Triplet and fluorescing states of the CP47 antenna complex of photosystem II studied as a function of temperature (original) (raw)
Related papers
Biochemistry, 2002
CP47 is a pigment-protein complex in the core of photosystem II that tranfers excitation energy to the reaction center. Here we report on a spectroscopic investigation of the isolated CP47 complex. By deconvoluting the 77 K absorption and linear dichroism, red-most states at 683 and 690 nm have been identified with oscillator strengths corresponding to ∼3 and ∼1 chlorophyll, respectively. Both states contribute to the 4 K emission, and the Stark spectrum shows that they have a large value for the difference polarizability between their ground and excited states. From site-selective polarized triplet-minus-singlet spectra, an excitonic origin for the 683 nm state was found. The red shift of the 690 nm state is most probably due to strong hydrogen bonding to a protein ligand, as follows from the position of the stretch frequency of the chlorophyll 13 1 keto group (1633 cm -1 ) in the fluorescence line narrowing spectrum at 4 K upon red-most excitation. We discuss how the 683 and 690 nm states may be linked to specific chlorophylls in the crystal structure [Zouni, A., Witt, H.-T.
The Journal of Physical Chemistry B, 2008
The CP43 protein complex of the core antenna of higher plant photosystem II (PSII) has two quasidegenerate "red" absorption states. It has been shown in the accompanying paper I (Dang, N. C., et al. J. Phys. Chem. B 2008, 112, 9921.) that the site distribution functions (SDFs) of red-states A and B are uncorrelated and the narrow holes are burned in subpopulations of chlorophylls (Chls) from states A and B that are the lowestenergy pigments in their particular CP43 complexes and cannot further transfer energy downhill. In this work, we present the results of a series of Monte Carlo simulations using the 3.0-Å structure of the PSII core complex from cyanobacteria (Loll, B., et al. Nature 2005, 303, 1040.) to model absorption, emission, persistent, and transient hole burned (HB) spectra. At the current structural resolution, we found calculated site energies (obtained from INDO/S calculations) to be only suggestive because their values are different for the two monomers of CP43 in the PS II dimer. As a result, to probe the excitonic structure, a simple fitting procedure was employed to optimize Chl site energies from various starting values corresponding to different A/B pigment combinations to provide simultaneously good fits to several types of optical spectra. It is demonstrated that the shape of the calculated absorption, emission, and transient/persistent hole-burned spectra is consistent with experimental data and our model for excitation energy transfer between two quasi-degenerate lowest-E states (A and B) with uncorrelated SDFs discussed in paper I. Calculations revealed that absorption changes observed near 670 nm in the non-line-narrowed persistent HB spectra (assigned to photoconversion involving Chl-protein hydrogen-bonding by Hughes (Biochemistry 2006, 45, 12345.) are most likely the result of nonphotochemical hole-burning (NPHB) accompanied by the redistribution of oscillator strength due to modified excitonic interactions. We argue that a unique redistribution of oscillator strength during the NPHB process helps to assign Chls contributing to the low-energy states. It is demonstrated that the 4.2 K asymmetric triplet-bottleneck (transient) hole is mostly contributed to by both A and B states, with the hole profile described by a subensemble of pigments, which are the lowest-energy pigments (B sand A s-type) in their complexes. The same lowest-energy Chls contribute to the observed fluorescence spectra. On the basis of our excitonic calculations, the best Chl candidates that contribute to the low-energy A and B states are Chl 44 and Chl 37, respectively.
Biophysical Journal, 1994
A key step in the photosynthetic reactions in photosystem 11 of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photosystem 11 reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The total triplet yield is found to be 0.83 ± 0.05 at 4 K, and it decreases upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a minor extent also from antenna chlorophyll present in the photosystem 11 reaction center. No carotenoid triplet states could be detected, demonstrating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 ± 0.02 at 10 K, and it decreases upon raising the temperature to reach a value of 0.05-0.06 at 60-70 K, increases upon raising the temperature to 0.07 at -165 K and decreases again upon further raising the temperature. The complex dependence of fluorescence quantum yield on temperature is explained by assuming the presence of one or more pigments in the photosystem 11 reaction center that are energetically degenerate with the primary electron donor P680 and below 60-70 K trap part of the excitation energy, and by temperature-dependent excited state decay above 165 K. A four-compartment model is presented that describes the observed triplet and fluorescence quantum yields at all temperatures and includes pigments that are degenerate with P680, temperature-dependent excited state decay and activated upward energy transfer rates. The eigenvalues of the model are in accordance with the lifetimes observed in fluorescence and absorption difference measurements by several workers. The model suggests that the free energy difference between singlet-excited P680 and the radical pair state P680+1is temperature independent, and that a distribution of free energy differences represented by at least three values of about 20, 40, and 80 meV, is needed to get an appropriate fit of the data.
Biochimica et biophysica acta, 2016
The identification of low-energy chlorophyll pigments in photosystem II (PSII) is critical to our understanding of the kinetics and mechanism of this important enzyme. We report parallel circular dichroism (CD) and circularly polarized luminescence (CPL) measurements at liquid helium temperatures of the proximal antenna protein CP47. This assembly hosts the lowest-energy chlorophylls in PSII, responsible for the well-known "F695" fluorescence band of thylakoids and PSII core complexes. Our new spectra enable a clear identification of the lowest-energy exciton state of CP47. This state exhibits a small but measurable excitonic delocalization, as predicated by its CD and CPL. Using structure-based simulations incorporating the new spectra, we propose a revised set of site energies for the 16 chlorophylls of CP47. The significant difference from previous analyses is that the lowest-energy pigment is assigned as Chl 612 (alternately numbered Chl 11). The new assignment is read...
1998
Spectral hole-burning experiments have been performed at liquid He temperature on the Q y-band of isolated reaction center complexes of photosystem II (PS II RC) containing five (RC-5) and six (RC-6) chlorophyll a (Chl a) molecules. The aim was to investigate the nature of the redmost shoulder in the absorption spectrum of RC-5 and to identify distributions of "trap" pigments. The "effective" homogeneous line width Γ′ hom was measured at 682 nm as a function of temperature between 1.2 and 4.2 K. It follows a T 1.3(0.1 power law in both complexes and extrapolates to the fluorescence lifetime-limited value, τ fl) (4 (1) ns, for T f 0. These results indicate that the redmost absorbing pigments act as "traps" for the excitation energy. The spectral distribution of these traps was reconstructed from the hole depth measured as a function of excitation wavelength λ exc and compared to that of RC-6 previously obtained by us (Groot, M. L.; et al. J. Phys. Chem. 1996, 100, 11488). The maximum of the RC-5 trap distribution lies at (682.9 (0.2) nm. We discovered a second distribution of fluorescing pigments centered at (673.4 (0.5) nm in both RC-5 and RC-6. The dependence of Γ′ hom on the delay time t d between burning and probing, for the red-and blue-absorbing pigments, is constant for t d e 1 s and increases linearly with log t d for longer delay times. The molecules absorbing at ∼674 nm, which can be chemically removed, are not free Chl a but are bound to a protein with the same mass as that of the RC complex.
On the Unusual Temperature-Dependent Emission of the CP47 Antenna Protein Complex of Photosystem II
The Journal of Physical Chemistry Letters, 2010
It is shown that the fluorescence origin band maximum (∼695 nm) of the intact CP47 antenna protein complex of PSII from spinach does not shift in the temperature range of 5-77 K. However, emission shifts continuously to shorter wavelengths (∼692 nm) if high fluence is used, and hole burning takes place. If permanent damage does not occur, this process is reversible by cycling the temperature. In contrast, the emission peaks previously observed near 685 and 691 nm are characteristic of destabilized complexes and cannot be eliminated by temperature cycling. We argue that the CP47 complex is extremely light sensitive at low temperatures and that its 695 nm emission band in the PSII core, in contrast to several literature reports, does not arise from excitations that are trapped on red-absorbing chlorophyll of the ∼690 nm band, as 5 K emission of intact (nonaggregated) CP47 also peaks near 695 nm.
Part of the fluorescence of chlorophyll a may originate in excited triplet states
Photosynthesis Research, 1999
Absorption and fluorescence spectra of chlorophyll a have been analyzed on the basis of an extended version of Kennard–Stepanov (KS) theory. It is proposed that at least one new electronic state lies just below the normal S1 − S0 transition (Qy), borrowing approximately 2–4% of its strength and contributing to the fluorescence in the tail. The KS anomalies leading to this hypothesis occur in a wide variety of cases, including chlorophyll a in solution and protein-bound chlorophyll a, suggesting that the phenomenon is an intrinsic property of the molecule. Natural candidates for the new state(s) are the second and third triplet states. The relationship of the fluorescence excitation spectrum to KS theory is investigated and applied to explain a red drop in yield.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2001
The Photosystem II (PSII) core antenna complexes, CP43 and CP47, were prepared from spinach (Spinacia oleracea L.). The absorption spectra in the red region at room temperature were recorded for the PSII core antenna samples after increased temperature treatment (up to 80³C). Derivative and difference spectra revealed the existence of two groups of chlorophyll a (Chl a) molecules in both CP43 and CP47. The one with the absorption peak in the shorter wavelength region was designated as CP43-669 and CP47-669, while the other with the absorption peak in the longer wavelength region was designated as CP43-682 and CP47-680. The results of the thermal treatment experiment demonstrated that CP43-669 and CP47-669 may exist as monomers of Chl a and that their binding sites on the polypeptides are insensitive to thermal treatment, whereas CP43-682 and CP47-680 may exist as dimers or multimers of Chl a and their binding regions in the polypeptide chains are more sensitive to heat treatment. The excitation energy transfer mechanism between these two different groups of Chl a molecules is also analyzed.
Journal of the Chemical Society, Faraday Transactions 2, 1988
CP1, the isolated reaction centre (RC) chlorophyll(ch1)-protein of plant photosystem I(PSI) containing P700 and ca. 40 antenna Chi has been isolated using sodium dodecyl sulphate and gel electrophoresis. It retained the triplet e.s.r. polarisation pattern characteristic of active charge separation and recombination. Low-temperature and time-resolved fluorescence emission spectra showed that at least two discrete antenna chl forms were present, and excitation energy transfer between them and P700 was studied by measuring chl sub-nanosecond fluorescence decay kinetics over a range of temperatures and emission wavelengths, using ca. 100 ps Ar-ion laser excitation pulses and single-photon detection, resulting in ca. 10 ps time resolution. The two forms are F720, emitting at 720nm (low-energy sites within the antenna) and F690, emitting at 690-695 nm. The latter form was only observed at short times (<200 ps) and at low temperatures. Decay kinetics were fitted to the sum of three exponentials. The two longer (> 1 ns) components were of small amplitude and have no significance for energy transfer. The lifetime of the shortest resolved component varied in a complex way with temperature between 30 and 150ps, also dependent on emission wavelength. At T > 200 K the lifetime was 40 f 10 ps, independent of wavelength, but on lowering the temperature it developed a strong wavelength dependence with a distinct minimum at 690-695 nm. A model is presented for energy transfer between the discrete chl antenna forms which accounts for the change of the observed lifetimes with temperature. In this model F690 forms a core antenna close to the RC and can transfer energy to P700 even at 10 K. Endothermic energy transfer out of F720, which is inhibited by low temperatures, gives rise to the observed temperature dependence of the F690 and F720 fluorescence lifetimes. The initial ultrafast events in plant photosynthesis [energy transfer within an antenna pigment bed and subsequent primary charge separation in a reaction centre(RC)] have been the subject of numerous picosecond fluorescence studies.' Progress in the study of bacterial photosynthesis has depended on the biochemical extraction of relatively simple and well defined complexes from the photosynthetic apparatus. However, for plant systems these methods have not been successful and the isolated chlorophyll(ch1)protein complexes are relatively large, containing many chl molecules, which are heterogeneous and can be distinguished spectroscopically.2 This inhomogeneity is inherent for the native complex and is not a result of isolation.2