 cm‐1 (v4) for 514.5 nm excitation. It was confirmed that two spectroscopically different β‐car molecules exist in the PSII RC. After light treatment for 20 min, band positions and bandwidths were unchanged. This indicates that carotenoid configuration is not the parameter that regulates photoprotection in the PSII RC.(Managing editor: Ping He)

Leishmania major encodes an unusual peroxidase that is a close homologue of plant ascorbate peroxidase: a novel role of the transmembrane domain

Haem-containing enzymes (peroxidase and catalase) are widely distributed among prokaryotes and eukaryotes and play a vital role in H2O2 detoxification. But, to date, no haem-containing enzymatic defence against toxic H2O2 has been discovered in Leishmania species. We cloned, expressed and purified an unusual plant-like APX (ascorbate peroxidase) from Leishmania major (LmAPX) and characterized its catalytic parameters under steady-state conditions. Examination of its protein sequence indicated approx. 30-60% identity with other APXs. The N-terminal extension of LmAPX is characterized by a charged region followed by a stretch of 22 amino acids containing a transmembrane domain. To understand how the transmembrane domain influences the structure-function of LmAPX, we generated, purified and extensively characterized a variant that lacked the transmembrane domain. Eliminating the transmembrane domain had no impact on substrate-binding affinity but slowed down ascorbate oxidation and increased resistance to H2O2-dependent inactivation in the absence of electron donor by 480-fold. Spectral studies show that H2O2 can quickly oxidize the native enzyme to compound (II), which subsequently is reduced back to the native enzyme by an electron donor. In contrast, ascorbate-free transmembrane domain-containing enzyme did not react with H2O2, as revealed by the absence of compound (II) formation. Our findings suggest that the single copy LmAPX gene may play an important role in detoxification of H2O2 that is generated by endogenous processes and as a result of external influences such as the oxidative burst of infected host macrophages or during drug metabolism by Leishmania.

Catalytic systems based on TiCl4/MgCl2/SiCl4-n(OR)n for olefin polymerization

The effect of the alcohol nature in TiCl4 catalysts supported on MgCl2, modified by the recrystallization technique with SiCl4, has been investigated in terms of activity and tacticity control. A series of alcoholic solutions of MgCl2, using 1-hexanol, t-butanol, butanol, iso-propanol, propanol and ethanol, were recrystallized with SiCl4 and the supports were subsequently impregnated with TiCl4. Also, as a way of comparison, catalysts were prepared from solutions of MgCl2 in 1-hexanol recrystallized by solvent evaporation and quick cooling. The supports and catalysts were characterized by infrared and energy dispersive scanning (EDS). Polymerization of ethylene and propylene was performed under different temperature and pressure conditions, using triethylaluminium as a cocatalyst in the absence of electron donors. The catalytic activity depended strongly on the length of the alkyl group and the nature of the isomeric alcohol used in the treatment of the support. The yield and isotacticity of the polypropylene are governed by polymerization parameters such as pressure and support treatment. The EDS results confirmed the presence of alkoxysilanes in the catalysts treated with SiCl4.

Spectral and kinetic studies on eosinophil peroxidase compounds I and II and their reaction with ascorbate and tyrosine

Eosinophil peroxidase, the major granule protein in eosinophils, is the least studied human peroxidase. Here, we have performed spectral and kinetic measurements to study the nature of eosinophil peroxidase intermediates, compounds I and II, and their reduction by the endogenous one-electron donors ascorbate and tyrosine using the sequential-mixing stopped-flow technique. We demonstrate that the peroxidase cycle of eosinophil peroxidase involves a ferryl/porphyrin radical compound I and a ferryl compound II. In the absence of electron donors, compound I is shown to be transformed to a species with a compound II-like spectrum. In the presence of ascorbate or tyrosine compound I is reduced to compound II with a second-order rate constant of (1.0±0.2)×10 6 M −1 s −1 and (3.5±0.2)×10 5 M −1 s −1, respectively (pH 7.0, 15°C). Compound II is then reduced by ascorbate and tyrosine to native enzyme with a second-order rate constant of (6.7±0.06)×10 3 M −1 s −1 and (2.7±0.06)×10 4 M −1 s −1, respectively. This study revealed that eosinophil peroxidase compounds I and II are able to react with tyrosine and ascorbate via one-electron oxidations and therefore generate monodehydroascorbate and tyrosyl radicals. The relatively fast rates of the compound I reduction demonstrate that these reactions may take place in vivo and are physiologically relevant.

ESR and UV-Vis studies of semiquinone radical anion and hydroquinone generated by irradiation of 15-Deacetyl-13-glycine substituted hypocrellin B

15-Deacetyl-13-glycine substituted hypocrellin B (GDHB) is a new type of hypocrellin derivative with enhanced red absorption longer than 600 nm and water solubility. When an anaerobic DMSO or DMSO-buffer (pH 7.4) solution of GDHB was illuminated with > 470 nm light, a strong electron spin resonance (ESR) signal was formed. The ESR signal was assigned to the semiquinone anion radical of GDHB (GDHB·-) based on a series of experiments. GDHB·- was predominantly photoproducted via the self-electron transfer between the excited- and ground-state species. Decay of this species, both in the presence and absence of electron donor, was consistent with second-order kinetics. In aqueous solution, the TEMPO counterspin experiment indicated the formation of GDHB·- that could not be detected by ESR method directly. The formation of GDHB·- and hydroquinone of GDHB (GDHBH·-) was also confirmed by spectrometric method. These findings suggested that GDHB was at least a favorable type I phototherapeutic agent.

Photodynamic action of a water-soluble hypocrellin derivative with enhanced absorptivity in the phototherapeutic window I: ESR and UV-Vis studies on the photogeneration of the semiquinone anion radical and hydroquinone

Cysteine-substituted hypocrellin B (Cys-HB) is a hypocrellin derivative with enhanced water solubility and absorption in the domain of phototherapeutic window (600–900 nm). Illumination of Cys-HB in DMSO or DMSO-buffer (pH 8.0) generated a strong Electron Spin Resonance (ESR) signal. The ESR signal was assigned to the semiquinone anion radical of Cys-HB (Cys-HB •−) based on a series of experimental results. Decay of Cys-HB •− in the presence and absence of electron donors, were both consistent with second-order kinetics. Spin counteraction of TEMPO by Cys-HB photosensitization indicated the formation of Cys-HB •− which couldn't be detected by ESR method directly in aqueous solution or acidic media. Spectrophotometric measurements showed that the absorption bands at 623 nm, and 504 nm (pH 8.0) or 520 nm (pH 11.0) were those of Cys-HB •− and hydroquinone of Cys-HB (Cys-HBH 2), respectively. The kinetical results of Cys-HB •− decay were in agreement with those by ESR measurements. Compared with hypocrellin B (HB), Cys-HB •− was more stable and less susceptible in strong alkaline media. Strong intramolecular hydrogen bonding was considered to be present in the chromophore of CysHB •−. The results in this study showed that Cys-HB was at least a favorable Type I phototherapeutic agent.

The general kinetic model of electron transfer in photosynthetic reaction centers activated by multiple flashes.

A new general kinetic model for the functioning of photosynthetic reaction centers (RC) of purple bacteria, under multiple flash activation, has been developed. The model includes the primary electron donor (P870) as well as the primary (QA) and secondary (QB) acceptor quinones. The new features of this general model include: (1) consideration of four different states of the QB binding site (vacant, occupied by QB, by QB- and by QBH2), (2) incorporation of the dark relaxation of the RC between flashes, (3) the assumption of fast exchange of quinones between the RC and quinone pool in detergent micelles or chromatophore membrane, (4) description of the kinetics of electron transfer in both oxidized (no donor for P870+) and reduced (in the presence of donor for P870+) conditions simultaneously, (5) the consideration of both single and multiple flash activation of the RC of purple bacteria and (6) consideration of the cumulative effects of all previous flashes of the series in the response induced by the current flash. This model is used to calculate and predict (1) flash-induced binary oscillations of the secondary acceptor semiquinone (QB-), (2) flash-induced behavior of P870+ in the presence and absence of electron donor and (3) the apparent equilibrium constant of electron transfer between QA and QB and others. Different characteristics of RC are analyzed as a function of flash intensity, time between flashes, concentration of electron donor, redox-potential of the medium, concentration of pool quinone and quinol, association and dissociation equilibrium constants for quinone and quinol at the QB binding site, equilibrium constants of electron transfer between QA- and QB and between QA- and QB-, as well as the rate constants of oxidation of QA- and QB- by redox mediators. The proposed model can be used as a basis for assays of kinetic behavior of native and mutant RC of purple bacteria and for determination of the factors influencing the release of QH2 from RC. The latter is needed for analysis of factors controlling light-activated electron transport in the cytochrome bc1 complexes of purple bacteria by quinol molecules released from RC. The developed general approach for parallel consideration of flash-induced transitions of RC and its following dark relaxation between flashes can also be used for kinetic description of photosynthetic RC of oxygenic photosynthesis.

UV-visible and electron spin resonance study of mercapto-substituted hypocrellin B: photogeneration of semiquinone radical anion and superoxide

Mercapto-substituted hypocrellin B derivatives (MHBDs), display, in the absence of electron donors, a characteristic absorbance peak and electron spin resonance (ESR) signal, which can be attributed to the semiquinone radical anion, formed by intermolecular electron transfer. In the presence of electron donors, the amplitude of the ESR signal increases significantly. Irradiation of MHBDs in the presence of oxygen leads to the generation of superoxide radicals, which may be registered by the spin trapping technique: the formation mechanism of superoxide in the absence or presence of electron donors is discussed. The free radicals and superoxide radicals formed on photoirradiation of MHBDs may play a hitherto unrecognized role in biological processes.