Different kinetics of photoinactivation of photosystem I-mediated electron transport and P700 in isolated thylakoid membranes

Photoinactivation kinetics of photosystem I (PSI)-mediated electron transport rate was compared to that of P700 content at room (22 degrees C) and low (4 degrees C) temperatures in isolated spinach thylakoid membranes. The high light treatment was carried out under aerobic and anaerobic conditions. At 22 degrees C the decrease of electron transport rate showed first order exponential kinetics. The amount of P700 decreased linearly, being less affected in the first hours of illumination. During photoinhibition at 4 degrees C in the presence of oxygen, the kinetics of inactivation of PSI photochemical activity and the content of P700 were different. It was found that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) had different protective effect on the electron transport rate and on P700 content at both temperatures. Treatment with high light intensity under N(2) atmosphere had no effect on the electron transport rate or P700 content. The possible degradation of PSI reaction centre proteins was determined using immunoblot methods. In the presence of linear electron transport at 22 degrees C correlation between formation of toxic hydroxyl radicals and inhibition of oxygen uptake was observed.     

Phosphate release from N ions irradiated 5′-CMP nucleotide and its kinetics

Ion beam irradiation-induced inorganic phosphate (Pi) release from 5′-CMP nucleotide could be divided into two parts: fast process and slow process. The yield of Pi released from the fast process not only had a non-linear response to irradiation dose, but was also influenced by post-irradiation treatments. It was found that heat treatment and alkali treatment increased the yield of Pi but acid treatment decreased it. With the heat treatment lower than 80°C, the Pi release process followed the first order reaction model, but under the conditions of 95°C treatment and alkali treatment the Pi release process followed the second order reaction model. Moreover, the kinetics equations and the rate constants of different models of the Pi release reactions were deduced.     

Photoageing of an electron beam cured polyurethane acrylate resin

Photoageing reactions and kinetics on long wavelength irradiation of an electron beam (EB) cured resin are presented. The material studied is an aliphatic polyurethane acrylate resin. The chemical photoreactivity of the polymer was followed by infrared spectroscopy and UV-visible spectrophotometry. Two periods of photo-oxidation were found. In the first stage the reaction of acrylate functions remaining after EB polymerization leads to additional crosslinking and to oxidized products. The tip of an atomic force microscope was used as an indenter to follow the variation of stiffness at the surface resulting from crosslinking. In the second stage, oxidation rate decreases and photodegradation products are formed at a constant rate. A heterogeneous distribution of residual acrylates was evidenced across the coating thickness (thick films of 100 μm). An oxidation profile was also observed in the depth of the film after photoageing.     

Interpretation of the experimental data on the reduction reaction of NO by CO on rhodium by Monte Carlo simulations and by solving the kinetic equations of the reaction mechanism

Some mechanisms of the reduction reaction of NO by CO on rhodium are analyzed and discussed, solving the kinetics equations and using Monte Carlo simulations, in terms of its ability to interpret the recent experiments of Zaera et al., who used a molecular beam method to study experimentally the kinetics of the reaction. Critical use is also made of the information on rate constants available for this system in the literature. Uniform catalytic surfaces and the statistical incipient percolation cluster (IPC) fractal are considered in the simulations.     

KINETICS STUDY ON MELT GRAFTING ACRYLIC ACID ONTO LLDPE USING REACTIVE EXTRUSION

The kinetics of melt grafting acrylic acid(AA)onto linear low density polyethylene(LLDPE)by using reactive extrusion was investigated.The polymeric peroxides(POOP and POOH)generated by electron beam irradiation were used to initiate the graft reaction.The samples taken out from the barrel at five ports along screw axis were analyzed by FTIR.The spectra show that both the graft copolymerization and homopolymerization proceed in two stages:the graft degree(or mass of homopolymer)increases linearly with the reaction time in the initial stage,and then gradually in the second stage.The rate of graft copolymerization R_g is always faster than that of homopolymerization R_h in the present system and the activation energy is 131 kJ·mol~(-1) for graft copolymerization and 127 kJ·mol~(-1) for homopolymeirzation.These results were interpreted in terms of solubility and diffusion of monomer,as well as the reactivity and the concentration of reactive species. The relationships between reaction rate and monomer concentration and peroxide concentration were found to he:R_g ∝ [M]~(1.46)[POOP+POOH]~(0.53) and R_h ∝[M]~(1.08)[POOH]~(0.51),which indicate that the addition of monomer to polymeric radicals is a slow step for the graft copolymerization.     

Reduction of chromium(VI) by phosphonic acid

The kinetics of electron transfer between chromium(VI) and H3PO3, yielding chromium(III), have been investigated in HClO4 and H2SO4 media by visible spectrophotometry. The rate of reaction increased with increasing [H2SO4] and [HClO4]. A rate law based on ester formation preceding the electron transfer has been established and a possible mechanism has been proposed. The mechanism and the derived rate law are consistent with the observed kinetics. This revised version was published online in June 2006 with corrections to the Cover Date.     

Transient absorption studies and numerical modeling of iodine photoreduction by nanocrystalline TiO2 films

We have used transient absorption spectroscopy to study the reaction between photogenerated electrons in a dye-free nanocrystalline titanium dioxide film and an iodine/iodide redox couple. Recombination kinetics was measured by recording the transient optical signal following band gap excitation by a UV laser pulse. In the presence of a methanol hole scavenger in the electrolyte, a long-lived (0.1-1 s) red/infrared absorbance is observed and assigned to photogenerated electrons forming Ti(3+) species. In the presence of iodine and excess iodide in the electrolyte, the signal decays on a millisecond-microsecond time scale, assigned to reduction of the redox couple by photogenerated electrons in the TiO(2). The electron lifetime decreases inversely with increasing iodine concentration, indicating that the back reaction is first order in [I(2)]. No evidence for I(2)(-) is observed, indicating that the reaction mechanism does not involve the formation of I(2)(-) as an intermediate. The shape of the kinetics evolves from monoexponential at low [I(2)] to stretched-exponential as [I(2)] increases. A Monte Carlo continuous-time random walk model is implemented to simulate the kinetics and its [I(2)] dependence and used to address the order of the recombination reaction with respect to electron density, n. The model incorporates the diffusion of oxidized species from the electrolyte toward the TiO(2) surface as well as electron trapping and transport in the TiO(2). In the limit of low [I(2)], the monoexponential kinetics is explained by the recombination reaction being rate limited by the diffusion of the oxidized species in the electrolyte. The stretched-exponential behavior at high [I(2)] can be explained by the reaction being rate limited by the transport of electrons through a distribution of trap states toward reactive sites at the TiO(2)-electrolyte interface, similar to the mechanism proposed previously for the kinetics of electron-dye cation recombination. Such trap-limited recombination can also explain the superlinear dependence of electron recombination rate on electron density, which has been reported elsewhere, without the need for a reaction mechanism that is second order in n. In contrast, a second-order reaction mechanism in a trap-free medium cannot explain the observed kinetics, although a second-order mechanism incorporating electron trapping cannot be conclusively ruled out by the data. We propose that the most likely reaction scheme, that is first order in both [I(2)] and n, is the dissociative reduction of I(2) onto the metal oxide surface, followed by a second electron reduction of the resulting adsorbed iodine radical, and that empirical second-order behavior of the electron lifetime is most likely explained by electron trapping rather than by a second-order recombination mechanism.     

Effect of electron beam irradiation on the formation of active sites in the Pt/H pentasil catalyst

The influence of preirradiation with an electron beam from a linear resonance electron accelerator in flowing argon on the structure and properties of the 1% Pt/H pentasil catalyst was studied. The support structure was studied by x-ray diffraction and low-temperature nitrogen adsorption. No changes in the crystal structure and pentasil specific surface area were observed in the irradiation dose range from 120 to 900 Mrad at an average electron energy of 7.7 MeV. The electron beam treatment resulted in a considerable increase in the catalytic activity of the Pt/H pentasil in gas-phase toluene hydrogenation as a model reaction. The results obtained by IR spectroscopy of adsorbed CO and X-ray diffraction data suggested that the increase in the catalyst activity after electron beam irradiation is due to changes in the size and charge of the Pt particles.     

The photokinetics of phenothiazine dyes with titanium trichloride in different solvents

The kinetic parameters of photoinduced electron transfer reaction of two phenothiazine dyes, methylene blue and methylene green with titanium trichloride, were determined in water and different aqueous-alcoholic solvents at different acidities by using a specially designed optical system. The rate of photoinduced electron transfer reaction was measured by determining the quantum yield of the reaction. The methylene green had a higher reactivity as compared to methylene blue with titanium trichloride. The graphical analysis showed that the reaction of dye with titanium trichloride follows pseudo–first-order kinetics. A reaction mechanism was proposed by considering the different excited states of dye and their possible interaction with the solvent and titanium trichloride. The different steps in the reaction mechanism were taken into consideration for deriving rate equations, which were used to determine the different rate constants in the reaction mechanism in different solvents.     

卟啉铁与抗坏血酸均相电子转移反应的动力学和机理

采用电子吸收光谱和光谱－电化学方法研究了中位－四（邻硝基苯基）四苯并卟啉的Fe(Ⅲ）配合物与抗坏血酸在DMF溶液中的均相电子转移反应的动力学和机理。结果表明此电子转移反应来源于抗坏血酸与铁卟啉中心铁离子的轴向配位作用,并将一个电子转移至铁离子。反应速度对铁卟啉和抗坏血酸均为一级,并与抗坏血酸的离解有关。     

二(乙酰丙酮根)合铜(Ⅱ)热分解动力学研究

金属有机化合物气相化学沉积(OMCTD)形成铜膜常用的母体化合物是铜(Ⅱ)的β-二酮类配合.本文首次采用CW二氧化碳激光研究二(乙酰丙酮根)合铜(Ⅱ)[Cu相似文献   

Simulation of the Kinetics of Methane Conversion in the Presence of Water in a Barrier Discharge

A method is proposed for the simulation of chemical kinetics in a dielectric barrier discharge (DBD) with the use of an effective rate constant of electron–molecule reaction and a simple expression for its estimation. The results of the simulation of the kinetics of methane conversion in DBD in the presence of water are presented.     

A new approach to determining gas-particle reaction probabilities and application to the heterogeneous reaction of deliquesced sodium chloride particles with gas-phase hydroxyl radicals

The reaction kinetics for gaseous hydroxyl radicals (OH) with deliquesced sodium chloride particles (NaCl(aq)) were investigated using a novel experimental approach. The technique utilizes the exposure of substrate-deposited aerosol particles to reactive gases followed by chemical analysis of the particles using computer-controlled scanning electron microscopy with energy-dispersive analysis of X-rays (CCSEM/EDX) capability. Experiments were performed at room temperature and atmospheric pressure with deliquesced NaCl particles in the micron size range at 70-80% RH and with OH concentrations in the range of 1 to 7 x 10(9) cm(-3). The apparent, pseudo first-order rate constant for the reaction was determined from measurements of changes in the chloride concentration of individual particles upon reaction with OH as a function of the particle loading on the substrate. Quantitative treatment of the data using a model that incorporates both diffusion and reaction kinetics yields a lower limit to the net reaction probability of gamma(net) > or = 0.1, with an overall uncertainty of a factor of 2.     

The kinetics of electron transfer reaction of methylene green and titanium trichloride in different solvents

The kinetics of the electron transfer reaction of methylene green and titanium trichloride was investigated in different solvents by spectrophotometry at different temperatures. The the reaction rate was determined by monitoring the absorbance as a function of time at λ_max 655 nm. The reaction is pseudo-first order, dependent only on the concentration of titanium trichloride at a fixed concentration of methylene green.     

Reaction-induced phase separation in polyethersulfone-modified epoxy-amine systems studied by temperature modulated differential scanning calorimetry

In epoxy-amine systems with a thermoplastic additive, the initially homogeneous reaction mixture can change into a multi-phase morphology as a result of the increase in molecular weight or network formation of the curing matrix. Temperature modulated DSC (TMDSC) allows the real-time monitoring of this reaction-induced phase separation. A linear polymerizing epoxy-amine (DGEBA–aniline) and a network-forming epoxy-amine (DGEBA–methylene dianiline), both with an amorphous engineering thermoplastic additive (polyethersulfone, PES), are used to illustrate the effects of phase separation on the signals of the TMDSC experiment. The non-reversing heat flow gives information about the reaction kinetics. The heat capacity signal also contains information about the reaction mechanism in combination with effects induced by the changing morphology and rheology such as phase separation and vitrification. In quasi-isothermal (partial cure) TMDSC experiments, the compositional changes resulting from the proceeding phase separation are shown by distinct stepwise heat capacity decreases. The heat flow phase signal is a sensitive indication of relaxation phenomena accompanying the effects of phase separation and vitrification. Non-isothermal (post-cure) TMDSC experiments provide additional real-time information on further reaction and phase separation, and on the effect of temperature on phase separation, giving support to an LCST phase diagram. They also allow measurement of the thermal properties of the in situ formed multi-phase materials.     

Remarkable influence of surface composition and structure of oxidized iron layer on orange I decomposition mechanisms

Although the decomposition of water pollutants in the presence of metallic iron is known, the reaction pathways and mechanisms of the decomposition of azo-dyes have been meagerly investigated. The interface phenomena taking place during orange I decomposition have been investigated with the use of infrared external reflection spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The studies presented in this paper establish that there are close relationships between the composition and structure of the iron surface oxidized layers and the kinetics and reaction pathway of orange decomposition. The influence of the molecular structure of azo-dye on the produced intermediates was also studied. There are remarkable differences in orange I decomposition between pH 3 and pH 5 at 30 degrees C. Decomposition at pH 3 is very fast with pseudo-first-order kinetics, whereas at pH 5 the reaction is slower with pseudo-zero-order kinetics. At pH 3, only one amine, namely 1-amino-4-naphthol, was identified as an intermediate that undergoes future decomposition. Sulfanilic acid, the second harmful reduction product, was not found in our studies. At pH 3, the iron surface is covered only by a very thin layer of polymeric Fe(OH)(2) mixed with FeO that ensures orange reduction by a combination of an electron transfer reaction and a catalytic hydrogenation reaction. At pH 5, the iron surface is covered up to a few micrometers thick, with a very spongy and porous layer of lepidocrocite enriched in Fe(2+) ions, which slows the electron transfer process. The fastest decomposition reaction was found at a potential near -300 mV (standard hydrogen electrode). An addition of Fe(2+) ions to solution, iron preoxidation in water, or an increase of temperature all result in an increasing decomposition rate. There is no single surface product that would inhibit the decomposition of orange. This information is crucial to perform efficient, clean and low cost waste water treatment. The findings presented here make the treatment of wastewater in the presence of metallic iron a very promising solution.     

Measuring the Electrode Kinetics of Vitamin B2 at a Constant Time Window of a Square Wave Voltammetric Experiment

The electrode kinetics of vitamin B2 was measured at a constant scan rate of a square‐wave (SW) voltammetric experiment by recently introduced method based on alteration of the height (amplitude) of the SW potential pulses. The electrode reaction mechanism was analyzed in the light of the simplified model considering two‐electron one step surface electrode mechanism, as well as to more appropriate EE surface reaction pathway consisting of two successive one‐electron quasireversible reactions. Theoretical analysis indicated that the evolution of the voltammetric response under variation of the SW amplitude enables estimation of the rate determining step of the EE mechanism.     

Effect of electron beam irradiation on CO2 reforming of methane over Ni/Al2O3 catalysts

The effect of electron beam irradiation on the CO₂ reforming of methane over Ni/Al₂O₃ was investigated. The conversion rate of CO₂ and CH₄ forming H₂ and CO using various catalysts irradiated with an absorbed dose greater than 2 MGy was 5–10% higher than when using an untreated catalyst. The Ni/O ratio on the catalyst surface increased after treatment with an electron beam, and was more prominent for catalysts with a higher Ni content. As such, based on XRD and XPS measurements, electron beam treatment was found to result in either the desorption of oxygen from NiO or the removal of OH groups from the outermost surface layer of the catalyst. In addition, the concentration of active sites, such as Ni²⁺ and NiO, or surface defects was also found to increase with the absorbed radiation dose, thereby increasing the conversion rate.     

Kinetics of the formation of copper β-octaphenylporphyrin complexes in pyridine and acetic acid

The formation kinetics of copper β-octaphenylporphyrin complexes in pyridine and acetic acid is reported and is compared with that of copper β-octamethylporphyrin and dodecaphenylporphyrin complexes. The introduction of electron-donating or electron-withdrawing substituents in the β-positions of the porphyrin macrocycle change the rate of the complexation reaction by at most one order of magnitude. On passing from the planar porphyrin macrocycle to the heavily distorted one, the rate of the reaction in pyridine (electron donor solvent) and acetic acid (electron acceptor solvent) increases and decreases, respectively, by several orders of magnitude.     

Variable-temperature rate coefficients for the electron transfer reaction N2+ + H2O measured with a coaxial molecular beam radio frequency ring electrode ion trap

The neutral molecule temperature dependence of the rate coefficient for the electron transfer reaction from H(2)O to N(2)(+) is determined using a coaxial molecular beam radio frequency ring electrode ion trap (CoMB-RET) method. The temperature of the N(2)(+) ions was maintained at 100 K, while the effusive water beam temperature was varied from 300 to 450 K. The result demonstrates the neutral molecule rotational/translational energy dependence on the rate coefficient of an ion-dipolar molecule reaction. It is found that the rate coefficient in the above temperature range follows the prediction of the simplest ion-dipole capture model. Use of different buffer gas collisional cooling in both the ion source and the RET reveals the effects of both translational and vibrational energy of the N(2)(+) ions.