Showing papers on "Photosynthetic reaction centre published in 1979"

Biphasic energy conversion kinetics and absorbance difference spectra of photosystem ii of chloroplasts. evidence for two different photosystem ii reaction centers

Abstract: — The continuous illumination induced kinetics of photochemical energy conversion at system II have been measured with isolated and 3-(3, 4-dichlorophenyl)-l, l-dimethylurea (DCMU) poisoned chloroplasts by means of absorbance difference spectroscopy in the UV and by the area growth over the fluorescence induction curve at room temperature. An optimal set of conditions was found in order to isolate absorbance changes caused by the reduction of the primary electron acceptor Q of PS II by suppressing other electron transfer processes. The light induced kinetics of Q- accumulation in the absorbance change measurements were found to be biphasic and strictly correlated with the kinetics of the area growth measured under the same conditions. From the resolution of the biphasic kinetics at different wavelengths in the UV region of the spectrum, it was found that both kinetic components in the system II photochemistry involve the reduction of a plastoquinone molecule to its plastosemiquinone anion. From the two kinetic components one was fast and non-exponential and the other relatively slow with an exponential time course. The initial rate difference in the kinetics of the two components was by a factor of approximately 3. A difference by a factor of about three was also found in the flash saturation curves of the two kinetic components. The results are explained by the hypothesis that in higher plant chloroplasts there are system II reaction centers embedded in a large pigment matrix with statistical energy transfer, and system II reaction centers embedded in separate, in terms of excitation energy transfer, units. The effective absorption cross section per reaction center for the centers in the statistical pigment bed is approximately 3 times larger than that of the reaction centers in the separate system II units. The two types of system II reaction centers have different yields of excitation trapping and charge stabilization properties.

Triplet states in photosystem I of spinach chloroplasts and subchloroplast particles.

Abstract: We report light-induced electron paramagnetic resonance triplet spectra from samples of chloroplasts or digitonin photosystem I particles that depend upon the dark redox state of the bound acceptors of photosystem I. If the reaction centers are prepared in the redox state P-700 A X- FdB-FdA-, then upon illumination at 11K we observe a polarized chlorophyll triplet species which we interpret as arising from radical pair recombination between P-700+ and A-. This chlorophyll triplet is apparently the analog of the PR state of photosynthetic bacteria [Parson, W.W. & Cogdell, R.J. (1975) Biochim. Biophys. Acta 416, 105-149]. If the reaction centers are prepared in the dark redox state P-700 A X FdB-FdA-, then upon illumination at 11K we observe a different triplet species of uncertain origin, possibly pheophytin or carotenoid. This species is closely associated with the photosystem I reaction center and it traps excitation when P-700 is oxidized.

Functional reconstitution of photosynthetic reaction centers in planar lipid bilayers.

Abstract: Planar lipid bilayers containing reaction centers from Rhodopseudomonas sphaeroides R-26 were formed by apposing two reaction center—lipid monolayers formed from a reaction center—lipid complex in hexane. Secondary donors (cytochrome c) and acceptors (ubiquinone-0) were added on opposite sides of the membrane. Upon illumination, this system generated transient and steady-state voltages and currents. The wavelength dependence of the photoresponse matched the absorption spectrum of reaction centers. A simple model based on the transfer of charges across the membrane that explains the salient features of the photoresponse is presented.

Chlorophyll a fluorescence of Gonyaulax polyedra grown on a light-dark cycle and after transfer to constant light.

Abstract: — The chlorophyll a fluorescence properties of Gonyaulax polyedra cells before and after transfer from a lightdark cycle (LD) to constant dim light (LL) were investigated. The latter display a faster fluorescence transient from the level ‘I’ (intermediary peak) to ‘D’ (dip) to ‘P’ (peak) than the former (3 s as compared to 10 s), and a different pattern of decline in fluorescence from ‘I’ to ‘D’ and from ‘P’ to the steady state level with no clearly separable second wave of slow fluorescence change, referred to as ‘s' (quasi steady state)‘M’ (maximum) ‘T’ (terminal steady state). The above differences are constant features of cells in LD and LL, and are not dependent on the time of day. They are interpreted as evidence for a greater ratio of photosystem II/photosystem I activity in cells in LL. After an initial photoadaptive response following transfer from LD to LL, the cell absorbance at room temperature and fluorescence emission spectra at 77 K for cells in LL and LD are comparable. The major emission peak is at 685–688 nm (from an antenna Chl a 680, perhaps Chl a-c complex), but, unlike higher plants and other algae, the emission bands at 696–698 nm (from Chl aII complex, Chl a 685, close to reaction center II) and 710–720 nm (from Chl a1, complexes, Chl a 695, close to reaction center I) are very minor and could be observed only in the fluorescence emission difference spectra of LL minus LD cells and in the ratio spectra of DCMU-treated to non-treated cells. Comparison of emission spectra of cells in LL and LD suggested that, in LL, there is a slightly greater net excitation energy transfer from the light-harvesting peridinin-Chl a (Chl a 670) complex, fluorescing at 675 nm, to the other antenna chlorophyll a complex fluorescing at 685–688 nm, and from the Chl a., complex to the reaction center II. Comparison of excitation spectra of fluorescence of LL and LD cells, in the presence of DCMU, confirmed that cells in LL transfer energy more extensively from the peridinin-Chl a complex to other Chl a complexes than do cells in LD.

An electrochemical investigation of the redox properties of bacteriochlorophyll and bacteriopheophytin in aprotic solvents

Abstract: Knowledge of solvent effects on the redox properties of bacteriochlorophyll (BChl) and bacteriopheophytin (BPhco) is important for understanding their possible role(s) as intermediate electron acceptors in the primary photochemistry of pho- tosynthetic bacteria. In the present study, an investigation of the electrochemical behavior of these compounds by cyclic vol- tammetry (CV) and cyclic differential pulse voltammetry (CDPV) in several aprotic solvents has shown that BChl aggregation and ligation interactions have a significant effect on its redox potentials. In methylene chloride, the one-electron reduction po- tential of BChl was found to shift positively by 200 mV to a value nearly identical with that of BPheo in the same solvent. The shift is most readily explained by the presence of BChl aggregates in this solvent. The one-electron oxidation potential is rela- tively unaffected by aggregation. In contrast, the formation of six-coordinate BChl in tetrahydrofuran (two molecules of sol- vent coordinated to the Mg atom of BChl) affects both the one-electron reduction and one-electron oxidation potential, with the greatest effect on the latter. Solvent effects on the redox properties of BPheo were found to be much smaller, a finding con- sistent with its inability to undergo aggregation and coordination interactions similar to those of BChl. During the past decade, considerable progress has been made toward understanding the primary photochemical events in photosynthetic bacteria.2 The reaction center (RC) complex has been purified3 and found to contain four bacteriochloro- phyll (BChl) and two bacteriopheophytin (BPheo) molecules in addition to one quinone and three polypeptides. (See recent reviews in ref 4-8.) Two of the BChl molecules, the special pair, which absorb at 870 nm (P870) are now generally accepted as the primary electron donor in photoinduced charge sepa- rati~n.~-'~ The quinone molecule has been shown to act as the primary electron ac~eptor'~,'~ as defined on a millisecond time scale.I5 However, with the advent of picosecond (ps) absorption spectroscopy techniques, an intermediate electron acceptor with a lifetime of 250 ps has been dete~ted.'~.'' The interme- diate was identified by Fajer and co-workers18 as one of the BPheo molecules present in the RC by a comparison of the optical properties of the intermediate with those of the BPheo anion radical prepared electrochemically in methylene chlo- ride. Subsequently, BPheo has been identified as the inter- mediate electron acceptor in a variety of photosynthetic bac- terial The picosecond data have recently been revie~ed,~~.~~ and possible artifacts of the method have been discu~sed.~~~~~ Although the role of the remaining two BChl molecules and the BPheo molecule in the charge separation process has not been unambiguously demonstrated, there are some experimental observations that suggest BChl (PSOO) may be involved in some manner in the transfer of an electron from P870 to PBheo. These observations include the picosecond absorption study of Shuvalov et a1.,28 which has been inter- preted by these authors to indicate that electron transfer pro- ceeds from P870 to P800 and then to BPheo within 35 ps. Steady-state optical spectra of the reduced intermediate have indicated the presence of a BChl anion radical as well as that of BP~~O.~~ Because BChl and BPheo participate directly in the initial photosynthetic charge separation, it is important that their redox properties be known. This information is necessary for a detailed understanding of the mechanism of charge separa- tion. For example, an estimate of the efficiency of the primary step in photochemical energy conversion has been made by comparing the electrochemical potential difference between the oxidized donor and reduced acceptor with the energy of a single photon absorbed by P87O.l8 In this calculation, the ox- idation potential of the special pair BChl, as determined by 0002-7863/79/150l-7605$0l .OO/O potentiometric titrations on in vivo preparations, was used as the donor potential. Since the reduction potential of BChl or BPheo has not been determined directly in the RC,30 values obtained electrochemically for these compounds in organic solvents were used for the acceptor potential. The results in- dicated an efficiency of 70% for electron transfer from P870 to BPheo and an efficiency of 90% for electron transfer from P870 to BChl. The former value is in good agreement with previous estimates of photosynthetic efficiency and constitutes additional support for the much stronger optical evidence of BPheo as the intermediate electron acceptor. If, however, P800 BChl functions as an extremely short-lived electron acceptor between P870 and BPheo as proposed by Shuvalov et it is necessary to consider the potential difference created by the transfer of an electron from P870 to BChl. An accurate value for the reduction potential of BChl is needed for this purpose. There have been several literature reports for the one-electron reduction potential of BChl in organic solvent^,^'-^^ but the effect of BChl interactions has not been considered. In the case of P870 BChl, the oxidation potential in vivo is lowered by approximately 200 mV relative to monomeric BChl in solution, no doubt as a result of the interaction between the two special pair BChl molecules. Therefore, it seems reasonable that the reduction potential of BChl or BPheo may be affected by their interactions in the RC, since there is strong coupling of the RC pigments as evidenced by CD studies.34 Consequently, we have endeavored to determine the effect of BChl aggregation and ligation interactions on its redox properties in organic solvents. The effect of solvent on BPheo redox properties was also ex- amined. The results, which are presented here, indicate both aggregation and ligation interactions affect the redox prop- erties of BChl, although in a significantly different manner.

The inhibition of photosynthetic electron transport in spinach chloroplasts by low osmolarity.

Abstract: The reversible inhibition, by low osmolarity, of the rate of electron transport through photosystem 1 has been investigated in spinach chloroplasts. By use of different electron donor systems to photosystem 1, inhibitors of plastocyanin, and by measurement of the extent of photooxidation of the photosystem 1 reaction center P700, the inhibition site has been localized on the electron donor side of this photosystem. From comparison of the influence of impermeant and permeant salts on the electron transport rate, and from the effect of ionic strength on the oxidation of externally added plastocyanin by subchloroplast preparations, it is concluded that low ionic strength within the thylakoids inhibits the photooxidation of endogenous plastocyanin by P700. The results are taken as evidence that plastocyanin is oxidized by P700 at the internal (lumen) side of the osmotic barrier in the thylakoid membrane.

Observations of chemically induced dynamic electron polarization in photosystem I of green plants and algae

Abstract: In the continuing study of the photoreactions occurring in photosynthetic reaction centers, the technique of flash photolysis with electron paramagnetic resonance (EPR) detection has been particularly usefu1.l The overall goal of such studies has been to identify the participants in the primary photochemistry occurring in bacterial

Ultrastructure - function relationship in Chlamydomonas reinhartii thylakoids, by means of a comparison between the wild type and the F34 mutant which lacks the photosystem II reaction center.

Abstract: The F3 4 mutant strain ofChlamydomonas reinhartii is deficient in photosystem II reaction centers. The E fracture faces of the thylakoid membranes of this mutant show a considerable reduction in the number of particles present and in their size compared with the wild type. We conclude that the polypeptides associated with the photosystem II reaction centers, which are missing in SDS polyacrylamide gel electrophoresis patterns of proteins from this mutant strain, are part of the EF particles and are required for assembly of these particles.