1On the role of the CP47 core antenna in the energy transfer and trapping dynamics of Photosystem II
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1
On the role of the CP47 core antenna in the energy transfer and trapping dynamics of Photosystem II
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The CP47–RC complex of photosystem II (PS II) has an antenna subunit of 16 chlorophyll a molecules attached to the reaction center (RC) at the side of its inactive branch.
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The X-ray structure of PS II revealed that the shortest interpigment distances between CP47 and RC are about 21 Å, which is two-three times larger than within each subunit.
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ConceptID: Obs1
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Such long distances may slow down the energy transfer from CP47 to RC.
Type: Hypothesis |
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In order to evaluate the influence of the CP47 antenna on the energy transfer and trapping dynamics in the RC we performed a comparative analysis of CP47–RC and RC complexes from spinach by transient difference absorption and time-resolved fluorescence spectroscopy at room temperature.
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Our data reveal a complex multiexponential decay of the excited states in both complexes.
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The main trapping lifetimes are 2–3, 30–40 and 360–460 ps in the RC and 2–6, 80–85 and 650–700 ps in the CP47–RC, the latter two phases being two to three times longer than in the RC.
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ConceptID: Res1
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The data could be fitted well with a kinetic model consisting of three reversible radical pair states, the first one being nearly isoenergetic with P680*, and identical energy levels and kinetics of processes occurring within the RC in both complexes.
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We conclude that there is no kinetic limitation of the energy transfer between CP47 and RC on the two slowest trapping phases and that this transfer occurs in 20 ps or less.
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The two factors that influence the observed slower trapping in CP47–RC are the highly reversible charge separation reaction in the RC and the presence in CP47 of states with energy lower than the primary electron donor P680.
Type: Conclusion |
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ConceptID: Con1
Introduction
11
In the thylakoid membranes of green plants, algae and cyanobacteria the reaction center (RC) of photosystem II (PS II) is surrounded by a number of pigment-protein complexes that harvest sunlight and efficiently deliver excitation energy to the RC.1
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The subunits most close to the RC are CP43 and CP47, which together with the RC and a number of extrinsic proteins involved in water oxidation constitute the PS II core complex.
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13
Structures of cyanobacterial PS II core complexes were obtained recently at 3.5–3.8 Å resolution.2–4
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The main features of the cyanobacterial structure match reasonably well those of green plants, so it is valid to use the X-ray structure of PS II from cyanobacteria as a model for the structure of the PS II core complex of higher plants.5
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According to the X-ray structure the RC (D1/D2/Cyt b559) complex comprises 8 chlorins and 2 carotenes, all of which participate in the energy transfer and/or electron transfer processes.
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Four chlorophyll (Chl) and two pheophytin (Pheo) molecules are arranged in two branches in the central part of the complex.
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One branch is known to be active in charge separation.9
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Two Chl molecules are bound at the periphery of the complex at distances of about 24 Å from the core pigments and do not participate in the primary charge separation process, though they can be involved in secondary electron transfer processes, just as the two β-carotene molecules and cytochrome b559.6
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The CP47 complex contains 16 Chls and two β-carotenes.4
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20
In the thylakoid membrane it is located at the side of the inactive branch of the RC.
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21
The closest interpigment distance between CP47 (Chl43 according to the nomenclature in ref. 2) and RC (PheoD2 and ChlZD2) is about 21 Å, which is considerably more than the ca. 9 Å average interpigment distance in CP47.
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22
In all PS II RC preparations obtained thusfar the secondary electron acceptors, the plastoquinones QA and QB, were removed from their binding sites during the isolation procedure, implying that photochemistry does not proceed beyond the primary radical pair.
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23
The CP47–RC complex differs from the RC mainly by the presence of the additional antenna subunit, because QA is also largely absent after isolation and purification.7
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24
This means that these two complexes represent a good target for comparative studies of the influence the CP47 core antenna on the energy transfer and charge separation processes in the RC.
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25
The kinetics of energy transfer and trapping in the RC have been studied by several groups using both ultrafast absorbance difference and fluorescence techniques.
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The results were reviewed elsewhere.8,9
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27
According to these studies the transfer of excitation energy within the six central chlorins of the RC is ultrafast, with a time constant of about 100 or 600 fs upon excitation into the blue or red edge of the Qy absorption band, respectively,10 though slower energy transfer components can probably not be excluded, in particular at low temperatures.11
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It was suggested12 that charge separation occurs from an excited state which at least at room temperature is distributed over a significant fraction of the six core chlorins, depending on the particular realisation of the disorder.
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The charge separation reactions are strongly multiphasic, with main components in the range of 0.4–3, 20–50 and 120–500 ps.
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30
The details of charge separation process are not precisely understood yet, especially with regard to the role of the “accessory” Chl on the active branch of the RC (ChlD1 according to the nomenclature in ref. 2).
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31
It has been proposed that at least at low temperatures ChlD1 acts as primary electron donor and that the ChlD1+ PheoD1− pair occurs first, followed by electron transfer from PD1 to ChlD1+.13
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33
Lifetimes of about 0.4, 3 and 8 ps were suggested for the primary charge separation.8,9
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34
The 20–50 ps processes were interpreted as formation of a secondary radical pair or primary charge separation limited by the slow transfer from the peripheral Chls to the core of RC.16–20
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36
It was proposed that the slow relaxation reactions of the radical pair may be induced by conformational changes of the protein, induced by the creation of the two charges.23
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37
Recent experiments indicated that most of the energy transfer dynamics in CP47 occurs within 2–3 ps.24
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38
In CP47 the lowest energetic state absorbs at 690 nm with an oscillator strength equal to that of 1 Chl.
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39
The 690 nm state gives a major contribution to the low temperature emission of CP47 and determines a 17 ps lifetime of energy transfer in CP47 at 77 K. The 690 nm pigment is most probably located at the periphery of CP47 not close to the RC and its population will slow down rather than speed up the energy transfer to the reaction center.25
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40
For PS II core complexes it was initially proposed21,26 that excitation energy equilibrates rapidly within several hundreds of femtoseconds between the core antenna and the RC, and that then charge separation occurs within several picoseconds (trap-limited model).
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41
Earlier fluorescence measurements performed on CP47–RC and larger PS II particles27,28 supported the fast energy equilibration between core antenna and RC.
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Type: Motivation |
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43
The lifetime of charge separation depends in this case on the slow energy delivery process from the core antenna to the RC (transfer-to-the-trap-limited model).
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44
In this contribution we present room temperature transient absorption and time-resolved fluorescence studies on PS II RC and CP47–RC complexes purified from spinach.
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Our goal was to record the energy transfer and trapping processes in RC and CP47–RC under similar experimental conditions to be able to make a reliable comparison of the obtained results.
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46
The results indicate that the presence of the CP47 antenna reduces the rate constants of trapping by a factor of 2–3.
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ConceptID: Res3
47
A fit based on a kinetic model suggests that the slower trapping observed in CP47–RC is caused by a shift of the excited states distribution towards the CP47 antenna, and that the intrinsic rate for energy transfer from CP47 to RC does not limit the overall trapping kinetics in the CP47–RC complex.
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Materials and methods
Sample preparation
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CP47–RC complexes were purified from spinach as described elsewhere.7
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For the spectroscopic measurements, all samples were diluted in a buffer containing 20 mM Bis-Tris (pH 6.5), 10 mM MgCl2, 10 mM CaCl2 and 0.03% n-dodecyl-β-d-maltoside (β-DM).
Type: Experiment |
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ConceptID: Exp1
51
Semi-anaerobic conditions in the samples were created by addition of catalase, glucose and glucose oxidase, in that order, to final concentrations of 120 μg ml−1, 10 mM and 120 μg ml−1, respectively.
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The optical density (OD) of the samples used for the transient absorption and fluorescence measurements was about 0.7 mm−1 and 0.8 cm−1, respectively, at the Qy absorption maximum.
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The steady state absorption of the samples was compared before and after measurement.
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Sample degradation was in all cases less than 1%.
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We also analyzed the CP47–RC complex before and after measurement by diode-array-assisted gel filtration chromatography.33
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This technique allows a quantitative estimation of the amount of free CP47 and free pigments in the samples.
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The results revealed a contamination of free CP47 complexes of 1% or less in the samples and no measurable amount of free chlorophylls (not shown), suggesting no significant contribution of unbound CP47 or free chlorophylls to the observed kinetics.
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ConceptID: Res4
Transient absorption
58
Absorption difference spectra were recorded with a femtosecond spectrophotometer, described in detail elsewhere.34
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In brief, the output of Ti:Sapphire oscillator (Coherent Mira) was amplified by means of chirped pulse amplification (Alpha-1000 US, B. M. Industries), generating 1 kHz, 800 nm, 60 fs pulses.
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Single-filament probe white light was generated in a 2 mm sapphire plate.
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Pump light at 400 nm was obtained by doubling the 800 nm fundamental via a SHG crystal.
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Pump light in the visible wavelength range was produced in a home-built, noncollinear optical parametric amplifier.
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63
After prism compression, the pulse duration was reduced to 50–60 fs.
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ConceptID: Exp2
64
Interference filters with transmission maxima at 662 and 695 nm and fwhm 12 and 15 nm, respectively, were used to select the wavelength of excitation.
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Transient absorption difference spectra were collected with probe and excitation beams oriented at magic angle.
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The excitation beam was focused in a spot with 350 μm diameter.
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67
The energy of excitation was 15 nJ pulse−1 upon 662 and 695 nm excitations, and 30 nJ pulse−1 upon 400 nm excitation.
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68
We estimated that about 0.25–0.5 photons were absorbed by each complex per laser shot upon 400 nm excitation, and 0.1–0.2 photons per complex upon 662 and 695 nm excitations.
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69
The cuvette (1 mm pathlength) was shaken in order to refresh the sample from shot to shot.
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70
The time resolution was typically 120 fs and the spectral resolution was 3 nm.
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71
The data were corrected for white light group velocity dispersion and instrument response, and fitted globally as described in .ref. 35
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72
A coherent coupling between pump and probe pulses36,37 was included into the fit function for the data obtained upon 662 and 695 nm excitations.
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Time-resolved emission
73
Time-resolved fluorescence emission spectra were recorded with a Hamamatsu C5680 synchroscan streak camera as described in .ref. 38
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ConceptID: Met9
74
In short, the output of a Ti:Sapphire oscillator (Coherent Mira-Rega), generating 125 kHz, 800 nm, 150 fs pulses, was doubled via an OPA (Coherent), producing 125 kHz, 400 nm, 150 fs pulses.
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ConceptID: Exp3
75
A 665 nm light was generated via the OPA (Coherent) and selected with the interference filter with fwhm = 5 nm.
Type: Experiment |
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ConceptID: Exp3
76
The sample was placed in a spinning cell (diameter 10 cm) with rotation frequency of 75 Hz.
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77
The excitation energy was 1–2 nJ/pulse and the excitation beam was focused in a spot with 150 μm diameter, corresponding to about 0.05–0.1 absorbed photons per complex.
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78
Fluorescence was collected at magic angle with respect to the polarization of the excitation beam.
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The time resolution was 3.5 ps for 400 nm excitation and 6 ps for 665 nm excitation, the spectral resolution was 5 nm.
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The fluorescence data were corrected for white light group velocity dispersion and instrument response, and fitted globally as described in ref. 38.
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Results
Steady-state absorption
81
Fig. 1 shows room temperature absorption spectra of the isolated RC and CP47–RC complexes.
Type: Observation |
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ConceptID: Obs3
82
The Qy absorption band peaks at 675 and 675.5 nm for the RC and CP47–RC respectively.
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ConceptID: Obs3
Type: Result |
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ConceptID: Res6
84
Thus, some selectivity can be achieved upon applying different wavelengths of excitations.
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ConceptID: Met12
85
The Qy band of isolated CP47 overlaps fully with that of the PS II RC and is even broader than this spectrum due to the presence in CP47 of Chls absorbing at energies higher and lower than those of the RC.
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86
Thus selective excitation of one of subunits within CP47–RC can not be achieved.
Type: Conclusion |
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ConceptID: Con2
87
The different lineshapes of the two preparations in the Soret region is explained by the relatively higher content in the RC of Pheo a and cytochrome b-559, which both have strong absorbances at 416 nm33.
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ConceptID: Res8
Transient absorption
88
Transient difference absorption spectra of RC and CP47–RC were measured on a time scale up to 4 ns upon excitation into the Soret band of Chl a (λex = 400 nm), and the blue and red edges of the Qy absorption band (λex = 662 and 695 nm, respectively).
Type: Experiment |
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ConceptID: Exp5
89
The decay-associated difference spectra (DADS) obtained after global analysis are shown in Fig. 2.
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ConceptID: Obs4
RC
90
The decay of delta absorption in the PS II RC complex could be fitted reasonably well with five components for 662 and 400 nm excitation and three components for 695 nm excitation.
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ConceptID: Res9
91
The first DADS has a lifetime of about 100 fs and has a very different shape for the three wavelengths of excitation.
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92
Upon 400 nm excitation it has a strongly positive amplitude (Fig. 2e), indicating that excited states of Qy absorption bands are populated during this time.
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ConceptID: Res10
93
This phase can therefore be attributed to Soret to Qy relaxation processes.
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ConceptID: Res10
94
The first DADS upon 662 nm excitation (Fig. 2c) has a negative peak at 670 nm and a positive peak at 683 nm.
Type: Observation |
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95
The conservative shape suggests energy transfer from species absorbing maximally at 670 nm to species absorbing maximally at 683 nm.
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ConceptID: Res11
96
Subpicosecond equilibration processes in PS II RC complexes were observed and extensively discussed before.10
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97
This fast equilibration must almost exclusively occur within the six central chlorines of the PS II RC, because there is evidence that the peripheral Chls transfer energy to the central pigments on a much slower timescale (10–30 ps).
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ConceptID: Res12
98
The second DADS has a lifetime of about 2–3 ps and has similar, but not identical shapes upon different wavelengths of excitation.
Type: Observation |
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ConceptID: Obs7
99
For 400 and 695 nm excitation it consists of a negative band peaking at about 683 nm and a positive band around 660–670 nm.
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ConceptID: Obs7
100
For 662 nm excitation, the negative band is slightly blue-shifted while no positive band between 660 and 670 is observed.
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ConceptID: Obs7
101
We attribute this phase to primary charge separation.
Type: Conclusion |
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ConceptID: Con3
102
Merry et al10. reported a 600 fs uphill energy transfer in PS II RC complexes upon 694 nm excitation, which was better resolved in data with a parallel orientation of pump and probe beams.
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103
It is possible that uphill energy transfer contributes to the 2 ps DADS in our magic angle data upon 695 nm excitation, but that it could not be resolved as a separate, 600 fs component.
Type: Hypothesis |
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ConceptID: Hyp2
104
The shape of the 2 ps DADS obtained upon 400 and 695 nm is similar to the 3 ± 1 ps DADS obtained upon 543 nm excitation into the Qx band of Pheo a (data not shown).
Type: Observation |
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ConceptID: Obs8
105
This gives evidence that Pheo contributes to the absorption around 680 nm and also confirms that Pheo participates in the initial charge separation process.
Type: Conclusion |
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ConceptID: Con4
106
The third DADS has a lifetime of about 20–30 ps and has a negative amplitude in the whole Qy range, except for a positive feature around 680 nm upon 662 and 400 nm excitation.
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ConceptID: Obs9
107
The negative contribution in the red edge of this DADS for a large part is due to the decay of stimulated emission.
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ConceptID: Res13
108
Stimulated emission must occur from excited states, implying that excited states still contribute to the observed absorption changes on the tens of ps timescale, in line with the fluorescence measurements (see below).
Type: Conclusion |
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ConceptID: Con5
109
The positive feature around 680 nm may occur as a result of several processes: energy transfer from peripheral chlorophylls peaking at 670 nm, electrochromic bandshifts caused by the positive and negative charges associated with PD1+ PheoD1− formation, like in the bacterial RC,41 and a blueshift of red exciton states caused by PD1+ PheoD1− formation.
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ConceptID: Res14
110
Excitation at 695 nm will prevent the first possibility, and the absence of the positive 680 nm feature upon 695 nm excitation suggests that energy transfer from peripheral chlorophylls at 670–680 nm states gives an important contribution to the overall kinetics.
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ConceptID: Res14
111
The fourth DADS has a very small amplitude in transient absorption, and has a lifetime of about 360–460 ps.
Type: Observation |
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ConceptID: Obs10
112
This phase was not observed upon 695 nm excitation, which could be due to its small amplitude.
Type: Result |
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ConceptID: Res15
113
This phase has a prominent amplitude in the fluorescence decay (see below), and can therefore be attributed to radical pair relaxation.
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ConceptID: Res15
114
The non-decaying (ND) absorbance within the time range of our experiments has a minimum at 680 nm and is positive at wavelengths longer than 700 nm, which is an indicator of presence of the PD1+PheoD1− radical pair, the final product of charge separation.18
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ConceptID: Res16
115
It is known that in the isolated RC the radical pair has a lifetime of about 50 ns at room temperature,42 which then recombines to the ground state or the P680* singlet or triplet state.
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ConceptID: Bac14
116
Its shape is identical upon all wavelengths of excitation.
Type: Observation |
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Novelty: None |
ConceptID: Obs11
CP47–RC
117
The global analysis revealed six, five and four components for CP47–RC upon 400, 662 and 695 nm excitation (Fig. 2b, d, f).
Type: Observation |
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ConceptID: Obs12
118
The first DADS (80–180 fs) have similar shapes as those observed in the RC and similar origins: Soret to Qy energy transfer for 400 nm excitation and energy equilibration between 670 and 680 nm absorbing species for 662 nm excitation.
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ConceptID: Res17
119
It is likely that the latter phase occurs not only in the RC, but also within CP47, because in isolated CP47 a strong 200 fs energy transfer from 670 to 680 nm absorbing species was found (at 77 K).24
Type: Result |
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ConceptID: Res17
120
In all traces one or two transients were found with lifetimes of 2–6 ps.
Type: Observation |
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ConceptID: Obs13
121
These lifetimes most probably reflect a mixture of at least two processes: downhill energy transfer within CP47, as was shown to occur at 77 K,24 and charge separation within the RC (like the 2.5 ps component in the RC).
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res18
122
It is also possible that energy equilibration between CP47 and RC contributes to this phase.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res18
123
The shapes of the 2–4 ps phases are similar to those of the corresponding phases in the RC, which suggests that direct charge separation within the RC contributes to a significant extent to these phases.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res19
124
The 6 ps phase upon 400 nm excitation has a shape consistent with downhill energy transfer within CP47.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs14
125
The third series of DADS have lifetimes of 75–90 ps and have roughly similar shapes as those of the 20–30 ps DADS in the RC, exept for the large negative feature caused by CP47 excited states.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs15
126
The negative amplitudes suggest that they all reflect a recovery of bleach, caused by a conversion of chlorophyll/pheophytin excited states into their oxidizes/reduced states.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res20
127
These DADS can therefore be attributed to the secondary trapping process, which in CP47–RC has an about threefold increased lifetime.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res20
128
A fourth series of DADS have lifetimes of 750–1000 ps.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs16
129
The amplitudes of these DADS are larger than those in the RC, and the lifetimes are two to three times longer.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs17
130
It is likely that a third step of radical pair relaxation contributes to these decays.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res21
131
The fact that these DADS are larger than in the RC can be explained by the fact that due to the presence of CP47 about 3x more excited states remain before this phase occurs.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res22
132
For all excitation wavelengths, the non-decaying (ND) components have similar shapes as those observed for the RC, and thus reflect the absorbance difference spectrum of the PD1+ PheoD1− radical pair.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res23
133
From the results of this section we conclude that in both complexes multiexponential trapping by charge separation occurs.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con6
134
The lifetimes of trapping are about 2–3, 23–30 and 360–460 ps for the RC and 2–4, 70–90 and 700–1000 ps for CP47–RC.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res24
135
The last two processes are about three times longer in CP47–RC than in the RC.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res24
Steady-state emission
136
Fig. 3 shows the steady-state emission spectra of RC and CP47–RC at room temperature which peak at 681 and 683 nm, respectively.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs18
137
Isolated CP47 peaks at 683 nm.44
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs19
138
The 2 nm red shift of the CP47–RC emission maximum is consistent with the fact that CP47 makes a significant contribution to the emission of the CP47–RC complex.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res25
Time-resolved emission
139
We have measured time-resolved fluorescence spectra of RC and CP47–RC upon 400 and 665 nm excitation, using a streak camera set-up.
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met13
140
This technique allows a better time and spectral resolution than the more commonly used single-photon timing technique, and was used before for an analysis of RC complexes with red excitation (F. van Mourik et al., manuscript in preparation).
Type: Method |
Advantage: Yes |
Novelty: New |
ConceptID: Met13
141
For our experiments, the time resolution was 3.5 ps for 400 nm excitation and 6 ps for 665 nm excitation.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp6
142
This was not sufficient to resolve subpicosecond processes with similar accuracy as in pump–probe experiments.
Type: Method |
Advantage: No |
Novelty: New |
ConceptID: Met13
143
Experimental traces are shown in Fig. 4.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs20
144
It is obvious from these pictures that CP47–RC is characterised by a longer decay of the emission than RC.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs20
145
The decay associated emission spectra (DAES) for RC and CP47–RC are shown in Figs. 5 and 6.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs21
RC
146
Apart from a poorly resolved sub-picosecond transient, four components were needed to fit the fluorescence decay of the RC for 400 and 665 nm excitation (Figs. 5a and 6a, respectively).
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac15
147
The first DAES has a lifetime of about 3 ps, and peaks considerably (400 and 695 nm excitation: F. van Mourik et al., manuscript in preparation) or slightly (665 nm excitation) more to the red than all subsequent decay phases.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs22
148
The DAES for 400 nm excitation has a positive peak at 690 nm and a small negative feature at 672 nm.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs23
149
The shape of this DAES is non-conservative (with a large positive and a small negative part).
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs23
150
The large positive part (decrease of fluorescence) reflects the trapping by charge separation from states emitting around 685–690 nm.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res26
151
The small negative part can be explained by uphill energy transfer.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res26
152
The following DAES have positive amplitudes and lifetimes of 39 and 360 ps, and reflect subsequent processes of trapping by charge separation.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res27
153
This subsequent trapping could only be observed in fluorescence measurements when excited states are still present in the complex, i.e. if trapping is intrinsically multi-exponential.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac16
154
One possible cause for multi-exponentiality is that trapping is reversible and that the initially formed radical pair ‘relaxes’, which will causes a decrease of excited states population resulting in a decrease of fluorescence.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac16
155
The 39 ps DAES upon 400 nm excitation and the 400 ps DAES upon both excitation wavelengths are similar in shape compared to the steady-state emission of the RC, suggesting that at least the 400 ps processes occur from excited states equilibrated over all pigments.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res28
156
Comparison of the 40 ps DAES upon 400 and 665 nm excitation (Fig. 7) reveals that this DAES upon 665 nm excitation has a larger contribution in the blue part around 670 nm.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs24
157
This suggests that the 40 ps trapping process occurs from a non-equilibrated state and thus can be a mixture of at least two processes: slow energy transfer from (peripheral) chlorophylls absorbing around 670 nm, and trapping by the formation of a secondary radical pair.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res29
158
The 400 ps DAES contributes significantly to the emission of the RC, consistent with radical pair relaxation.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs25
159
The last DAES have a lifetime more than 15 ns which can be attributed to the lifetime of radical pair.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs26
CP47–RC
160
The DAES obtained for CP47–RC upon 400 and 665 nm excitation are shown in Figs. 5b and 6b, respectively.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs27
161
The first DAES has a lifetime of about 5 ps, but a very different shape for both excitation wavelengths.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs28
162
Upon 665 nm excitation, it has a positive peak at 675 nm and negative peak at 695 nm, consistent with energy transfer within CP47.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs29
163
The non-conservative character of this DAES suggests that trapping also occurs on the same time scale.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con7
164
Upon 400 nm excitation, the shape of this component is similar to the 3 ps component in the RC.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs30
165
Its amplitude, however, is smaller than that of the next DAES in the CP47–RC complex.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res30
166
This phase must be largely due to charge separation.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con8
167
The following two DAES have positive amplitudes and reflect trapping by charge separation with lifetimes of about 80–85 ps and 650–700 ps.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res31
168
These times are similar to those observed in transient absorption, and the amplitudes suggest that both processes contribute about equally to the trapping.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res31
169
The large amplitudes of these DAESes suggest that the decay of CP47 excited states largely contributes to these components.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con9
170
The last DAES has a lifetime longer than 15 ns which can attributed to the lifetime of radical pair.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res32
Kinetic model
171
The analysis of the time-resolved fluorescence and transient difference absorption data allowed us to make the following conclusions.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac17
172
In the PS II RC complex, the excitation energy is rapidly (about 100 fs and less than 2–3 ps upon excitation in the blue and red edges of the Qy absorption band, respectively) redistributed among the central six pigments of the RC between states absorbing at around 670 and 680 nm.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res33
173
Then the initial trapping process occurs with a timeconstant of 2–3 ps.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res33
174
The following 30–40 ps process is a mixture of at least two: secondary radical pair formation and transfer of excitation energy from blue-shifted Chls.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res33
175
A 360–460 ps trapping phase can be assigned to further radical pair relaxation.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res33
176
The relaxed radical pair lives longer than 15 ns.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res33
177
In CP47–RC, the first trapping phase takes place on the 2–6 ps timescale, but its kinetics can not easily be followed because of the presence of a major energy transfer process within CP47 in the same time range.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res34
178
Also energy transfer between CP47 and RC can in principle occur on this timescale.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res34
179
Just as in the RC there are two subsequent trapping phases.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res34
180
Both are about three times longer in CP47–RC than in RC.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res34
181
Thus, the addition of the CP47 antenna results in a lengthening of the two slow phases in excited state decay by a factor of about three.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con10
182
These experimental observations can be explained in at least two ways: (1) the intrinsic rate constant of energy transfer between CP47 and RC is slow due to long distances between the pigments of CP47 and RC; this slow transfer would slow down the subsequent process of trapping in the RC, and (2) the intrinsic rate constant of energy transfer between CP47 and RC is fast, while the excited states distribution is shifted towards CP47 because of red-absorbing states.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con11
183
The equilibration of the excited states over a larger antenna will then cause the slower trapping lifetimes observed in the experiments.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con11
184
These two possibilities can be summarized as transfer-to-the-trap-limited and trap-limited kinetics, respectively.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con11
185
For the RC, we used a model with five compartments (Fig. 8a).
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
186
Compartments ‘A*’ and ‘RC*’ correspond to the average excited state energy levels of blue 670 Chls and bulk 680 chlorins, respectively.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
187
The former may correspond to the two distant Chls in the RC; the latter to the six central chlorins of the RC core.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
188
Compartment ‘RP1’ represents the energy level of the first radical pair.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
189
The subsequent relaxations of the radical pair are represented by the compartments ‘RP2’ and ‘RP3’.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
190
For simplicity, we discard fast equilibration within the RC core, and thus assume that charge separation takes place from a state equilibrated over all core pigments.
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met14
191
We only modelled the fluorescence data upon 665 nm excitation.
Type: Method |
Advantage: None |
Novelty: None |
ConceptID: Met14
192
All compartments are connected by forward and backward energy transfer.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
193
The back transfer rate was calculated via the Boltzmann factor: kji = kij(Ni/Nj)exp(−Δij/kT), in which Ni is the number of chlorophylls in compartment i, Δij is the energy difference between compartment i and compartment j, k is Boltzmann’s constant, T is the absolute temperature.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
194
The losses including emission are expressed via the value kfl−1 which is of the order of the lifetime of fluorescence of free Chl.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
195
The total emission decay is proportional to the sum of decays of populations of excited states ‘A*’ and ‘RC*’.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
196
In order to exclude the influence of apparatus response functions on the experimentally measured kinetics, we used data obtained from global analysis of the fluorescence measurements, and integrated over all wavelengths.
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met14
197
The latter simplification is justified, because apart from the 2–3 ps phase all decay components have similar spectra.
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met14
198
The model presented in Fig. 8a gives a good fit of the fluorescence decay in the PS II RC.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs31
199
The fit for the 665 nm excitation data is shown in Fig. 9a.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs32
200
The parameters of the fit are listed in Table 1 together with the reciprocal eigenvalues obtained from the solution of the master equation describing the model.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs33
201
The reciprocal eigenvalues are close to the time constants obtained in the experiment.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs34
202
Values of 25.7 and 37.4 ps can not be resolved separately, and give a single value of 23–30 ps in pump–probe and 39 ps in fluorescence.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res35
203
The energy gap between ‘A*’ and ‘RC*’ is 284 cm−1, assuming that blue Chls and bulk chlorins absorb maximally at 670 and 683 nm respectively.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res36
204
Slight changes in these energy levels did not significantly influence the fit.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs35
205
We assumed that ‘RC*’ and ‘RP1’ are nearly isoenergetic.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
206
Slightly higher or lower energy levels for ‘RP1’ relatively to ‘RC*’ did not increase the quality of the fit.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs36
207
The two subsequent relaxations give an energy differences between ‘RC*’ and the final charge separated state ‘RP3’ of 76.5 meV.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res37
208
This value has been measured to lie between 46 and 110 meV.23,43–46
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac18
209
The fact that the initial radical pairs are not much below the excited state in free energy explains why the emission can be observed over such a long time window.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con12
210
The contribution of each compartment of the model to the total decay of emission is shown in Fig. 10a.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs37
211
It is clear from the picture that ‘RC*’ and ‘RP1’ fully equilibrate in about 180 ps.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs38
212
After that the populations of the both states remain similar.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs38
213
The contribution of the blue chls (compartment ‘A*’) to the total decay is essential only up to about 100 ps.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs38
214
For the CP47–RC we added an additional compartment ‘B*’ (Fig. 8b) corresponding to the average energy level of CP47. ‘B*’ is reversibly connected with the compartments ‘A*’ and ‘RC*’.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
215
In this case compartment ‘A*’ corresponds to the average energy level of only one distant Chl from RC, the closest to CP47.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
216
The total emission decay is proportional to the sum of decays of populations of excited states ‘A*’, ‘B*’ and ‘RC*’.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
217
The fit of CP47–RC fluorescence upon 665 nm excitation is shown in Fig. 9b.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs39
218
For the processes occurring within the RC, we used the same energy levels and rate constants as found for the isolated RC complex.
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met15
219
The parameters of the fit and the eigenvalues are listed in Table 2.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs40
220
To obtain a good fit of the experimental data it was necessary to set level ‘B*’ lower in energy than ‘RC*’.
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met16
221
A small energy gap of 42.9 cm−1 (see Table 2) still allows efficient uphill energy transfer from ‘B*’ to ‘RC*’ at room temperature.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res38
222
Furthermore, a good fit of the data can only be obtained if the intrinsic rate constant of energy transfer from CP47 to RC is sufficiently fast.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res39
223
We found an upper limit for this transfer rate of (20 ps)−1.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res39
224
Slower rates result in a too slow decay of the excited states, which can only be corrected for by changing the energy levels of the secondary and tertiary radical pairs in the RC.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res39
225
The contribution of each compartment to the total decay of the emission is shown in Fig. 10b.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs41
226
The ingrowth of population of each radical pair in CP47–RC is now much slower than in RC, which is because the excited states distribution is shifted towards CP47.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res40
227
The main conclusion of this section is that good fits of the excited state decay can be obtained by a reversible radical pair model coupled to two subsequent relaxed radical pair states.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con13
228
If the energy levels of the three radical pairs do not change as a result of the addition of CP47, then the energy transfer between CP47 and RC does not limit the overall kinetics.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con13
229
The slowing down of the trapping kinetics upon the addition of CP47 is explained by a shift of the excited state distribution towards CP47.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con13
Discussion
RC
230
The analysis of transient difference absorption and time-resolved emission data revealed a multiexponential decay of the excited states in the PS II RC.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con14
231
Typical trapping lifetimes in the RC are 2–3, 30–40 and 350–450 ps.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res41
232
Multiexponential trapping in the PS II RC was also observed in earlier experiments, as well as in CP47–RC (this work) and PS II core preparations.47
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac19
233
Initially it was thought that the wide spread of trapping times is due to sample heterogeneity and/or to a wide distribution of the free energy difference of radical pair formation.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac20
234
Later it was suggested that radical pair formation induces a reorganization of the protein surroundings, which in turn reduces the energy level of the radical pair.23
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac20
235
Also secundary electron transfer reactions, e.g., from PD1 to ChlD1+, may reduce the free energy of the radical pair.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac20
236
Our data are consistent with the following model.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac21
237
Excitation energy rapidly equilibrates within the six central chlorines of RC.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res42
238
Some or most equilibration takes place in about 100 fs, the remainder within a few ps at most.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res42
239
Then initial charge separation takes place in 2–3 ps.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res42
240
The red shifted position of the 2–3 ps component in the transient absorption (400, 695 and 543 nm excitation) and time-resolved emission (400 and 680 nm excitation) indicates that initial charge separation starts from a red exciton state in the RC.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res42
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met17
242
It is most likely that the first radical pair consists of the ChlD1+PheoD1− pair.13–15
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con15
243
The first charge separation generates a radical pair state of about equal energy as that of the singlet excited state of the primary electron donor.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res43
244
This makes recombination back to excited state highly possible.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con16
245
After charge recombination the excitation is equilibrated over all central chlorines in RC.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res44
246
This defines the blue shift (relatively to the 2–3 ps DAS) of the 30–40 ps component in fluorescence which is observed at 400, 665 and 680 nm excitation.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res44
247
The process of energy transfer from the peripheral Chls also contributes to this phase.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res44
248
It is possible that the 30–40 ps trapping phase originates from electron transfer from PD1 to ChlD1+, thus creating the PD1+PheoD1− radical pair.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res44
249
If this interpretation is correct, then this process would be considerably slower than in the purple bacterial RC, where this process takes place in about 3 ps.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con17
250
It would also mean that the 3 ps phases in the RCs of PS II and purple bacteria have different origins.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con17
251
The third trapping lifetime on the 350–450 ps time scale can be interpreted by a lowering of the energy level of PD1+PheoD1− radical pair due to conformational changes of the protein surroundings.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con18
252
The fact that this phase, rather prominent in the time-resolved emission experiments, gives rise to only very minor absorbance changes is consistent with this idea.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con19
CP47–RC
253
The data of the X-ray structures2–4 indicate that the chlorophylls of CP47 are located at considerable distances from the chlorophylls and pheophytins of the RC.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac22
254
The shortest distance is 21.3 Å, between Chl43 of CP47 and PheoD2 of the PS II RC.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac22
255
This will lead to slow energy transfer between CP47 and RC, also because the excitation energy will be rapidly delocalized over the 16 Chls of CP47 and the six central Chls and Pheos of the PS II RC.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac22
256
Thus, one should expect to observe this component as a relatively slow phase in the time-resolved spectroscopy data.
Type: Hypothesis |
Advantage: None |
Novelty: None |
ConceptID: Hyp3
257
We indeed observed slower lifetimes for the excited states decays in CP47–RC.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs42
258
However, the data analysis based on the kinetic model leads to the conclusion that the intrinsic time for energy transfer from CP47 to RC does not limit the trapping kinetics.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con20
259
According to this analysis, the upper limit for this transfer is about 20 ps.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res45
260
This analysis was based on the assumption that the energy levels and rate constants between the various compartments was identical for processes within the RC in the PS II RC and CP47–RC preparations.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
261
In other words, charge separation is assumed to occur in identical ways in both preparations.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
262
While there is no solid experimental evidence that would argue in favour or against this assumption, it does reflect the most simple model for charge separation in PS II.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
263
An upper limit of about 20 ps was defined for the intrinsic time constant for energy transfer from CP47 to the RC.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
264
Only a few ‘linker’ chlorophylls from CP47 are directly coupled with the RC chlorines via energy transfer.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac23
265
To obtain the time constant of energy transfer between the coupled pigments the value 20 ps must be corrected for the number of Chls in CP47 and the number of ‘linker’ chlorophylls, which gives 2.6 ps assuming 16 Chls in CP47, two of which are ‘linker’ Chls.
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met18
266
This value is of the order of the general equilibration time in the CP47 antenna.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs43
267
It may become slightly longer if the excited states of the ‘linker’ chlorophylls are populated to a larger extent than expected from a random distribution, for instance because they are red-shifted or are involved in exciton states that delocalize the main exciton state predominantly on these chlorophylls (which would also be caused by their relatively low site energies).
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con21
268
According to the X-ray structures, the linker chlorophylls are ideally oriented for energy transfer to PheoD2,30 supporting our conclusion about the fast rate of this process.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac24
269
We calculated the difference between energy transfer rates from the nearest linker chl of CP47 (Chl43) to PheoD2, and from ChlZD2 to PheoD2, based on the Förster formula.
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met19
270
The shorter distance and the more favorable orientation of the first pair of molecules suggests a ten times faster energy transfer rate from Chl43 to PheoD2, in agreement with the parameters of our model.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res46
271
We note, however, that local differences in refractive index can have a significant influence on these rates.49,50
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac25
272
Because the charge separation in the RC is highly reversible and the rate of excitation equilibration between CP47 and the RC is fast, the excited states of the CP47 antenna remain populated all the time the excitation occurs in the CP47–RC complex.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con22
273
The spectral properties of the CP47 antenna are also very important.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac26
274
The fact that the average energy level of CP47 is lower than that of the RC helps to distribute the excited states towards the core antenna.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs44
275
We have found that the intrinsic rate constant of energy transfer is of the same order as the initial rate of charge separation.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res47
276
These results indicate that energy transfer and trapping in CP47–RC is not transfer-to-the-trap limited, as it is in the purple bacterial RC-LH1 complex.51
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con23
277
However, it is also not really trap-limited, because some of the energy transfer processes occur on the same timescale, or are even slower than that of the primary charge separation reaction.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con23
The role of CP47 antenna
278
A unique property of Photosystem II is that it has a high oxidation potential.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac27
279
It would be reasonable to assume that to prevent oxidation of the antenna components and leaking away of the positive charge from the site of water oxidation, these components must be located at long distances from the RC.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac28
280
The available structural data have shown that the CP47 chlorophylls are indeed located at distances of 21 Å or more.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac28
281
Our analysis, however, has shown that this does not prevent a fast excitation energy transfer between CP47 and RC, highlighting the light-harvesting function of CP47.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con24
282
It is also clear that CP47 does not play a direct role in the stabilization of the charge separation.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs45
283
Crucial roles in the charge stabilization are currently explained by electron transfer, in the 20–30 ps time range, from PD1 to ChlD1+ and by conformational changes, in about 300 ps, of the charged protein surroundings.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res48
284
At high light intensities, the rate of photosynthetic electron transport reaches a maximum.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac29
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac29
286
Our results indicate that CP47 reduces the apparent rate constant of charge separation and acts as a temporary excitation storage system.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con25
287
The excited levels of CP47 gradually contribute to the ingrowth of radical pair populations.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con25
288
It is likely that the other PS II antenna complexes play similar roles.
Type: Hypothesis |
Advantage: None |
Novelty: None |
ConceptID: Hyp4
289
So, the fast rate constant of energy transfer between CP47 and RC helps to keep the excitation energy in the antenna system and the possibility to regulate the light need for PS II by means of the antenna.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con26
290
This means that the antenna has a dual function: (1) it increases the absorption cross-section, i.e. it increases the amount of collected light; and (2) it reduces the oversaturation of energy transfer chain.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con26
291
We remark that these properties are not unique for PS II.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con27
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac30