Red Chlorophyll a Antenna States of Photosystem I of the Cyanobacterium Synechocystis sp. PCC 6803

Abstract

The antenna chlorophyll a (Chl a) molecules of photosystem I of green plants and cyanobacteria that absorb further to the red than P700, the special pair of the reaction center, have long been of considerable interest. Recently, the results of nonphotochemical hole burning experiments at liquid helium temperatures, which included use of high pressure and external electric (Stark) fields, led to the conclusion that the cyanobacterium Synechocystis sp. PCC 6803 possesses two "red" antenna states whose S 0 (ground state) f Q y (S 1 ) origin absorption bands are at 714 and ≈708 nm (Ra ¨tsep et al. J. Phys. Chem B 2000, 104, 836). The results indicated that the 714 nm state is due to strongly coupled Chl a molecules (C-714), a dimer or, possibly, a trimer. It was concluded that the 714 nm state is responsible for the fluorescence origin band at 720 nm. Presented here are the results of theoretical simulations of the dependence of the hole spectra on burn wavelength and burn fluence that are consistent with the conclusions of Ra ¨tsep et al. They lead to a more detailed characterization of the two states, including determination of their site distribution functions (SDF) and the electron-phonon coupling parameters of the S 0 f Q y transitions. The higher energy state is found to lie closer to 706 nm than 708 nm. The electron-phonon coupling of the 714 nm transition is strong with a total Huang-Rhys factor (S t ) of 2.0 due to low-frequency modes at 18 and ≈70 cm -1 . The coupling of the 706 nm transition is weaker by a factor of 1.5. It is concluded that both C-714 and C-706 are, at a minimum, dimers that are not in close proximity to each other. The large widths, ≈300 cm -1 , of the SDF suggest that the structures of C-706 and C-714 are fragile. The spectroscopic properties of P700, C-706, and C-714 are compared and discussed in terms of excitation energy transfer at low temperatures. A new model that explains why only about half of the PS I complexes undergo irreversible charge separation in the low-temperature limit following excitation of the higher energy bulk antenna states is presented.

Protein dynamics-induced variation of excitation energy transfer pathways

“…The pool F699 has its fluorescence emission maximum at 699 nm and was determined by SMS (24). C706 and C714 were proposed based on holeburning spectroscopy and named according to their absorption maxima (26)(27)(28)30). The fluorescence emission maxima for C706 and C714 are expected at Ϸ710 and Ϸ720 nm, respectively (26).…”

“…C706 and C714 were proposed based on holeburning spectroscopy and named according to their absorption maxima (26)(27)(28)30). The fluorescence emission maxima for C706 and C714 are expected at Ϸ710 and Ϸ720 nm, respectively (26). The effect of spectral diffusion differs for these 3 contributions (17,24).…”

“…Under these conditions, the excitation energy is partially released as fluorescence light (22) with the maximum of the fluorescence emission found at wavelengths Ͼ715 nm. The number and the spectroscopic characteristics of these traps vary remarkably between PSI from different organisms (9,17,(23)(24)(25)(26)(27)(28)(29)(30). The efficient energy transfer from high-energy antenna states to the red pools is very advantageous in SMS because the whole fluorescence emission of PSI can be observed without interference by the excitation light, and the released fluorescence light contains information on the exciton transfer within PSI.…”

Characterization of Low-Energy Chlorophylls in the PSI-LHCI Supercomplex from Chlamydomonas reinhardtii. A Site-Selective Fluorescence Study

“…4,17 Indeed, at least four red Chl pools were reported: two for LHCI 20 and two for PSI core. 17 It should be emphasized that the spectral parameters obtained and discussed in this contribution cannot be directly compared to those obtained from hole-burning spectroscopy 9,15,16 since it was shown that these two techniques may look selectively at different pools of red Chls in the PSI-LHCI complex. 17 Interestingly, the hole-burning and selective fluorescence studies of red Chls reveal preferentially low-frequency phonons of ∼20 cm -1 and high-frequency phonons of ∼100 cm -1 , respectively.…”

“…Additional red Chls pools at 714-715 nm were reported for PSI cores from Synechocystis PCC 6803 and T. elongatus on the basis of hole burning spectroscopy. 9,15,16 Several pools of red Chls were observed in PSI-LHCI complexes from green plants. 4,17 The lowest energy Chls are located in the peripheral antenna, LHCI.…”