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21.
The electron-transfer reactions and thermodynamic equilibria involving the quinone acceptor complex in bacterial reaction centers from R. sphaeroides were investigated. The reactions are described by the scheme: We found that the charge recombination pathway of D+QAQ?B proceeds via the intermediate state D+Q?AQB, the direct pathway contributing less than approx. 5% to the observed recombination rate. The method used to obtain this result was based on a comparison of the kinetics predicted for the indirect pathway (given by the product kAD-times the fraction of reaction centers in the Q?AQB state) with the observed recombination rate, kobsD+ →D. The kinetic measurements were used to obtain the pH dependence (6.1 ? pH ? 11.7) of the free energy difference between the states Q?AQB and QAQ?B. At low pH (less than 9) QAQ?B is stabilized relative to Q?AQB by 67 meV, whereas at high pH Q?AQB is energetically favored. Both Q?A and Q?B associate with a proton, with pK values of 9.8 and 11.3, respectively. The stronger interaction of the proton with Q?B provides the driving force for the forward electron transfer. 相似文献
22.
Radical-pair decay kinetics and molecular triplet quantum yields at various magnetic fields are reported for quinone-depleted reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides R26. The radical-pair decay is observed by picosecond absorption spectroscopy to be a single exponential to within the experimental uncertainty at all fields. The decay time increases from 13 ns at zero field to 17 ns at 1 kG, and decreases to 9 ns at 50 kG. The orientation averaged quantum yield of formation of the molecular triplet of the primary electron donor, 3P, drops to 47% of its zero-field value at 1 kG and rises to 126% at 50 kG. Combined analysis of these data gives a singlet radical-pair decay rate constant of 5 · 107s?1, a lower limit for the triplet radical-pair decay rate constant of 1 · 108s?1 and a lower limit for the quantum yield of radical-pair decay by the triplet channel of 38% at zero field. The upper limit of the quantum yield of 3P formation at zero field is measured to be 32%. In order to explain this apparent discrepancy, decay of the radical pair by the triplet channel must lead to some rapid ground state formation as well as some 3P formation. It is proposed that the triplet radical pair decays to a triplet charge-transfer state which is strongly coupled to the ground state by spin-orbit interactions. Several possibilities for this charge-transfer state are discussed. 相似文献
23.
The amino-terminal sequences have been determined by Edman degradation for the reaction center polypeptides from a carotenoidless mutant of Rhodopseudomonas capsulata. Individual polypeptides were isolated by preparative electrophoresis and electroelution. By comparison with the sequences deduced from the DNA (Youvan, D.C., Alberti, M., Begush, H., Bylina, E.J. and Hearst, J.E. (1984) Proc. Natl. Acad. Sci. USA 81, 189–192) we conclude that the M and L subunits are processed so as to remove the amino-terminal methionine, whereas the H subunit is not processed at the amino-terminus after translation. None of the subunits is synthesized with a significant amino-terminal extension peptide. 相似文献
24.
A.C. Van Bochove T. Swarthoff H. Kingma R.M. Hof R. Van Grondelle L.N.M. Duysens J. Amesz 《BBA》1984,764(3):343-346
A reaction-center pigment-protein complex of the green bacterium Prosthecochloris aestuarii was studied by means of nanosecond-flash spectroscopy. In this complex electron transfer between the primary and secondary acceptor is blocked. The spectra and kinetics of the absorption changes induced by a short flash indicated the formation of the radical pair P-840+I?, which decayed in 20–35 ns, mainly to the triplet state of the primary electron donor P-840. The absorption difference spectrum of the initial absorption change indicated that the primary acceptor I is either bacteriopheophytin c or another pigment with absorption maximum at 665 nm. 相似文献
25.
The ratio of Photosystem (PS) II to PS I electron-transport capacity in spinach chloroplasts was compared from reaction-center and steady-state rate measurements. The reaction-center electron-transport capacity was based upon both the relative concentrations of the PS IIα, PS IIβ and PS I centers, and the number of chlorophyll molecules associated with each type of center. The reaction-center ratio of total PS II to PS I electron-transport capacity was about 1.8:1. Steady-state electron-transport capacity data were obtained from the rate of light-induced absorbance-change measurements in the presence of ferredoxin-NADP+, potassium ferricyanide and 2,5-dimethylbenzoquinone (DMQ). A new method was developed for determining the partition of reduced DMQ between the thylakoid membrane and the surrounding aqueous phase. The ratio of membrane-bound to aqueous DMQH2 was experimentally determined to be 1.3:1. When used at low concentrations (200 μM), potassium ferricyanide is shown to be strictly a PS I electron acceptor. At concentrations higher than 200 μM, ferricyanide intercepted electrons from the reducing side of PS II as well. The experimental rates of electron flow through PS II and PS I defined a PS II/PS I electron-transport capacity ratio of 1.6:1. 相似文献
26.
The 688 nm absorption changes (ΔA688), indicating the photochemical turnover of chlorophyll aII (Chl aII) have been investigated under repetitive laser flash excitation conditions in spinach chlorplasts. It was found that under steady state conditions about 50–60% of the photo-oxidized primary donor of Photosystem II (PS II), Chl a+II, becomes re-reduced with a biphasic kinetics in the nanosecond time scale with half-life times of about 50 ns and 400 ns. The remaining Chl a+II becomes re-reduced in the microsecond range. 相似文献
27.
28.
Resonance Raman spectra of the π-cation of bacterio-chlorophyll a in solution at 30 K are reported and discussed. Outer CC bonds of the pyrroles and the methine bridges are weakened by the ionization, while CN and Mg-N bonds remain essentially unaffected. Resonance Raman spectra of reaction centers suggest that the positive charge on P-870+ should be localized on a single bacteriochlorophyll molecule by the lifetime of the scattering process (≈ 10?13 s). 相似文献
29.
A two-frequency oscillator model for the primary photochemical reaction bacteriorhodopsin batho-bacteriorhodopsin (K610) is proposed. According to this model two conformational changes in the reaction are considered to take place: the first one is a distortion of the retinal in the bacteriorhodopsin active site and the second one is a conformational transition of the bacterioopsin, affecting the native structure hydrogen bonds. On the basis of this model the temperature dependences of the rate constants for normal and deuterated reactants are calculated in good agreement with the available experimental data. The relations of the reaction considered to the primary photochemical reaction of vision are discussed. 相似文献
30.
The properties of Photosystem II electron donation were investigated by EPR spectrometry at cryogenic temperatures. Using preparations from mutants which lacked Photosystem I, the main electron donor through the Photosystem II reaction centre to the quinone-iron acceptor was shown to be the component termed Signal II. A radical of 10 G line width observed as an electron donor at cryogenic temperatures under some conditions probably arises through modification of the normal pathway of electron donation. High-potential cytochrome b-559 was not observed on the main pathway of electron donation. Two types of PS II centres with identical EPR components but different electron-transport kinetics were identified, together with anomalies between preparations in the amount of Signal II compared to the quinone-iron acceptor. Results of experiments using cells from mutants of Scenedesmus obliquus confirm the involvement of the Signal II component, manganese and high-potential cytochrome b-559 in the physiological process leading to oxygen evolution. 相似文献