Light harvesting in photosystem II

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.

Charge separation is virtually irreversible in photosystem II core complexes with oxidized primary quinone acceptor

X-ray structures of the Photosystem II (PSII) core revealed relatively large interpigment distances between the CP43 and CP47 antenna complexes and the reaction center (RC) with respect to the interpigment distances in a single unit. This finding questions the possibility of fast energy equilibration among the antenna and the RC, which has been the basic explanation for the measured PSII fluorescence kinetics for more than two decades. In this study, we present time-resolved fluorescence measurements obtained with a streak-camera setup on PSII core complexes from Thermosynechococcus elongatus at room temperature (RT) and at 77 K. Kinetic modeling of the RT data obtained with oxidized quinone acceptor Q(A), reveals that the kinetics are best described by fast primary charge separation at a time scale of 1.5 ps and slow energy transfer from the antenna into the RC, which results in an energy equilibration time between the antenna and the RC of about 44 ps. This model is consistent with structure-based computations. Primary radical pair formation was found to be a virtually irreversible process. Energy equilibration within the CP43 and CP47 complexes is shown to occur at a time scale of 8 ps. Kinetic modeling of the 77 K data reveals similar energy transfer time scales in the antenna units and among the antenna and the RC as at RT, respectively, 7 and 37 ps. We conclude that the energy transfer from the CP43/CP47 antenna to the RC is the dominant factor in the total charge separation kinetics in intact PSII cores.

Toward understanding molecular mechanisms of light harvesting and charge separation in photosystem II

Conversion of light energy in photosynthesis is extremely fast and efficient, and understanding the nature of this complex photophysical process is challenging. This review describes current progress in understanding molecular mechanisms of light harvesting and charge separation in photosystem II (PSII). Breakthroughs in X-ray crystallography have allowed the development and testing of more detailed kinetic models than have previously been possible. However, due to the complexity of the light conversion processes, satisfactory descriptions remain elusive. Recent advances point out the importance of variations in the photochemical properties of PSII in situ in different thylakoid membrane regions as well as the advantages of combining sophisticated time-resolved spectroscopic experiments with atomic level computational modeling which includes the effects of molecular dynamics.

Charge separation and energy transfer in the photosystem II core complex studied by femtosecond midinfrared spectroscopy

The core of photosystem II (PSII) of green plants contains the reaction center (RC) proteins D1D2-cytb559 and two core antennas CP43 and CP47. We have used time-resolved visible pump/midinfrared probe spectroscopy in the region between 1600 and 1800 cm(-1) to study the energy transfer and charge separation events within PSII cores. The absorption difference spectra in the region of the keto and ester chlorophyll modes show spectral evolution with time constants of 3 ps, 27 ps, 200 ps, and 2 ns. Comparison of infrared (IR) difference spectra obtained for the isolated antennas CP43 and CP47 and the D1D2-RC with those measured for the PSII core allowed us to identify the features specific for each of the PSII core components. From the presence of the CP43 and CP47 specific features in the spectra up to time delays of 20-30 ps, we conclude that the main part of the energy transfer from the antennas to the RC occurs on this timescale. Direct excitation of the pigments in the RC evolution associated difference spectra to radical pair formation of PD1+PheoD1- on the same timescale as multi-excitation annihilation and excited state equilibration within the antennas CP43 and CP47, which occur within approximately 1-3 ps. The formation of the earlier radical pair ChlD1+PheoD1-, as identified in isolated D1D2 complexes with time-resolved mid-IR spectroscopy is not observed in the current data, probably because of its relatively low concentration. Relaxation of the state PD1+PheoD1-, caused by a drop in free energy, occurs in 200 ps in closed cores. We conclude that the kinetic model proposed earlier for the energy and electron transfer dynamics within the D1D2-RC, plus two slowly energy-transferring antennas C43 and CP47 explain the complex excited state and charge separation dynamics in the PSII core very well. We further show that the time-resolved IR-difference spectrum of PD1+PheoD1- as observed in PSII cores is virtually identical to that observed in the isolated D1D2-RC complex of PSII, demonstrating that the local structure of the primary reactants has remained intact in the isolated D1D2 complex.

Mixing of exciton and charge-transfer states in Photosystem II reaction centers: modeling of Stark spectra with modified Redfield theory

We propose an exciton model for the Photosystem II reaction center (RC) based on a quantitative simultaneous fit of the absorption, linear dichroism, circular dichroism, steady-state fluorescence, triplet-minus-singlet, and Stark spectra together with the spectra of pheophytin-modified RCs, and so-called RC5 complexes that lack one of the peripheral chlorophylls. In this model, the excited state manifold includes a primary charge-transfer (CT) state that is supposed to be strongly mixed with the pure exciton states. We generalize the exciton theory of Stark spectra by 1), taking into account the coupling to a CT state (whose static dipole cannot be treated as a small parameter in contrast to usual excited states); and 2), expressing the line shape functions in terms of the modified Redfield approach (the same as used for modeling of the linear responses). This allows a consistent modeling of the whole set of experimental data using a unified physical picture. We show that the fluorescence and Stark spectra are extremely sensitive to the assignment of the primary CT state, its energy, and coupling to the excited states. The best fit of the data is obtained supposing that the initial charge separation occurs within the special-pair PD1PD2. Additionally, the scheme with primary electron transfer from the accessory chlorophyll to pheophytin gave a reasonable quantitative fit. We show that the effectiveness of these two pathways is strongly dependent on the realization of the energetic disorder. Supposing a mixed scheme of primary charge separation with a disorder-controlled competition of the two channels, we can explain the coexistence of fast sub-ps and slow ps components of the Phe-anion formation as revealed by different ultrafast spectroscopic techniques.

Photophysical Behavior and Assignment of the Low-Energy Chlorophyll States in the CP43 Proximal Antenna Protein of Higher Plant Photosystem II

We have employed absorption, circular dichroism (CD), and persistent spectral hole-burning measurements at 1.7 K to study the photoconversion properties and exciton coupling of low-energy chlorophylls (Chls) in the CP43 proximal antenna light-harvesting subunit of photosystem II (PSII) isolated from spinach. These approximately 683 nm states act as traps for excitation energy in isolated CP43. They "bleach" at 683 nm upon illumination and photoconvert to a form absorbing in the range approximately 660-680 nm. We present new data that show the changes in the CD spectrum due to the photoconversion process. These changes occur in parallel with those in absorption, providing evidence that the feature undergoing the apparent bleach is a component of a weakly exciton-coupled system. From our photoconversion difference spectra, we assign four states in the Chl long-wavelength region of CP43, two of which are the known trap states and are both highly localized on single Chls. The other two states are associated with weak exciton coupling (maximally approximately 50 cm(-)(1)) to one of these traps. We propose a mechanism for photoconversion that involves Chl-protein hydrogen bonding. New hole-burning data are presented that indicate this mechanism is distinct to that for narrow-band spectral hole burning in CP43. We discuss the photophysical behavior of the Chl trap states in isolated CP43 compared to their behavior in intact PSII preparations. The latter represent a more intact, physiological complex, and we find no clear evidence that they exhibit the photoconversion process reported here.

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

The mechanism and kinetics of electron transfer in isolated D1/D2-cyt(b559) 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 approximately 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 approximately 200-ps component represents the electron transfer to the Q(A) acceptor. In isolated RCs, the apparent lifetimes of primary and secondary charge separation are approximately 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 Chl(acc 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).

The polyphasic chlorophyll a fluorescence rise measured under high intensity of exciting light

Chlorophyll a fluorescence rise caused by illumination of photosynthetic samples by high intensity of exciting light, the O-J-I-P (O-I₁-I₂-P) transient, is reviewed here. First, basic information about chlorophyll a fluorescence is given, followed by a description of instrumental set-ups, nomenclature of the transient, and samples used for the measurements. The review mainly focuses on the explanation of particular steps of the transient based on experimental and theoretical results, published since a last review on chlorophyll a fluorescence induction [Lazár D (1999) Biochimica et Biophysica Acta 1412, 1-28]. In addition to 'old' concepts (e.g. changes in redox states of electron acceptors of photosystem II (PSII), effect of the donor side of PSII, fluorescence quenching by oxidised plastoquinone pool), 'new' approaches (e.g. electric voltage across thylakoid membranes, electron transport through the inactive branch in PSII, recombinations between PSII electron acceptors and donors, electron transport reactions after PSII, light gradient within the sample) are reviewed. The K-step, usually detected after a high-temperature stress, and other steps appearing in the transient (the H and G steps) are also discussed. Finally, some applications of the transient are also mentioned.

Initial electron donor and acceptor in isolated Photosystem II reaction centers identified with femtosecond mid-IR spectroscopy

Despite the apparent similarity between the plant Photosystem II reaction center (RC) and its purple bacterial counterpart, we show in this work that the mechanism of charge separation is very different for the two photosynthetic RCs. By using femtosecond visible-pump-mid-infrared probe spectroscopy in the region of the chlorophyll ester and keto modes, between 1,775 and 1,585 cm(-1), with 150-fs time resolution, we show that the reduction of pheophytin occurs on a 0.6- to 0.8-ps time scale, whereas P+, the precursor state for water oxidation, is formed after approximately 6 ps. We conclude therefore that in the Photosystem II RC the primary charge separation occurs between the "accessory chlorophyll" Chl(D1) and the pheophytin on the so-called active branch.

Origin of the F685 and F695 fluorescence in photosystem II

The emission spectra of CP47-RC and core complexes of Photosystem II (PS II) were measured at different temperatures and excitation wavelengths in order to establish the origin of the emission and the role of the core antenna in the energy transfer and charge separation processes in PS II. Both types of particles reveal strong dependences of spectral shape and yield on temperature. The results indicate that the well-known F-695 emission at 77 K arises from excitations that are trapped on a red-absorbing CP47 chlorophyll, whereas the F-685 nm emission at 77 K arises from excitations that are transferred slowly from 683 nm states in CP47 and CP43 to the RC, where they are trapped by charge separation. We conclude that F-695 at 77 K originates from the low-energy part of the inhomogeneous distribution of the 690 nm absorbing chlorophyll of CP47, while at 4 K the fluorescence originates from the complete distribution of the 690 nm chlorophyll of CP47 and from the low-energy part of the inhomogeneous distribution of one or more CP43 chlorophylls.