scholarly journals New Perspectives on Photosystem II Reaction Centres

2020 ◽  
Vol 73 (8) ◽  
pp. 669 ◽  
Author(s):  
Jeremy Hall ◽  
Rafael Picorel ◽  
Nicholas Cox ◽  
Robin Purchase ◽  
Elmars Krausz
Keyword(s):  
Photosystem Ii ◽  
Charge Separation ◽  
Cd Spectra ◽  
Negative Component ◽  
Ps Ii ◽  
Pheophytin A ◽  
Chl A ◽  
Special Pair ◽  
Reaction Centres ◽  
Weak Luminescence

We apply the differential optical spectroscopy techniques of circular polarisation of luminescence (CPL) and magnetic CPL (MCPL) to the study of isolated reaction centres (RCs) of photosystem II (PS II). The data and subsequent analysis provide insights into aspects of the RC chromophore site energies, exciton couplings, and heterogeneities. CPL measurements are able to identify weak luminescence associated with the unbound chlorophyll-a (Chl-a) present in the sample. The overall sign and magnitude of the CPL observed relates well to the circular dichroism (CD) of the sample. Both CD and CPL are reasonably consistent with modelling of the RC exciton structure. The MCPL observed for the free Chl-a luminescence component in the RC samples is also easily understandable, but the MCPL seen near 680nm at 1.8K is anomalous, appearing to have a narrow, strongly negative component. A negative sign is inconsistent with MCPL of (exciton coupled) Qy states of either Chl-a or pheophytin-a (Pheo-a). We propose that this anomaly may arise as a result of the luminescence from a transient excited state species created following photo-induced charge separation within the RC. A comparison of CD spectra and modelling of RC preparations having a different number of pigments suggests that the non-conservative nature of the CD spectra observed is associated with the ‘special pair’ pigments PD1 and PD2.

scholarly journals Redox potential of pheophytin a in photosystem II of two cyanobacteria having the different special pair chlorophylls

2010 ◽  
Vol 107 (8) ◽  
pp. 3924-3929 ◽  
Author(s):  
Suleyman I. Allakhverdiev ◽  
Tatsuya Tomo ◽  
Yuichiro Shimada ◽  
Hayato Kindo ◽  
Ryo Nagao ◽  
...  
Keyword(s):  
Photosystem Ii ◽  
Redox Potential ◽  
Pheophytin A ◽  
Special Pair

Kinetic response of photosystem II photochemistry in the cyanobacterium Spirulina platensis to high salinity is characterized by two distinct phases

1999 ◽  
Vol 26 (3) ◽  
pp. 283 ◽  
Author(s):  
Congming Lu ◽  
Giuseppe Torzillo ◽  
Avigad Vonshak
Keyword(s):  
Electron Transport ◽  
Quantum Yield ◽  
Photosystem Ii ◽  
Spirulina Platensis ◽  
High Salinity ◽  
Photochemical Quenching ◽  
Second Phase ◽  
Ps Ii ◽  
Reaction Centres ◽  
Two Phases

The kinetic response of photosystem II (PS II) photochemistry in Spirulina platensis(Norstedt M2 ) to high salinity (0.75 M NaCl) was found to consist of two phases. The first phase, which was independent of light, was characterized by a rapid decrease (15–50%) in the maximal efficiency of PS II photochemistry (Fv /Fm), the efficiency of excitation energy capture by open PS II reaction centres (Fv′/Fm′), photochemical quenching (qp) and the quantum yield of PS II electron transport (Φ PS II) in the first 15 min, followed by a recovery up to about 80–92% of their initial levels within the next 2 h. The second phase took place after 4 h, in which further decline in above parameters occurred. Such a decline occurred only when the cells were incubated in the light, reaching levels as low as 45–70% of their initial levels after 12 h. At the same time, non-photochemical quenching (qN) and Q B -non-reducing PS II reaction centres increased significantly in the first 15 min and then recovered to the initial level during the first phase but increased again in the light in the second phase. The changes in the probability of electron transfer beyond QA (ψo) and the yield of electron transport beyond QA (φ Eo), the absorption flux (ABS/RC) and the trapping flux (TRo /RC) per PS II reaction centre also displayed two different phases. The causes responsible for the decreased quantum yield of PS II electron transport during the two phases are discussed.

The origin of chlorophyll fluorescence In vivo and its quenching by the photosystem II reaction centre

1989 ◽  
Vol 323 (1216) ◽  
pp. 227-239 ◽  
Keyword(s):  
Chlorophyll Fluorescence ◽  
Photosystem Ii ◽  
Electron Acceptor ◽  
Charge Separation ◽  
Bright Light ◽  
Sodium Dithionite ◽  
Reaction Centre ◽  
Primary Charge Separation ◽  
Ps Ii ◽  
Variable Yield

Isolated chlorophyll a , in contrast to when it is dissolved in organic solvents, shows a lower and variable yield of fluorescence when bound to protein and embedded in the thylakoid membrane of photosynthetic organisms. There are two current theories that attempt to explain the origin of this variable yield of fluorescence, (i) It may be emitted directly from the photosystem II (PSII) antenna system and therefore in competition with photochemical trapping (prompt fluorescence), (ii) It may be derived from a recombination reaction between oxidized P 680 and reduced pheophytin within the PS II reaction centre (delayed fluorescence). We have isolated a PS II reaction centre complex that binds only four chlorophyll a molecules and can carry out primary charge separation. The complex contains no plastoquinone and therefore is devoid of the secondary electron acceptor Q A . It does, however, contain two pheophytin a molecules, and one of these acts as a primary electron acceptor. The electron donor is P 680 , which is either a monomeric or dimeric form of chlorophyll a . The isolated PS II reaction centre fluoresces at room temperature with a maximum at 683 nm, and the intensity of this emission is almost totally quenched when reduced pheophytin (bright light plus sodium dithionite) or oxidized P 680 (bright light plus silicomolybdate) is photoaccumulated. The photo-induced quenching of chlorophyll fluorescence when sodium dithionite is present is also observed in intact PS II preparations containing plastoquinone Q A . In the latter case Q A is chemically reduced in the dark by dithionite. Bearing in mind the above two postulates for the origin of variable chlorophyll fluorescence it has been possible to investigate the relative quantum yields for the photoproduction of the P 680 Pheo - state either in the absence (with isolated PS II reaction centres) or presence (with PSII-enriched membranes) of reduced Q A . It has been shown that in the absence of Q - A the quantum efficiency for production of the P 680 Pheo - is several orders of magnitude greater than when Q - A is present. This difference probably partly reflects the coulombic restraints on primary charge separation when Q A is reduced and would suggest that under these conditions the PS II reaction centre is a less efficient trap. Such a conclusion is therefore consistent with postulate (i) that the increase inyield of chlorophyll fluorescence as Q A becomes reduced is not due to a back reaction between P + 680 and Pheo - but rather to a decrease in competition between emission and trapping. The results do emphasize however, that the P 680 Pheo - and P + 680 Pheo states are quenchers of chlorophyll fluorescence. In addition to the above, it has been noted that at 77 K fluorescence from the isolated PS II reaction centre reaches a maximum at 685 nm and does not have a peak at 695 nm. This observation appears to invalidate the postulate that the 695 nm emission is from the pheophytin of the PS II reaction centre.

Further Studies on Quantifying Photosystem II in vivo by Flash-Induced Oxygen Yield From Leaf Discs

1991 ◽  
Vol 18 (4) ◽  
pp. 397 ◽  
Author(s):  
WS Chow ◽  
AB Hope ◽  
JM Anderson
Keyword(s):  
Steady State ◽  
Photosystem Ii ◽  
Red Light ◽  
Light Activation ◽  
Ps Ii ◽  
Dark Incubation ◽  
Leaf Discs ◽  
Reaction Centres ◽  
Oxygen Yield

It was shown briefly [W. S. Chow, A. B. Hope and J. M. Anderson (1989), Biochirnica et Biophysics Acta, 973, 105-8] that the oxygen evolved per flash from leaf discs, under steady-state flashing conditions and in the presence of background far-red light, gave a valid measure of the number of functional photosystem II (PS II) reaction centres. Further work on this direct and convenient method has been done to optimise conditions for making reliable measurements. It is found that, to obtain the higher estimates of [PS II], corresponding to functionality of practically all PS II reaction centres that bind herbicides, a form of 'light activation' is necessary after a prolonged dark pre-incubation. Without a sufficient number of flashes being given following a long dark incubation, the number of functional PS II reaction centres was underestimated. Provided light activation had occurred, the measured number of functional PS II reaction centres was independent of flash frequencies up to at least 40 Hz. The results strongly suggest that, in steady-state, light-limited photosynthesis, there does not exist any sub- stantial fraction of non-functional or 'slow' PS II reaction centres.

scholarly journals A TDDFT investigation of the Photosystem II reaction center: Insights into the precursors to charge separation

2020 ◽  
Vol 117 (33) ◽  
pp. 19705-19712
Author(s):  
Maeve A. Kavanagh ◽  
Joshua K. G. Karlsson ◽  
Jonathan D. Colburn ◽  
Laura M. C. Barter ◽  
Ian R. Gould
Keyword(s):  
Photosystem Ii ◽  
Oscillator Strength ◽  
Reaction Center ◽  
Water Oxidation ◽  
Charge Separation ◽  
Density Functional ◽  
Molecular Orbitals ◽  
Type Model ◽  
Absorption Bands ◽  
Ps Ii

Photosystem II (PS II) captures solar energy and directs charge separation (CS) across the thylakoid membrane during photosynthesis. The highly oxidizing, charge-separated state generated within its reaction center (RC) drives water oxidation. Spectroscopic studies on PS II RCs are difficult to interpret due to large spectral congestion, necessitating modeling to elucidate key spectral features. Herein, we present results from time-dependent density functional theory (TDDFT) calculations on the largest PS II RC model reported to date. This model explicitly includes six RC chromophores and both the chlorin phytol chains and the amino acid residues <6 Å from the pigments’ porphyrin ring centers. Comparing our wild-type model results with calculations on mutant D1-His-198-Ala and D2-His-197-Ala RCs, our simulated absorption-difference spectra reproduce experimentally observed shifts in known chlorophyll absorption bands, demonstrating the predictive capabilities of this model. We find that inclusion of both nearby residues and phytol chains is necessary to reproduce this behavior. Our calculations provide a unique opportunity to observe the molecular orbitals that contribute to the excited states that are precursors to CS. Strikingly, we observe two high oscillator strength, low-lying states, in which molecular orbitals are delocalized over ChlD1and PheD1as well as one weaker oscillator strength state with molecular orbitals delocalized over the P chlorophylls. Both these configurations are a match for previously identified exciton–charge transfer states (ChlD1+PheD1−)* and (PD2+PD1−)*. Our results demonstrate the power of TDDFT as a tool, for studies of natural photosynthesis, or indeed future studies of artificial photosynthetic complexes.

Fluorescence and Spectroscopic Studies of Exciton Trapping and Electron Transfer in Photosystem II of Higher Plants

1995 ◽  
Vol 22 (2) ◽  
pp. 167 ◽  
Author(s):  
G Renger ◽  
HJ Eckert ◽  
A Bergmann ◽  
J Bernarding ◽  
B Liu ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Photosystem Ii ◽  
Excitation Energy ◽  
Rate Constant ◽  
Charge Separation ◽  
Higher Plants ◽  
Radical Pair ◽  
Isotope Effects ◽  
Primary Charge Separation ◽  
Reaction Centres

Measurements of time-resolved fluorescence decay, laser-flash-induced absorption changes in the UV and at 820 nm and of the relative fluorescence quantum yield in different preparations (thylakoids, photosystem II (PSII) membrane fragments and PSII core complexes) from spinach led to a number of conclusions. (1) Light is transformed into Gibbs energy with trapping times of 250 ps and 130 ps in open reaction centres of PSII membrane fragments and PSII core complexes, respectively. Assuming rapid Boltzmann distribution of excitation energy and taking into account the antenna properties (size and spectral distribution), the molecular rate constant of primary charge separation is estimated to be about (3 ps)-1. (2) The electron transfer from Pheo- to QA is characterised by a rate constant of (300 ps)-1. (3) The QA- reoxidation kinetics are significantly retarded in D2O suspensions. These H/D isotope effects are interpreted as to reflect hydrogen-bond dependent changes in the protein dynamics that are relevant to electron transfer. (4) In PSII reaction centres closed for photochemical trapping the yield of a primary radical pair with lifetimes exceeding 1 ns is comparatively small (c 30%) at room temperature. Short illumination in the presence of Na2S2O4 changes the radical pair dynamics. (5) Photoinhibition under aerobic conditions impairs the primary charge separation and leads to formation of quencher(s) of excitation energy.

scholarly journals Plant Photochemistry, Reactive Oxygen Species, and Photoprotection

Photochem ◽  
2021 ◽  
Vol 2 (1) ◽  
pp. 5-8
Author(s):  
Michael Moustakas
Keyword(s):  
Reactive Oxygen Species ◽  
Photosystem Ii ◽  
Photosystem I ◽  
Charge Separation ◽  
Light Harvesting ◽  
Oxygen Species ◽  
Light Harvesting Complexes ◽  
Cytochrome B6f ◽  
Reaction Centres ◽  
Electron Carriers

Light energy, absorbed as photons by chlorophylls and other pigment molecules consisting of light-harvesting complexes (LHCs), is transferred to the reaction centres (RCs), where, through charge separation, electrons flow from photosystem II (PSII) through cytochrome b6f and diffusible electron carriers to photosystem I (PSI) [...]

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