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1.
The rate of electron transfer between reduced cytochrome ƒ and plastocyanin (both purified from parsley) has been measured as k = 3.6 · 10 7 M −1 · s −1, at 298 °K and pH 7.0, with activation parameters Δ H‡ = 44 kJ · mole −1 and Δ S‡ = +46 J · mole −1 · °K −1. Replacement of cytochrome ƒ with red algal cytochrome c-553, Pseudomonas cytochrome c-551 and mammalian cytochrome c gave rates at least 30 times slower: k = 5 · 10 5, 7.5 · 10 5 and 1.0 · 10 6 M −1 · s −1, respectively. Similar measurements made with azurin instead of plastocyanin gave k = 6 · 106 and approx. 2 · 107 M−1 · s−1 for reaction of reduced azurin with cytochrome ƒ and algal cytochrome respectively. Rate constants of 115 and 80 M−1 · s−1 were found for reduction of plastocyanin by ascorbate and hydroquinone at 298 °K and pH 7.0. The rate constants for the oxidation of plastocyanin, cytochrome ƒ, Pseudomonas cytochrome c-551 and red algal cytochrome c-553 by ferricyanide were found to be between 3 · 104 and 8 · 104 M−1 · s−1. The results are discussed in relation to photosynthetic electron transport. 相似文献
2.
Under physiological pH conditions (pH 7.2-7.4) the rate constant of the reaction NO + O 2 yielding peroxonitrite (ONOO) was determined as k = (3.7 ± 1.1) × 10 7 M 1 s 1. The decay of peroxonitrite at this pH follows first order kinetics with a rate constant of 1.4 s 1. At alkaline pH peroxonitrite is practically stable.
Possible consequences of these reactions for the biological lifetime of EDRF will be discussed. 相似文献
3.
The fluorescence induction and other fluorescence properties of spinach chloroplasts at room temperature were probed utilizing two 30-ps wide laser pulses (530 nm) spaced Δ t (s) apart in time (Δ t = 5–110 ns). The energy of the first pulse (P 1) was varied (10 12–10 16 photons · cm −2), while the energy of the second (probe) pulse (P 2) was held constant (5 · 10 13 photons · cm −2). A gated (10 ns) optical multichannel analyzer-spectrograph system allowed for the detection of the fluorescence generated either by P 1 alone, or by P 2 alone (preceded by P 1). The dominant effect observed for the fluorescence yield generated by P 1 alone is the usual singlet-singlet exciton annihilation which gives rise to a decrease in the yield at high energies. However, when the fluorescence yield of dark-adapted chloroplasts is measured utilizing P 2 (preceded by pulse P 1) an increase in this yield is observed. The magnitude of this increase depends on Δ t, and is characterized by a time constant of 28 ± 4 ns. This rise in the fluorescence yield is attributed to a reduction of the oxidized (by P 1) reaction center P-680 + by a primary donor. At high pulse energies (P 1 = 4 · 10 14 photons · cm −2) the magnitude of this fluorescence induction is diminished by another quenching effect which is attributed to triplet excited states generated by intense P 1 pulses. Assuming that the P 1 pulse energy dependence of the fluorescence yield rise reflects the closing of the reaction centers, it is estimated that about 3–4 photon hits per reaction center are required to close completely the reaction centers, and that there are 185–210 chlorophyll molecules per Photosystem II reaction center. 相似文献
4.
1. 1. The superoxide anion radical (O2−) reacts with ferricytochrome c to form ferrocytochrome c. No intermediate complexes are observable. No reaction could be detected between O2− and ferrocytochrome c. 2. 2. At 20 °C the rate constant for the reaction at pH 4.7 to 6.7 is 1.4 · 106 M−1 · s−1 and as the pH increases above 6.7 the rate constant steadily decreases. The dependence on pH is the same for tuna heart and horse heart cytochrome c. No reaction could be demonstrated between O2− and the form of cytochrome c which exists above pH ≈ 9.2. The dependence of the rate constant on pH can be explained if cytochrome c has pKs of 7.45 and 9.2, and O2− reacts with the form present below pH 7.45 with k = 1.4 · 106 M−1 · s−1, the form above pH 7.45 with k = 3.0 · 105 M−1 · s−1, and the form present above pH 9.2 with k = 0. 3. 3. The reaction has an activation energy of 20 kJ mol−1 and an enthalpy of activation at 25 °C of 18 kJ mol−1 both above and below pH 7.45. It is suggested that O2− may reduce cytochrome c through a track composed of aromatic amino acids, and that little protein rearrangement is required for the formation of the activated complex. 4. 4. No reduction of ferricytochrome c by HO2 radicals could be demonstrated at pH 1.2–6.2 but at pH 5.3, HO2 radicals oxidize ferrocytochrome c with a rate constant of about 5 · 105–5 · 106 M−1 · s−1
. 相似文献
5.
Thermally stimulated luminescence glow curves of CaB 4O 7:Dy samples after β‐irradiation showed glow peaks at ~335, 530 and 675 K, with a heating rate of 2 K/s. The main peak at 530 K was analyzed using the T max–T stop method and was found to be composed of at least five overlapping glow peaks. A curve‐fitting program was used to perform computerized glow curve deconvolution (CGCD) analysis of the complex peak of the dosimetric material of interest. The kinetic parameters, namely activation energy ( E) and frequency factor ( s), associated with the main glow peak of CaB 4O 7:Dy at 520 K were evaluated using peak shape (PS) and isothermal luminescence decay (ILD) methods. In addition, the kinetics was determined to be first order ( b =1) by applying the additive dose method. The activation energies and frequency factors obtained using PS and ILD methods are calculated to be 0.72 and 0.72 eV and 8.76 × 10 5 and 1.44 × 10 6/s, respectively. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
6.
In this paper a number of experiments with the purple bacteria Rhodospirillum rubrum and Rhodopseudomonas capsulata is described in which the total fluorescence yield and/or the total fraction of reaction centers closed after a picosecond laser pulse were measured as a function of the pulse intensity. The conditions were such that the reaction centers were either all in the open or all in the closed state before the pulse arrived. These experiments are analysed using the theoretical formalism discussed in the preceding paper (Den Hollander, W.T.F., Bakker J.G.C., and Van Grondelle, R., Biochim. Biophys. Acta 725, 492–507). From the experimental results the number of connected photosynthetic units, λ, the rate of energy transfer between neighboring antenna molecules, kh, and the rate of trapping by an open reaction center, kot, can be estimated. For R. rubrum it is found that λ = 14−17, kh = (1−2)·10 12 s −1 and kot = (4−6)·10 11 s −1, for Rps. capsulata λ ≈ 30, kh ≈ 4·10 11 s −1 and kot ≈ 3·10 11 s −1. The findings are discussed in terms of current models for the structure of the antenna and the kinetic properties of the decay processes occurring in these purple bacteria. 相似文献
7.
1. Rate constants for reduction of paraquat ion (1,1′-dimethyl-4,4′-bipyridy-lium, PQ 2+) to paraquat radical (PQ +·) by e−aq and CO 2−· have been measured by pulse radiolysis. Reduction by e−aq is diffusion controlled ( k = 8.4·10 10 M −1·s −1) and reduction by CO 2−· is also very fast k = 1.5·10 10 M −1·s −1). 2. The reaction of paraquat radical with oxygen has been analysed to give rate constants of 7.7·108 M−1·s−1 and 6.5·108 M−1·s−1 for the reactions of paraquat radical with O2 and O2−·, respectively. The similarity in these rate constants is in marked contrast to the difference in redox potentials of O2 and O2−· (− 0.59 V and + 1.12 V, respectively). 3. These rate constants, together with that for the self-reaction of O2−·, have been used to calculate the steady-state concentration of O2−· under conditions thought to apply at the site of reduction of paraquat in the plant cell. On the basis of these calculations the decay of O2−· appears to be governed almost entirely by its self-reaction, and the concentration 5 μm away from the thylakoid is still 90% of that at the thylakoid itself. Thus, O2−· persists long enough to diffuse as far as the chloroplast envelope and tonoplast, which are the first structures to be damaged by paraquat treatment. O2−· is therefore sufficiently long-lived to be a candidate for the phytotoxic product formed by paraquat in plants. 相似文献
8.
We have used picosecond absorption spectroscopy with low intensity (5 · 10 11–5 · 10 12 photons · pulse −1 · cm −2) continuously tunable infrared (800–900 nm) pulses to study the energy transfer dynamics in the isolated B800–850 pigment-protein complex of Rhodobacter sphaeroides. Our results suggest the following picture of the energy transfer dynamics: (i) a fast transfer, within approx. 1 ps, from BChl 800 to BChl 850; (ii) transfer among different BChl 800's with a rate which is at the most of the same order of magnitude as that of BChl 800 → BChl 850 transfer; (iii) very fast transfer ( k > 1 · 10 12 s −1) between BChl 850 molecules. Assuming Förster type of energy transfer maximum distances of about 22 and 15 Å are obtained for the BChl 800–BChl 850 and BChl 850–BChl 850 separations, respectively. 相似文献
9.
The reduction of spinach ferredoxin by V(II)-EDTA, Eu(II)-DPTA and propylene viologen was monitored electrochemically. The rates of these reactions were found to be 3.0 · 10 4, 3.2 · 10 5 and 1.2 · 10 5 M −1 · s −1, respectively, by the use of chronoamperometry, pulse polarography, differential pulse polarography and rotating-disk voltammetry. These reaction rates were analyzed by tunneling theory for electron transfers, and the comparisons between the theoretical and experimental values were quite good. The tunneling theory also worked quite well with other ferredoxin electron-transfer rates that are available in the literature. In addition to that, the activation enthalpy and entropy compared well with the tunneling theory. 相似文献
10.
The kinetics and equilibria of complex formation by Ga(III) with NCS − in aqueous solution have been measured over a range of acidities and temperatures, the contributing paths to the reaction resolved, and their rate constants and activation parameters determined. The hydrolysis equilibria required to carry out this resolution of kinetic behaviour have also been measured. Unlike the other reported complexation reactions of Ga(III) in aqueous solution, the separate reaction pathways can be assigned with no ambiguity. At 25 °C and ionic strength 0.5 M, the observed forward rate constant for the complex formation is described by {k1 + k2K1h/[H+] + k3K1hK2h/[H+]2} M−1 s−1. For these conditions, the first and second successive hydrolysis constants of Ga(H2O)63+ are given by pK1h = 3.69 ± 0.01 and pK2h = 3.74 ± 0.04. The rate constants corresponding to the reactions of the species Ga(H2O)63+, Ga(H2O)5(OH)2+ and Ga(H2O)4(OH)2+ with NCS− are k1 = 57 ± 4 M−1 −1, k2 = (1.08 ± 0.01) × 105 M−1 s−1 and k3 = 3 × 106 M−1 s−1 respectively. The complexation equilibrium quotient [GaNCS2+]/([Ga3+][NCS−]) has been independently determined by spectrophotometric titration to be 20.8 ± 0.3 M−1 at 25 °C and ionic strength 0.5 M. These kinetic results lead to an interpretation of the data, and a reinterpretation of other data for aquo-Ga(III) complex formation kinetics from the literature which support the assignment of a dissociative interchange mechanism for these reactions rather than the associative activation mode sometimes proposed. 相似文献
11.
Carotenoicls in chloroform and carbon tetrachloriclc photobleach upon nanosecond laser flash photolysis in two steps: instantaneously and in a second-order reaction. The rate constant for second-order reaction (first-order in a solvent derived radical and first-order in (excess) ccirotenoid) is largest for carotenes (9.8·10 8 M -1 s -1 for β-carotene), intermediate for hydroxylated carotenoids, and smallest for carbonyl containing carotenoids (1.0·10 8 M -1 s -1 for astaxanthin) in chloroform at 20°C. Near infrared, ibsorbing transients are formed concomitant with pliotohleaching in chloroform (not detected in cxbon tetrachloride). A species formed instantaneously is tentatively identified as either a carotenoid/solvent adduct or an ion-pair. A second species is formed by decay of the instantaneously formed species and is identified as the carotenoid radical cation. This species is formed in a first-order reaction with a rate constant of approx. 5·10 4 s -1 and absorbing at longer wavelength than the precursor. The lifetime (second-order decay) of the interniediates appears to be longest for the carotenoids with the longest conjugated system. The results indicate that carotenes are better antioxidants than xantliophylls as the carotenes, at least in the present lipophilic solvents, react faster with free radicals. 相似文献
12.
1. Single reduced methyl viologen (MV .+) acts as an electron donor in a number of enzyme systems. The large changes in extinction coefficient upon oxidation (λ max 600 nm; MV .+, = 1.3 · 10 4 M −1 · cm −1; oxidised form of methyl viologen (MV 2+), = 0.0) make it ideally suited to kinetic studies of electron transfer reactions using stopped-flow and standard spectrophotometric techniques. 2. A convenient electrochemical preparation of large amounts of MV.+ has been developed. 3. A commercial stopped-flow apparatus was modified in order to obtain a high degree of anaerobicity. 4. The reaction of MV.+ with O2 produced H2O2 (k > 5 · 106 M−1 · s−1, pH 7.5, 25 °C). H2O2 subsequently reacted with excess MV.+ (k = 2.3 · 103 M−1 · s−1, pH 7.5, 25 °C) to produce water. The kinetics of this reaction were complex and have only been interpreted over a limited range of concentrations. 5. The results support the theory that the herbicidal action of methyl viologen (Paraquat, Gramoxone) is due to H2O2 (or radicals derived from H2O2) induced damage of plant cell membrane. 相似文献
13.
Direct evidence obtained by means of the technique of pulse radiolysis-kinetic spectrometry, with measurements in the time range 10 −6 to 1 s, is presented that, consequent upon reaction of a single H-atom with a single molecule of ferricytochrome c, a reducing equivalent is transmitted via the protein structure to the ferriheme moiety. Such transmission accounts for at least 70% of the total reduction of the ferri to the ferro state of cytochrome c. The remainder of the total reduction takes place without stages resolvable on the time scale of these experiments. Reduction brought about by H atoms appears to follow a different course than reduction by hydrated electrons. In the latter case, intramolecular transmission of reducing equivalents could not be demonstrated (Lichtin, N. N., Shafferman, A. and Stein, G. (1973) Biochim. Biophys. Acta 314, 117–135). Not every H-atom reacts with ferricytochrome c at a site which results in conversion of the Fe(III) state to the Fe(II) state. Approximately half of reacting H-atoms do not produce reduction. The following second order rate constants have been determined in solutions of low ionic strength at 20±2 °C: k[H+ferricytochrome c] = (1.0±0.2) · 1010 M−1 · s−1 at pH 3.0 and 6.7; k[H+ferrocytochrome c] = (1.3±0.2) · 1010 M−1 · s−1 at pH 3.0; k[e−aq + ferrocytochrome c] = (1.9±0.4) · 1010 M−1 · s−1 at pH 6.7. 相似文献
14.
Magnetic field-dependent recombination measurements together with magnetic field-dependent triplet lifetimes (Chidsey, E.D., Takiff, L., Goldstein, R.A. and Boxer, S.G. (1985) Proc. Natl. Acad. Sci USA 82, 6850–6854) yield a free energy change Δ G(P +H − − 3P*) = 0.165 eV ±0.008 at 290 K. This does not depend on whether nuclear spin relaxation in the state 3P* is assumed to be fast or slow compared to the lifetime of this state. This value, being (almost) temperature independent, indicates Δ G(P +H − − 3P*) Δ H(P +H − − 3P*) and is consistent with Δ G( 1P* − P +H −) and Δ H( 1P* − 3P*) from previous delayed fluorescence and phosphorescence data, implying Δ G Δ H for all combinations of these states. 相似文献
15.
Comparative measurements of bacterial total counts and volumes of flow cytometry (FCM), transmission electron (TEM), and epifluorescence microscopy (EFM), were undertaken during a four week mesocosm experiment. Total counts of bacteria measured by TEM, EFM, and FCM were in the range of 1 · 10 6−6 cells ml −1, 1 · 10 6−3 · 10 16 cells ml −1, and 5 · 10 5 cells ml −1 respectively. The mean volume of the bacterial community, measured by means of EFM and TEM, increased from 0.12–0.15 μm 3 at the start of the experiment to 0.39–0.53 μm 3 at the end. Generally, there was good agreement between the two methods and regression analyses gave r = 0.87 ( p < < 0.01) for cell volume and r = 0.97 ( p < < 0.01) for cell number. DAPI stained bacteria with volumes less than 0.2 μm 3 were not detected by flow cytometry and these were generally an order of magnitude lower than counts made by TEM and EFM. For samples where the mean bacterial cell volume was longer than 0.3 μm 3, all three methods were in agreement both with respect to counts and volume estimates. 相似文献
16.
Delayed fluorescence from isolated reaction centers of Rhodopseudomonas sphaeroides was measured to study the energetics of electron transfer from the bacteriochlorophyll complex ( P-870, or P) to the primary and secondary quinones (Q A and Q B). The analysis was based on the assumption that electron transfer between P and Q reaches equilibrium quickly after flash excitation, and stays in equilibrium during the lifetime of the P +Q − radical pair. Delayed fluorescence of 1Q reaction centers (reaction centers that contain only Q A) has a lifetime of about 0.1 s, which corresponds to the decay of P +Q −A. 2Q reaction centers (which contain both Q A and Q B) have a much weaker delayed fluorescence, with a lifetime that corresponds to that of P +Q −B (about 1 s). In the presence of o-phenanthroline, the delayed fluorescence of 2Q reaction centers becomes similar in intensity and decay kinetics to that of 1Q reaction centers. From comparisons of the intensities of the delayed fluorescence from P +Q −A and P +Q −B, the standard free energy difference between P +Q −A and P +Q −B is calculated to be 78 ± 8 meV. From a comparison of the intensity of the delayed fluorescence with that of prompt fluorescence, we calculate that P +Q −A is 0.86 ± 0.02 eV below the excited singlet state of P in free energy, or about 0.52 eV above the ground state PQ A. The temperature dependence of the delayed fluorescence indicates that P +Q −A is about 0.75 eV below the excited singlet state in enthalpy, or about 0.63 eV above the ground state. 相似文献
17.
Low-molecular-weight chitosan were prepared using 85% phosphoric acid at different reaction temperatures and reaction time. At room temperature, the viscosity average-molecular weights ( Mv) of chitosan decreased to 7.1×10 4 from 21.4×10 4 after 35 days treatment. The degradation rate decreased with increasing hydrolysis time. The yields of chitosan also continuously decreased from 68.4 to 40.2% after 35 days. At 40, 60 and 80 °C, the molecular weight decreased to 3.70×10 4, 3.50×10 4 and 2.00×10 4 on 8 h hydrolysis, respectively. The yields of chitosan remain at a high level compared with that at room temperature and were 86.5, 71.4 and 61.3% at 40, 60 and 80 °C treatment, respectively. The different reaction time gave chitosan with different molecular weights. At 60 °C, the molecular weight of products decreased to 7.40×10 4 from 21.4×10 4 within 4 h, then decreased slowly to 1.90×10 4 in 15 h. It was also found that the water-solubility of chitosan increased as the molecular weight decreased. Results show the changes in yields and molecular weight of chitooligomers were strongly dependent on the reaction temperature and reaction time. 相似文献
18.
Cytosol extracts of human amnion tissue contained high affinity binding of cortisol ( Ka = 2.48 ± 1.06 × 10 9 M −1; N = 30) and low capacity binding of cortisol ( Nmax = 279 ± 15.5 fmol mg −1 protein). Kinetic studies of cortisol binding resulted in a similar value of Ka to that obtained by Scatchard analysis. Nuclear extracts of amnion tissue contained high affinity binding of cortisol ( Ka = 5.8 ± 1.91 × 10 7 M −1) and low binding capacity ( Nmax = 91.4±21.4 fmol mg −1 protein). Ka values were an order of magnitude higher in cytosol than in blood serum when amnion and blood were obtained from the same individuals. Differences in competitive ligand binding, especially dexamethasone, were observed between the amnion receptor and transcortin in serum. Gel permeation chromatography gave only one peak at 320 kDa for amnion receptor and only one peak at 48 kDa for transcortin from serum. When amnion tissue was incubated with or without cortisol, cytosol receptor activity was significantly lower in cortisol treated tissue than in control. The nuclear extracted receptor activity was significantly higher in cortisol treated tissue than control. The Ka values from cortisol treated tissue were significantly lower from control. Together the data support the presence of a specific cortisol receptor in the human amnion that is different from transcortin. 相似文献
19.
Thermoluminescence glow curves of gamma-irradiated samarium-doped lithium borosilicate glass were investigated. The number of overlapping peaks was determined using the repeated initial rise method. The glow curves were deconvoluted into four overlapping peaks. The trapping parameters such as activation energy E, frequency factor (s), and kinetic order (b) for each peak were determined. The obtained results indicated that the lithium borosilicate glass doped with samarium had four electron trap levels with the average activation energies of 0.82, 1.01, 1.21, and 1.31 eV. Thermal fading analysis of the individual peaks based on the deconvolution data was performed. The obtained results showed high thermal fading of the first peak, but high thermal stability of the second and third peaks compared with the other peaks. These results could be used to explain some observed properties such as high thermal fading and light sensitivity for this thermoluminescent material. Moreover, the obtained results may be helpful in minimizing fading corrections in dosimetric applications. 相似文献
20.
The reaction between a cytochrome oxidase from Pseudomonas aeruginosa and oxygen has been studied by a rapid mixing technique. The data indicate that the heme d1 moiety of the ascorbate-reduced enzyme is oxidized faster than the heme c component. The oxidation of heme d1 is accurately second order with respect to oxygen and has a rate constant of 5.7 · 10 4 M −1 · s −1 at 20 °C. The oxidation of the heme c has a first-order rate constant of about 8 s −1 at infinite concentration of O 2. The results indicate that the rate-limiting step is the internal transfer of electrons from heme c to heme d1. These more rapid reactions are followed by more complicated but smaller absorbance changes whose origin is still not clear. The reaction of ascorbate-reduced oxidase with CO has also been studied and is second order with a rate constant of 1.8 · 104 M−1 · s−1. The initial reaction with CO is followed by a slower reaction of significantly less magnitude. The equilibrium constant for the reaction with CO, calculated as a dissociation constant from titrimetric experiments with dithionite-reduced oxidase, is about 2.3 · 10−6 M. From these data a rate constant of 0.041 s−1 can be calculated for the dissociation of CO from the enzyme. 相似文献
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