摘要:

The present issue ofPhotochemistry and Photobiologydeals with the “ultra‐fast reactions” in the primary process of photosynthesis. The definition of what one calls “ultra‐fast reactions” was left to the individual authors; it ranged from picoseconds (ps) to microseconds (μs). All of the papers were invited for this issue; these include those presented by the various speakers (to be cited at appropriate places) at the symposium on “Primary Photoprocesses in Photosynthesis: Ultrafast Reactions” held at the 5th annual meeting of the American Society for Photobiology, May 11–15, 1977, in San Juan, Puerto Rico. All the papers in this issue were reviewed by three authorities in the field, and the authors have incorporated as many of their suggestions as possible. I thank all the reviewers for their help and their names will be included in the annual list of the reviewers of the entire journal ofPhotochemistr

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07732.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Abstract.Investigations on the bacterial photosynthetic reaction center have recently made several important steps forward. Progress has been made in measuring the time course of the light‐driven reactions, and in understanding the thermodynamics of these processes, in determining the chemical‐structural properties of the protein and its constituents, and in elucidating the functional relationship of the reaction center with the chromatophore membrane. Although the well‐characterizedRhodopseudomonas sphaeroidesreaction center has been the main exploratory vehicle in many of these studies, we now have an ever increasing body of information from bacteria of other species and genera. This work is providing information from which we can underline features that are common to bacterial reaction centers, but it also reveals differences which may reflect different selection pressures on the separate species.In this report we shall describe the early photochemical events of the reaction center, summarize the comparative biology of the reaction center, and discuss some of the current physical‐chemical problems pertaining to the redox components of the reaction

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07733.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Abstract.As a model for the primary reactions of photosynthesis, we studied photochemical electron transfer from bacteriopheophytin (BPh) to methyl viologen (MVC12) and tom‐dinitrobenzene (m‐DNB) in solution. Both MVC12andm‐DNB cause reductions in the lifetime of the first excited singlet state of BPh (BPh*), in the fluorescence quantum yield, and in the quantum yield of the triplet state, BPh+. The quenching of BPh* probably results from electron transfer, which generates short‐lived radical pairs involving the BPh radical cation (BPh+) and the reduced form of the quencher. Electron transfer from BPh* is thermodynamically favorable, but that from BPhTis not. From the magnitude of the quenching, we calculate rate constants for electron transfer in collision complexes formed between BPh* and MVC12orm‐DNB. Measurements of the quantum yield of the free BPh+radical indicate that about 3/4 of the [BPh+MV+] radical pairs decay by reverse electron transfer, rather than dissociating to give the free radicals. Essentially all of the [BPh+m‐DNB+] radical pairs must decay by reverse electron transfer, because free BPh+cannot be detected in this case. From these data, we estimate the rate constants for the reverse electron transfer reactions. The higher probability of dissociation in the [BPh+MV+] radical pair can be explained by coulombic repulsion.The rate of the primary electron transfer reaction in photosynthetic bacteria is comparable to that of forward electron transfer in the BPh* collision complexes. Reverse electron transfer, however, is at least 103‐times slower in the radical pair formed in the bacterial reaction center than it is in [BPh+m‐DNB−], and more than 104‐times slower than in [BPh+MV+]. The explanation for this dramatic and crucially important difference remains unclear, but several possibil

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07734.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Abstract.New results are presented on the effects of mono‐ and divalent cations on concurrent changes in the microsecond yields and kinetics of chlorophyllafluorescence and delayed light emission, and the light saturation curve for the latter at 100 μs, following a 10 ns flash at 337 nm. (1) The fluorescence yield increases exponentially from 3 to 30 μs (lifetime, τ, 6.4 ± 0.6/μs), and decays biphasically between 50 and 800μs. (2) The delayed light emission decays biphasically with two exponential phases: fast phase,T= 7–10μs, and slow phase,T= 33–40μs. (3) The light saturation curve for 100μs delayed light emission is satisfactorily represented by a one‐hit Poisson saturation curve. (4) Addition of 5mMNaCl to salt‐depleted chloroplasts decreases (by as much as 40%) the yields of μs fluorescence and delayed light emission, and the subsequent addition of 5mMMgCl2increases the yields (≤2 × over samples with only NaCl). (5) The fluorescence yield rise and delayed light emission decay kinetics are independent of low concentrations of cations. The lifetime of the fast phase of fluorescence decay changes from ˜90μs to ˜160μs, when Na+or Na++ Mg2+are added.Based on a detailed analysis presented in this paper, the following conclusions regarding the effects of low concentrations (few mM) of mono‐and divalent cations in sucrose‐washed chloroplasts at room temperature are made: (a) Na+decreases (˜6%) and Mg2+increases (˜ 20% compared with the Na+sample) the sensitization of photosystem II photochemistry: this effect is small, but significant. (b) Na+increases and Mg2+decreases the efficiency for radiationless transitions in singlet excited Chlain the antenna and closed reaction center of PS II; this includes non‐radiative energy transfer to PS I, intramolecular intersystem crossing and internal conversion. The ratio of the sum of the rate constants for radiationless transitions to that for fluorescence increases by ˜ 2‐fold upon the addition of Na+, and is completely reversed by the addition of Mg2+. (c) The rate constant for the re‐oxidation of Q‐decreases (about 50%) in the presence of Na+or Na++ Mg2+. These conclusions imply that cations produce multiple changes in the primary photoprocesses of PS II at physiological temperatures. It is proposed that these changes

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07735.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Abstract.In this paper we review picosecond fluorescence studies of exciton dynamics in photosynthesis. We discuss some of the exciton interactions that led to artifacts in early picosecond data and outline procedures for avoiding their presence. In the case of high intensity single pulse excitation (>1013photons cm2), the dominant mechanism is singlet‐singlet fusion, manifesting itself by a decrease in the observed lifetime and quantum efficiency of fluorescence. The manner in which excitons interactin vivoprovides an indicator of the topology of the photosynthetic unit (PSU). The shape of the fluorescence quenching curve, as a function of intensity, in particular, can be used to test various models. In addition to fluorescence quenching curves, we also report the results of fluorescence decay following ps laser flashes, using an ultrafast streak camera in four types of systems: (1) organic crystal anologues, (2) chromatophores of various mutants of the photosynthetic bacteria,Rhodopseudomonas sphaeroides, (3) intact cells of the green alga,Chlorellaand (4) chloroplasts of higher plants (e.g. spinach

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07736.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Abstract.Analysis of the effect of multiple excitations on chlorophyllafluorescence yields in the green algaChlorellareveals several distinct reactions. The first excitation in dark‐z‐adapted units produces photochemistry and the high yield state with a rise time of 35 ns. It is ascribed to a change in coupling between the antenna pigments and the photochemical trap. The second hit produces with the same quantum yield a quenched state which changes to the high yield state with a rise time of 4μs. This is ascribed to the formation and the decay of a particular carotenoid triplet state near the funnel or antenna‐trap junction. Further hits produce enhanced quenching assigned to mobile triplets with lifetimes in the order of 100 ns. The fluorescence yield decreases monotonically with increasing excitations during the 7 ns pulse. This effect can be adequately ascribed to annihilation of excitations with lifetimes longer than the trapping time, or by a unique model of a multi‐trapped unit. The latter model is favored by arguments based both on the absence of a local maximum in the graph of fluorescence yield vs excitation energy and on the fact that the high yield state shows a different behaviour on multiple excitation, fit by a single‐trapped unit. This analysis is related to that used in experiments with ps flashes and is applied to the qualitatively different bacter

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07737.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Abstract.The fluorescence yield, φ, as a function of single picosecond laser pulse intensity was experimentally studied in spinach chloroplasts and for chlorophyllain ethyl ether solution. The progressive decrease in φ with increasing incident intensity forin vivochlorophyll was found to be adequately explained within the context of continuum bimolecular kinetics with a singlet‐singlet fusion rate constant of γ=5×‐9cm‐3s‐1at room temperature. We discuss qualitatively how the fluorescence quantum yield depends on the duration and intensity of the incident pulse. The identity of φ vsl(the number of absorbed quanta) curves at the emission maxima of 685 nm and 735 nm for single picosecond pulse mode of excitation is explained within the context of Butler's tripartite model of the fluorescence of chloroplasts at 77 K. Various models relating γ to the singlet exciton diffusion coefficient and the Förster energy transfer rate are used to infer lower bounds to these physical parameters. Predictions and supporting experimental evidence for the tripartite model

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07738.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Abstract.An overview of the reported chlorophyllafluorescence lifetimes from green plant photosystems is presented and the problems encountered in the measurement of fluorescence lifetime using two currently available picosecond techniques are discussed.The fluorescence intensity of spinach chloroplasts exposed to 10 ps flashes was measured as a function of time after the flash and wavelength of observation by the ultrafast Kerr shutter technique. Using a train of 100 pulses separated by 6ns and with an average photon flux per pulse of ˜2 times 1014photons/cm2, the fluorescence intensity at 685 nm (room temperature) was found to decay with two components, a fast one with a 56 ps lifetime, and a slow one with a 220 ps lifetime. The 730 nm fluorescence intensity at room temperature decays as a single exponential with a 100 ps lifetime. The 730 nm fluorescence lifetime was found to increase by a factor of 6 when the temperature was lowered from room temperature to 90 K while the lifetime of 685 and 695 nm fluorescence were unchanged. At room temperature, the fast and slow components at 685 nm are attributed to the emission from pigment system I (PS I) and PS II, respectively. It is likely that the absolute values of lifetimes, reported here, may increase if single ps low intensity flashes are used for these measurements

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07739.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Abstract.The published data concerning the fluorescence kinetics of chlorophyllain various photosynthetic species are reviewed. The effects of singlet‐singlet and singlet‐triplet annihilation induced by excessively high incident light intensities are discussed and related to the changes produced in the fluorescence lifetimes and quantum yields. We also review the fluorescence lifetimes ofChlorella pyrenoidosaand spinach chloroplast fragments under a variety of experimental conditions; these measurements were performed at single pulse excitation intensities of less than 5 × 1013photons cm–2where distortion due to annihilation processes is negligible. Evidence for and against a time dependent rate equation for energy migration will be discussed with reference to the authors' work onin vitros

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07740.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Abstract.Under conditions that drive the reaction centers (RC's) into the “closed” state, the lifetime (T) of the fluorescence emitted by antenna molecules increases from 80 to 200 ps in PS I, from 300 to 600 ps in PS II, and from 200 to 500 ps in bacterial chromatophores. InRhodopseudomonas sphaeroidesstrain 1760‐1, the decay curve for fluorescence from the RC's has a component withT2= 15 ps due to the bacteriochlorophyll of the RC, and a second component withT2= 250 ps due to bacteriopheophytin.Data on electron transfer at low temperatures and under different redox conditions are analyzed. along with the ps fluorescence kinetics. The hypothesis is discussed that electron transfer in RC's is coupled to conformation changes in the interacting mole

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1978.tb07741.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY