Fluorescence Spectroscopy of Excitation Transfer in Photosystem I (original) (raw)
Related papers
Science Access, 2001
A remarkable feature of Photosystem I in higher plants and green algae are chlorophylls absorbing and emitting at energies lower than P700. A dominant portion of the red fluorescence comes from a peripheral light-harvesting antenna (LHCI), specifically from its LHCI-730 subpopulation. One of the constituents of the LHCI730 heterodimer, the Lhca4 subunit, was found to harbor the low energy (red) pigments emitting at 730 nm (Tjus et al. 1995; Schmid et al. 1997; Knoetzel et al. 1998). Excitation energy transfer processes in the LHCI-730 and molecular organization of the red pigments that lead to the excitation energy localization are poorly understood. 77 K transient absorption difference spectra of Lhca4 upon excitation of Chl b revealed the presence of several ultrafast energy transfer processes with yet-unresolved lifetimes (Melkozernov et al. 2000a). Two major energy transfer processes include a 400-600 fs energy transfer between spectral forms of Chl b and Chl a followed by 3-5 p...
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1995
The main infrared absorption band of the antenna of the purple bacterium Rps. viridis is investigated using low-temperature absorption and site-selected fluorescence spectroscopy. Low-temperature absorption spectra show that at least three bands are present in the near-infrared. Gaussian fits indicate that the maxima are located around 1015, 1030 and 1045 nm. Fluorescence measurements using narrow-band laser excitation in the red flank of the absorption band show that the emission is highly polarized (p > 0.3). This was found for membranes and (isolated) core complexes. In isolated core complexes, the polarization of the emission increases smoothly as a function of the excitation wavelength, starting in the center of the absorption band, whereas in membranes the increase is abrupt and occurs in the extreme red edge. These results differ significantly from similar measurements performed on core light-harvesting complexes from Rhodobacter sphaeroides Energy transfer and aggregate size effect in the inhomogeneously broadened core light-harvesting complex of Rhodobacter sphaeroides, Chem. Phys. Lett. 193, and cannot be interpreted by assuming a single inhomogeneously broadened pool of antenna pigments. Moreover, the low-temperature absorption spectrum gives clear evidence for a long-wavelength spectral form. Therefore, it is concluded that the antenna is heterogeneous, with a minor long-wavelength spectral form (B1045). The fluorescence measurements show that this B1045 band is inhomogeneously broadened.
Biophysical Journal, 1995
Fluorescence emission and triplet-minus-singlet (T-S) absorption difference spectra of the CP47 core antenna complex of photosystem II were measured as a function of temperature and compared to those of chlorophyll a in Triton X-1 00. Two spectral species were found in the chlorophyll T-S spectra of CP47, which may arise from a difference in ligation of the pigments or from an additional hydrogen bond, similar to what has been found for Chi molecules in a variety of solvents. The T-S spectra show that the lowest lying state in CP47 is at -685 nm and gives rise to fluorescence at 690 nm at 4 K. The fluorescence quantum yield is 0.11 ± 0.03 at 4 K, the chlorophyll triplet yield is 0.16 ± 0.03. Carotenoid triplets are formed efficiently at 4 K through triplet transfer from chlorophyll with a yield of 0.15 ± 0.02. The major decay channel of the lowest excited state in CP47 is internal conversion, with a quantum yield of about 0.58. Increase of the temperature results in a broadening and blue shift of the spectra due to the equilibration of the excitation over the antenna pigments. Upon increasing the temperature, a decrease of the fluorescence and triplet yields is observed to, at 270 K, a value of about 55% of the low temperature value. This decrease is significantly larger than of chlorophyll a in Triton X-100. Although the coupling to low-frequency phonon or vibration modes of the pigments is probably intermediate in CP47, the temperature dependence of the triplet and fluorescence quantum yield can be modeled using the energy gap law in the strong coupling limit of Englman and Jortner (1970. J. Mol. Phys. 18:145-164) for non-radiative decays. This yields for CP47 an average frequency of the promoting/accepting modes of 350 cm-' with an activation energy of 650 cm-1 for internal conversion and activationless intersystem crossing to the triplet state through a promoting mode with a frequency of 180 cm-'. For chlorophyll a in Triton X-1 00 the average frequency of the promoting modes for non-radiative decay is very similar, but the activation energy (300 cm-1) is significantly smaller.
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.
Biophysical Journal, 2001
Ultrafast transient absorption spectroscopy was used to probe excitation energy transfer and trapping at 77 K in the photosystem I (PSI) core antenna from the cyanobacterium Synechocystis sp. PCC 6803. Excitation of the bulk antenna at 670 and 680 nm induces a subpicosecond energy transfer process that populates the Chl a spectral form at 685-687 nm within few transfer steps (300-400 fs). On a picosecond time scale equilibration with the longest-wavelength absorbing pigments occurs within 4-6 ps, slightly slower than at room temperature. At low temperatures in the absence of uphill energy transfer the energy equilibration processes involve low-energy shifted chlorophyll spectral forms of the bulk antenna participating in a 30-50-ps process of photochemical trapping of the excitation by P 700. These spectral forms might originate from clustered pigments in the core antenna and coupled chlorophylls of the reaction center. Part of the excitation is trapped on a pool of the longest-wavelength absorbing pigments serving as deep traps at 77 K. Transient hole burning of the ground-state absorption of the PSI with excitation at 710 and 720 nm indicates heterogeneity of the red pigment absorption band with two broad homogeneous transitions at 708 nm and 714 nm (full-width at half-maximum (fwhm) ϳ 200-300 cm Ϫ1). The origin of these two bands is attributed to the presence of two chlorophyll dimers, while the appearance of the early time bleaching bands at 683 nm and 678 nm under excitation into the red side of the absorption spectrum (Ͼ690 nm) can be explained by borrowing of the dipole strength by the ground-state absorption of the chlorophyll a monomers from the excited-state absorption of the dimeric red pigments.
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
Biophysical Journal, 2007
In this work the spectroscopic properties of the special low-energy absorption bands of the outer antenna complexes of higher plant Photosystem I have been investigated by means of low-temperature absorption, fluorescence, and fluorescence line-narrowing experiments. It was found that the red-most absorption bands of Lhca3, Lhca4, and Lhca1-4 peak, respectively, at 704, 708, and 709 nm and are responsible for 725-, 733-, and 732-nm fluorescence emission bands. These bands are more red shifted compared to ''normal'' chlorophyll a (Chl a) bands present in light-harvesting complexes. The lowenergy forms are characterized by a very large bandwidth (400-450 cm À1 ), which is the result of both large homogeneous and inhomogeneous broadening. The observed optical reorganization energy is untypical for Chl a and resembles more that of BChl a antenna systems. The large broadening and the changes in optical reorganization energy are explained by a mixing of an Lhca excitonic state with a charge transfer state. Such a charge transfer state can be stabilized by the polar residues around Chl 1025. It is shown that the optical reorganization energy is changing through the inhomogeneous distribution of the red-most absorption band, with the pigments contributing to the red part of the distribution showing higher values. A second red emission form in Lhca4 was detected at 705 nm and originates from a broad absorption band peaking at 690 nm. This fluorescence emission is present also in the Lhca4-N-47H mutant, which lacks the 733-nm emission band.
Chemical Physics, 2002
Hole-burning and single photosynthetic complex spectroscopy were used to study the excitonic structure and excitation energy-transfer processes of cyanobacterial trimeric Photosystem I (PS I) complexes from Synechocystis PCC 6803 and Thermosynechococcus elongatus at low temperatures. It was shown that individual PS I complexes of Synechocystis PCC 6803 (which have two red antenna states, i.e., C706 and C714) reveal only a broad structureless fluorescence band with a maximum near 720 nm, indicating strong electronphonon coupling for the lowest energy C714 red state. The absence of zero-phonon lines (ZPLs) belonging to the C706 red state in the emission spectra of individual PS I complexes from Synechocystis PCC 6803 suggests that the C706 and C714 red antenna states of Synechocystis PCC 6803 are connected by efficient energy transfer with a characteristic transfer time of ∼5 ps. This finding is in agreement with spectral holeburning data obtained for bulk samples of Synechocystis PCC 6803. The importance of comparing the results of ensemble (spectral hole burning) and single-complex measurements was demonstrated. The presence of narrow ZPLs near 710 nm in addition to the broad fluorescence band at ∼730 nm in Thermosynechococcus elongatus (Jelezko et al. J. Phys. Chem. B 2000, 104, 8093-8096) has been confirmed. We also demonstrate that high-quality samples obtained by dissolving crystals of PS I of Thermosynechococcus elongatus exhibit stronger absorption in the red antenna region than any samples studied so far by us and other groups.
Excited state dynamics in chlorophyll-based antennae: the role of transfer equilibrium
Biophysical Journal, 1994
We present computer simulations of excited state dynamics in models of PS I and PS 11 which are based upon known structural and spectral properties of the antennae. In particular, these models constrain the pigment binding sites to three-dimensional volumes determined from molecular properties of the antenna complexes. The simulations demonstrate that within a 10-30 ps after light absorption, rapid energy transfer among coupled antenna chlorophylls leads to a quasiequilibrium state in which the fraction of the excited state on any antenna chlorophyll, normalized to the total excited state remaining on the model, remains constant with time. We describe this quasiequilibrium state as a "transfer equilibrium" (TE) state because of its dependence on the rates of processes that couple excited state motion and quenching in the antenna as well as on the individual antenna site energies and temperature. The TE state is not a true equilibrium in that loss of the excited state primarily due to photochemistry (but also due to fluorescence, thermal emission, and intersystem crossing) continues once TE is established. Depending on the dynamics of the system, the normalized distribution of excited state at TE may differ substantially from the Boltzmann distribution (the state of the model at infinite time in the absence of any avenues for decay of excited state). The models predict lifetimes, equilibration times, and photochemical yields that are in agreement with experimental data and affirm trap-limited dynamics in both photosystems. The rapid occurrence of TE states implies that any excited state dynamics that depends on antenna structure and excitation wavelength must occur before the TE state is established. We demonstrate that the excited state distribution of the TE state is central to determining the excited state lifetime and quantum efficiency of photochemistry.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1993
Fluorescence emission spectra for the seven chlorophyll-protein complexes comprising the antenna system of Photosystem II (PS II) have been measured. All four outer antenna complexes (LHC II, CP24, CP26, CP29) have relatively greater emission near 648 nm and 680 nm with respect to the inner antenna complexes (CP43, CP47, D1/D2/cyt b-559). The emission spectra for both outer and inner antenna were calculated from the measured emission spectra of the single chlorophyll-protein complexes, using as weighting factors the excited state population at thermodynamic equilibrium in the various chlorophyll-protein complexes suggested by Jennings et al. (Jennings, R.C., Bassi, R., Garlaschi, F.M., Dainese, P. and Zucchelli, G. (1993) Biochemistry 32, 3203-3210). Subsequently, the overall emission spectra for the total PS II antenna (i.e., outer plus inner antenna) were calculated for situations in which varying excited state levels were assumed for the inner and outer antenna. In an attempt to determine the steady-state distribution of excited states between outer and inner antenna these calculated fluorescence spectra were compared with those measured for (a), PS II particles prepared from maize and (b), chloroplasts of wild-type barley and the chlorina F2 mutant. From this comparison it is concluded that at steady-state fluorescence emission, between 28% and 38% of the excited states in PS II are associated with the inner antenna and between 62% and 72% with the outer antenna. These results suggest that the PS II antenna is organised as a very shallow energy funnel. This antenna organisation is discussed in terms of the generation of non-photochemical quenching mechanisms which are designed to protect PS II from high light stress. * Corresponding author. Fax: + 39 2 2361070.