Energy transfer and trapping in photosystem I.

A theory for calculating time– and frequency–domain optical spectra of pigment–protein complexes is presented using a density matrix approach. Non-Markovian effects in the exciton–vibrational coupling are included. A correlation function is deduced from the simulation of 1.6 K fluorescence line narrowing spectra of a monomer pigment–protein complex (B777), and then used to calculate fluorescence line narrowing spectra of a dimer complex (B820). A vibrational sideband of an excitonic transition is obtained, a distinct non-Markovian feature, and agrees well with experiment on B820 complexes. The theory and the above correlation function are used elsewhere to make predictions and compare with data on time–domain pump–probe spectra and frequency–domain linear absorption, circular dichroism and fluorescence spectra of Photosystem II reaction centers.

/pdf/on-the-relation-of-protein-dynamics-and-exciton-relaxation-k46qulgbra.pdf

On the relation of protein dynamics and exciton relaxation in pigment–protein complexes: An estimation of the spectral density and a theory for the calculation of optical spectra

The mechanism and kinetics of electron transfer in isolated D1/D2-cytb559 photosystem (PS) II reaction centers (RCs) and in intact PSII cores have been studied by femtosecond transient absorption and kinetic compartment modeling. For intact PSII, a component of ≈1.5 ps reflects the dominant energy-trapping kinetics from the antenna by the RC. A 5.5-ps component reflects the apparent lifetime of primary charge separation, which is faster by a factor of 8–12 than assumed so far. The 35-ps component represents the apparent lifetime of formation of a secondary radical pair, and the ≈200-ps component represents the electron transfer to the QA acceptor. In isolated RCs, the apparent lifetimes of primary and secondary charge separation are ≈3 and 11 ps, respectively. It is shown (i) that pheophytin is reduced in the first step, and (ii) that the rate constants of electron transfer in the RC are identical for PSII cores and for isolated RCs. We interpret the first electron transfer step as electron donation from the primary electron donor Chlacc D1. Thus, this mechanism, suggested earlier for isolated RCs at cryogenic temperatures, is also operative in intact PSII cores and in isolated RCs at ambient temperature. The effective rate constant of primary electron transfer from the equilibrated RC* excited state is 170–180 ns−1, and the rate constant of secondary electron transfer is 120–130 ns−1.

https://www.pnas.org/content/pnas/103/18/6895.full.pdf

Kinetics and mechanism of electron transfer in intact photosystem II and in the isolated reaction center: Pheophytin is the primary electron acceptor

Water oxidation in photosynthesis takes place in photosystem II (PSII). This photosystem is built around a reaction center (RC) where sunlight-induced charge separation occurs. This RC consists of various polypeptides that bind only a few chromophores or pigments, next to several other cofactors. It can handle far more photons than the ones absorbed by its own pigments and therefore, additional excitations are provided by the surrounding light-harvesting complexes or antennae. The RC is located in the PSII core that also contains the inner light-harvesting complexes CP43 and CP47, harboring 13 and 16 chlorophyll pigments, respectively. The core is surrounded by outer light-harvesting complexes (Lhcs), together forming the so-called supercomplexes, at least in plants. These PSII supercomplexes are complemented by some “extra” Lhcs, but their exact location in the thylakoid membrane is unknown. The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light. In this review, we will provide a short overview of the relation between the structure and organization of pigment-protein complexes in PSII, ranging from individual complexes to entire membranes and experimental and theoretical results on excitation energy transfer and charge separation. It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes. At the end, an overview will be given of unanswered questions that should be addressed in the near future.

/pdf/light-harvesting-in-photosystem-i-1u5jlmr8vt.pdf

Light-harvesting in photosystem I

▪ Abstract Photosystem I is the light-driven plastocyanin-ferredoxin oxidoreductase in the thylakoid membranes of cyanobacteria and chloroplasts. In recent years, sophisticated spectroscopy, molecular genetics, and biochemistry have been used to understand the light conversion and electron transport functions of photosystem I. The light-harvesting complexes and internal antenna of photosystem I absorb photons and transfer the excitation energy to P700, the primary electron donor. The subsequent charge separation and electron transport leads to the reduction of ferredoxin. The photosystem I proteins are responsible for the precise arrangement of cofactors and determine redox properties of the electron transfer centers. With the availability of genomic information and the structure of photosystem I, one can now probe the functions of photosystem I proteins and cofactors. The strong reductant produced by photosystem I has a central role in chloroplast metabolism, and thus photosystem I has a critical role in...

PHOTOSYSTEM I: Function and Physiology

Results of 4.2 K hole burning, chemical reduction (sodium dithionite, in dark and with illumination), and oxidation (ferricyanide) experiments are reported for the isolated PS II reaction center containing five chlorophyll (Chl) molecules (RC-5). Qy states at 679.6 and 668.3 nm are identified as being highly localized on pheophytin a of the D1 branch (Pheo1) and pheophytin a of the D2 branch (Pheo2), respectively. The Pheo1−Qx and Pheo2−Qx transitions were found to lie on the low and high energy sides of the single Pheo−Qx absorption band, at 544.4 and 541.2 nm, respectively. The Qy band of the 684 nm absorbing Chl, which is more apparent in absorption in RC-5 than in RC-6 samples, is assigned to the peripheral Chl on the D1 side. The results are consistent with that peripheral Chl being Chlz (Stewart, D. H.; Cua, A.; Chisholm, D. A.; Diner, B. A.; Bocian, D. F.; Brudvig, G. W. Biochemistry 1998, 37, 10040). The results indicate that P680, the primary electron donor, is the main acceptor for energy transf...

Excited States of the 5-Chlorophyll Photosystem II Reaction Center

Low-temperature absorption, fluorescence, and persistent hole-burned spectra are reported for aggregates of the trimeric light-harvesting antenna complex of photosystem II (LHC II) . The lowest energy Qy-state was found to lie at 681.5 nm on the basis of hole spectra, which corresponds to a 2 nm red shift relative to the isolated LHC II trimer (Pieper et al. J. Phys. Chem. B 1999, 2412, accompanying paper). The electron−phonon coupling of the 681.5 nm state is characterized by S ∼ 0.8 and coupling to phonons with a mean frequency of ∼ 20 cm-1 which is very similar to that of the isolated trimer. This coupling is consistent with the 4.2 K Stokes shift of the fluorescence originating from the 681.5 nm state. An adjacent state at 680.0 nm is assigned. On the basis of the results of Pieper et al. for the isolated trimer, a state at ∼ 678.5 nm is inferred. These three lowest energy Qy-states are associated with the lowest energy chlorophyll a state of the subunit of the isolated LHC II trimer. Their degeneracy...

Effects of Aggregation on Trimeric Light-Harvesting Complex II of Green Plants: A Hole-Burning Study

High-pressure and stark hole-burning studies of chlorosome antennas from Chlorobium tepidum.

Low temperature (4.2 K) absorption and hole-burned spectra are presented for the trimeric (wild-type, WT) photosystem I complex of the cyanobacterium Synechocystis sp. PCC 6803, its monomeric form, and mutants deficient in the PsaF, K, L, and M protein subunits. High-pressure- and Stark-hole-burning data for the WT trimer are presented as well as its temperature-dependent Qy-absorption and -fluorescence spectra. Taken as a whole, the data lead to assignment of a new and lowest energy antenna Qy-state located at 714 nm at low temperatures. It is this state that is responsible for the fluorescence in the low-temperature limit and not the previously identified antenna Qy-state near 708 nm. The data indicate that the 714 nm state is associated with strongly coupled chlorophyll a molecules (perhaps a dimer) and possesses significant charge transfer character. The red chlorophylls absorbing at 708 and 714 nm do not appear to be directly bound to any of the above protein subunits. The results are consistent with...

The Red-Absorbing Chlorophyll a Antenna States of Photosystem I: A Hole-Burning Study of Synechocystis sp. PCC 6803 and Its Mutants

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Excited States of the 5-Chlorophyll Photosystem II Reaction Center

Effects of Aggregation on Trimeric Light-Harvesting Complex II of Green Plants: A Hole-Burning Study

High-pressure and stark hole-burning studies of chlorosome antennas from Chlorobium tepidum.

The Red-Absorbing Chlorophyll a Antenna States of Photosystem I: A Hole-Burning Study of Synechocystis sp. PCC 6803 and Its Mutants