Pigment organization and their interactions in reaction centers of photosystem II: optical spectroscopy at 6 K of reaction centers with modified pheophytin composition

Intraband Exciton Transitions in Photosynthetic Complexes Revealed by Novel Five-Wave-Mixing Spectroscopy

We calculate the χ(4) optical response of an oriented photosystem II reaction center of purple bacteria described by the Frenkel exciton model using nonlinear exciton equations (NEE). This approach treats each chromophore as an anharmonic oscillator and provides an intuitive quasiparticle picture of nonlinear spectroscopic signals of interacting excitons. It provides a computationally powerful description of nonlinear spectroscopic signals that avoids complete diagonalization of the total Hamiltonian. Expressions for the second- and the fourth-order nonlinear signals are derived. The NEE have been successfully employed in the past to describe even-order-wave-mixing. Here, we extend them to aggregates with broken inversion symmetries. Even-order susceptibilities require the introduction of permanent dipoles, which allow to directly probe low-frequency intraband transitions of excitons.

Short-Range Effects in the Special Pair of Photosystem II Reaction Centers: The Nonconservative Nature of Circular Dichroism

Photosystem II reaction centers extract electrons from water, providing the basis of oxygenic life on earth. Among the light-sensitive pigments of the reaction center, a central chlorophyll a dimer, known as the special pair, so far has escaped a complete theoretical characterization of its excited state properties. The close proximity of the special pair pigments gives rise to short-range effects that comprise a coupling between local and charge transfer (CT) excited states as well as other intermolecular quantum effects. Using a multiscale simulation and a diabatization technique, we show that the coupling to CT states is responsible for 45% of the excitonic coupling in the special pair. The other short-range effects cause a nonconservative nature of the circular dichroism spectrum of the reaction center by effectively rotating the electric transition dipole moments of the special pair pigments inverting and strongly enhancing their intrinsic rotational strength.

Triplet Delocalization over the Reaction Center Chlorophylls in Photosystem II

The triplet state of chlorophyll formed by charge recombination in photosystem II (PSII) is a precursor of harmful singlet oxygen. Although main localization of the triplet state on the monomeric chlorophyll, Chl_D1, at cryogenic temperatures has been suggested, how the triplet state is delocalized on other chlorophylls remains unclear. Here, we investigated the distribution of the triplet state of chlorophyll in PSII using light-induced Fourier transform infrared (FTIR) difference spectroscopy. Measurements of triplet-minus-singlet FTIR difference spectra with PSII core complexes from cyanobacterial mutants, D1-V157H, D2-V156H, D2-H197A, and D1-H198A, in which the interactions of the 13¹-keto C═O groups of the reaction center chlorophylls, P_D1, P_D2, Chl_D1, and Chl_D2, respectively, were perturbed, identified the 13¹-keto C═O bands of the individual chlorophylls and showed that the triplet state is delocalized over all of these chlorophylls. It is suggested that the triplet delocalization plays important roles in the photoprotection and photodamage mechanisms in PSII.

The origin of unidirectional charge separation in photosynthetic reaction centers: nonadiabatic quantum dynamics of exciton and charge in pigment-protein complexes

Exciton charge separation in photosynthetic reaction centers from purple bacteria (PbRC) and photosystem II (PSII) occurs exclusively along one of the two pseudo-symmetric branches (active branch) of pigment-protein complexes. The microscopic origin of unidirectional charge separation in photosynthesis remains controversial. Here we elucidate the essential factors leading to unidirectional charge separation in PbRC and PSII, using nonadiabatic quantum dynamics calculations in conjunction with time-dependent density functional theory (TDDFT) with the quantum mechanics/molecular mechanics/polarizable continuum model (QM/MM/PCM) method. This approach accounts for energetics, electronic coupling, and vibronic coupling of the pigment excited states under electrostatic interactions and polarization of whole protein environments. The calculated time constants of charge separation along the active branches of PbRC and PSII are similar to those observed in time-resolved spectroscopic experiments. In PbRC, Tyr-M210 near the accessary bacteriochlorophyll reduces the energy of the intermediate state and drastically accelerates charge separation overcoming the electron-hole interaction. Remarkably, even though both the active and inactive branches in PSII can accept excitons from light-harvesting complexes, charge separation in the inactive branch is prevented by a weak electronic coupling due to symmetry-breaking of the chlorophyll configurations. The exciton in the inactive branch in PSII can be transferred to the active branch via direct and indirect pathways. Subsequently, the ultrafast electron transfer to pheophytin in the active branch prevents exciton back transfer to the inactive branch, thereby achieving unidirectional charge separation.

New Perspectives on Photosystem II Reaction Centres

We apply the differential optical spectroscopy techniques of circular polarisation of luminescence (CPL) and magnetic CPL (MCPL) to the study of isolated reaction centres (RCs) of photosystem II (PS II). The data and subsequent analysis provide insights into aspects of the RC chromophore site energies, exciton couplings, and heterogeneities. CPL measurements are able to identify weak luminescence associated with the unbound chlorophyll-a (Chl-a) present in the sample. The overall sign and magnitude of the CPL observed relates well to the circular dichroism (CD) of the sample. Both CD and CPL are reasonably consistent with modelling of the RC exciton structure. The MCPL observed for the free Chl-a luminescence component in the RC samples is also easily understandable, but the MCPL seen near 680nm at 1.8K is anomalous, appearing to have a narrow, strongly negative component. A negative sign is inconsistent with MCPL of (exciton coupled) Qy states of either Chl-a or pheophytin-a (Pheo-a). We propose that this anomaly may arise as a result of the luminescence from a transient excited state species created following photo-induced charge separation within the RC. A comparison of CD spectra and modelling of RC preparations having a different number of pigments suggests that the non-conservative nature of the CD spectra observed is associated with the ‘special pair’ pigments PD1 and PD2.

Electrostatic Asymmetry in the Reaction Center of Photosystem II

The exciton Hamiltonian of the chlorophyll (Chl) and pheophytin (Pheo) pigments in the reaction center (RC) of photosystem II is computed based on recent crystal structures by using the Poisson-Boltzmann/quantum-chemical method. Computed site energies largely confirm a previous model inferred from fits of optical spectra, in which Chl_D1 has the lowest site energy, while that of Pheo_D1 is higher than that of Pheo_D2. The latter assignment has been challenged recently under reference to mutagenesis experiments. We argue that these data are not in contradiction to our results. We conclude that Chl_D1 is the primary electron donor in both isolated RCs and intact core complexes at least at cryogenic temperatures. The main source of asymmetry in site energies is the charge distribution in the protein. Because many small contributions from various structural elements have to be taken into account, it can be assumed that this asymmetry was established in evolution by global optimization of the RC protein.

Conformational control of cofactors in nature - the influence of protein-induced macrocycle distortion on the biological function of tetrapyrroles

Tetrapyrrole-containing proteins are one of the most fundamental classes of enzymes in nature and it remains an open question to give a chemical rationale for the multitude of biological reactions that can be catalyzed by these pigment-protein complexes. There are many fundamental processes where the same (i.e., chemically identical) porphyrin cofactor is involved in chemically quite distinct reactions. For example, heme is the active cofactor for oxygen transport and storage (hemoglobin, myoglobin) and for the incorporation of molecular oxygen in organic substrates (cytochrome P450). It is involved in the terminal oxidation (cytochrome c oxidase) and the metabolism of H2O2 (catalases and peroxidases) and catalyzes various electron transfer reactions in cytochromes. Likewise, in photosynthesis the same chlorophyll cofactor may function as a reaction center pigment (charge separation) or as an accessory pigment (exciton transfer) in light harvesting complexes (e.g., chlorophyll a). Whilst differences in the apoprotein sequences alone cannot explain the often drastic differences in physicochemical properties encountered for the same cofactor in diverse protein complexes, a critical factor for all biological functions must be the close structural interplay between bound cofactors and the respective apoprotein in addition to factors such as hydrogen bonding or electronic effects. Here, we explore how nature can use the same chemical molecule as a cofactor for chemically distinct reactions using the concept of conformational flexibility of tetrapyrroles. The multifaceted roles of tetrapyrroles are discussed in the context of the current knowledge on distorted porphyrins. Contemporary analytical methods now allow a more quantitative look at cofactors in protein complexes and the development of the field is illustrated by case studies on hemeproteins and photosynthetic complexes. Specific tetrapyrrole conformations are now used to prepare bioengineered designer proteins with specific catalytic or photochemical properties.

Circularly polarized luminescence spectroscopy reveals low-energy excited states and dynamic localization of vibronic transitions in CP43

Circularly polarized luminescence (CPL) spectroscopy is an established but relatively little-used technique that monitors the chirality of an emission. When applied to photosynthetic pigment assemblies, we find that CPL provides sensitive and detailed information on low-energy exciton states, reflecting the interactions, site energies and geometries of interacting pigments. CPL is the emission analog of circular dichroism (CD) and thus spectra explore the optical activity only of fluorescent states of the pigment-protein complex and consequently the nature of the lowest-energy excited states (trap states), whose study is a critical area of photosynthesis research. In this work, we develop the new approach of temperature-dependent CPL spectroscopy, over the 2-120 K temperature range, and apply it to the CP43 proximal antenna protein of photosystem II. Our results confirm strong excitonic interactions for at least one of the two well-established emitting states of CP43 named "A" and "B". Previous structure-based models of CP43 spectra are evaluated in the light of the new CPL data. Our analysis supports the assignments of Shibata et al. [Shibata et al. J. Am. Chem. Soc. 135 (2013) 6903-6914], particularly for the highly-delocalized B-state. This state dominates CPL spectra and is attributed predominantly to chlorophyll a's labeled Chl 634 and Chl 636 (alternatively labeled Chl 43 and 45 by Shibata et al.). The absence of any CPL intensity in intramolecular vibrational sidebands associated with the delocalized "B" excited state is attributed to the dynamic localization of intramolecular vibronic transitions.

Low-temperature (77 K) phosphorescence of triplet chlorophyll in isolated reaction centers of photosystem II

Phosphorescence characterized by the main emission band at 952 ± 1 nm (1.30 eV), the lifetime of 1.5 ± 0.1 ms and the quantum yield nearly equal to that for monomeric chlorophyll a in aqueous detergent dispersions, has been detected in isolated reaction centers (RCs) of spinach photosystem II at 77 K. The excitation spectrum shows maxima corresponding to absorption bands of chlorophyll a, pheophytin a, and β-carotene. The phosphorescence intensity strongly depends upon the redox state of RCs. The data suggest that the phosphorescence signal originates from the chlorophyll triplet state populated via charge recombination in the radical pair [Formula: see text].

Primary radical ion pairs in photosystem II core complexes

Ultrafast absorption spectroscopy with 20-fs resolution was applied to study primary charge separation in spinach photosystem II (PSII) reaction center (RC) and PSII core complex (RC complex with integral antenna) upon excitation at maximum wavelength 700-710 nm at 278 K. It was found that the initial charge separation between P680* and ChlD1 (Chl-670) takes place with a time constant of ~1 ps with the formation of the primary charge-separated state P680* with an admixture of: P680*((1-δ)) (P680(δ+)ChlD1(δ-)), where δ ~ 0.5. The subsequent electron transfer from P680(δ+)ChlD1(δ-) to pheophytin (Pheo) occurs within 13 ps and is accompanied by a relaxation of the absorption band at 670 nm (ChlD1(δ-)) and bleaching of the PheoD1 bands at 420, 545, and 680 nm with development of the Pheo(-) band at 460 nm. Further electron transfer to QA occurs within 250 ps in accordance with earlier data. The spectra of P680(+) and Pheo(-) formation include a bleaching band at 670 nm; this indicates that Chl-670 is an intermediate between P680 and Pheo. Stimulated emission kinetics at 685 nm demonstrate the existence of two decaying components with time constants of ~1 and ~13 ps due to the formation of P680(δ+)ChlD1(δ-) and P680(+)PheoD1(-), respectively.

Probing Electron-Transfer Times in Photosynthetic Reaction Centers by Hole-Burning Spectroscopy

A brief discussion is presented of transient hole-burned (HB) spectra (and the information that they provide) obtained for isolated reaction centers (RCs) from wild-type (WT) Rhodobacter sphaeroides, RCs containing zinc-bacteriochlorophylls (Zn-BChls), and RCs of Photosystem II (PSII) from spinach and Chlamydomonas reinhardtii . The shape of the spectral density and the strength of electron-phonon coupling in bacterial RCs are discussed. We focus, however, on heterogeneity of isolated PS II RCs from spinach and, in particular, Chlamydomonas reinhardtii , site energies of active (electron acceptor) and inactive pheophytins, the nature of the primary electron donor(s), and the possibility of multiple charge-separation (CS) pathways in the isolated PSII RC. We conclude with comments on current efforts in HB spectroscopy in the area of photosynthesis and future directions in HB spectroscopy.

Primary electron donor(s) in isolated reaction center of photosystem II from Chlamydomonas reinhardtii

Isolated reaction centers (RCs) from wild-type Chlamydomonas (C.) reinhardtii of Photosystem II (PSII), at different levels of intactness, were studied to provide more insight into the nature of the charge-separation (CS) pathway(s). We argue that previously studied D1/D2/Cytb559 complexes (referred to as RC680), with ChlD1 serving as the primary electron donor, contain destabilized D1 and D2 polypeptides and, as a result, do not provide a representative model system for the intact RC within the PSII core. The shapes of nonresonant transient hole-burned (HB) spectra obtained for more intact RCs (referred to as RC684) are very similar to P(+)QA(-) - PQA absorbance difference and triplet minus singlet spectra measured in PSII core complexes from Synechocystis PCC 6803 [Schlodder et al. Philos. Trans. R. Soc. London, Ser. B2008, 363, 1197]. We show that in the RC684 complexes, both PD1 and ChlD1 may serve as primary electron donors, leading to two different charge separation pathways. Resonant HB spectra cannot distinguish the CS times corresponding to different paths, but it is likely that the zero-phonon holes (ZPHs) observed in the 680-685 nm region (corresponding to CS times of ∼1.4-4.4 ps) reveal the ChlD1 pathway; conversely, the observation of charge-transfer (CT) state(s) in RC684 (in the 686-695 nm range) and the absence of ZPHs at λB > 685 nm likely stem from the PD1 pathway, for which CS could be faster than 1 ps. This is consistent with the finding of Krausz et al. [Photochem. Photobiol. Sci.2005, 4, 744] that CS in intact PSII core complexes can be initiated at low temperatures with fairly long-wavelength excitation. The lack of a clear shift of HB spectra as a function of excitation wavelength within the red-tail of the absorption (i.e., 686-695 nm) and the absence of ZPHs suggest that the lowest-energy CT state is largely homogeneously broadened. On the other hand, in usually studied destabilized RCs, that is, RC680, for which CT states have never been experimentally observed, ChlD1 is the most likely electron donor.

Site energies of active and inactive pheophytins in the reaction center of Photosystem II from Chlamydomonas reinhardtii

It is widely accepted that the primary electron acceptor in various Photosystem II (PSII) reaction center (RC) preparations is pheophytin a (Pheo a) within the D1 protein (Pheo(D1)), while Pheo(D2) (within the D2 protein) is photochemically inactive. The Pheo site energies, however, have remained elusive, due to inherent spectral congestion. While most researchers over the past two decades placed the Q(y)-states of Pheo(D1) and Pheo(D2) bands near 678-684 and 668-672 nm, respectively, recent modeling [Raszewski et al. Biophys. J. 2005, 88, 986 - 998; Cox et al. J. Phys. Chem. B 2009, 113, 12364 - 12374] of the electronic structure of the PSII RC reversed the assignment of the active and inactive Pheos, suggesting that the mean site energy of Pheo(D1) is near 672 nm, whereas Pheo(D2) (~677.5 nm) and Chl(D1) (~680 nm) have the lowest energies (i.e., the Pheo(D2)-dominated exciton is the lowest excited state). In contrast, chemical pigment exchange experiments on isolated RCs suggested that both pheophytins have their Q(y) absorption maxima at 676-680 nm [Germano et al. Biochemistry 2001, 40, 11472 - 11482; Germano et al. Biophys. J. 2004, 86, 1664 - 1672]. To provide more insight into the site energies of both Pheo(D1) and Pheo(D2) (including the corresponding Q(x) transitions, which are often claimed to be degenerate at 543 nm) and to attest that the above two assignments are most likely incorrect, we studied a large number of isolated RC preparations from spinach and wild-type Chlamydomonas reinhardtii (at different levels of intactness) as well as the Chlamydomonas reinhardtii mutant (D2-L209H), in which the active branch Pheo(D1) is genetically replaced with chlorophyll a (Chl a). We show that the Q(x)-/Q(y)-region site energies of Pheo(D1) and Pheo(D2) are ~545/680 nm and ~541.5/670 nm, respectively, in good agreement with our previous assignment [Jankowiak et al. J. Phys. Chem. B 2002, 106, 8803 - 8814]. The latter values should be used to model excitonic structure and excitation energy transfer dynamics of the PSII RCs.

Quantum chemical description of absorption properties and excited-state processes in photosynthetic systems

The theoretical description of the initial steps in photosynthesis has gained increasing importance over the past few years. This is caused by more and more structural data becoming available for light-harvesting complexes and reaction centers which form the basis for atomistic calculations and by the progress made in the development of first-principles methods for excited electronic states of large molecules. In this Review, we discuss the advantages and pitfalls of theoretical methods applicable to photosynthetic pigments. Besides methodological aspects of excited-state electronic-structure methods, studies on chlorophyll-type and carotenoid-like molecules are discussed. We also address the concepts of exciton coupling and excitation-energy transfer (EET) and compare the different theoretical methods for the calculation of EET coupling constants. Applications to photosynthetic light-harvesting complexes and reaction centers based on such models are also analyzed.

Optical properties, excitation energy and primary charge transfer in photosystem II: theory meets experiment

In this review we discuss structure-function relationships of the core complex of photosystem II, as uncovered from analysis of optical spectra of the complex and its subunits. Based on descriptions of optical difference spectra including site directed mutagenesis we propose a revision of the multimer model of the symmetrically arranged reaction center pigments, described by an asymmetric exciton Hamiltonian. Evidence is provided for the location of the triplet state, the identity of the primary electron donor, the localization of the cation and the secondary electron transfer pathway in the reaction center. We also discuss the stationary and time-dependent optical properties of the CP43 and CP47 subunits and the excitation energy transfer and trapping-by-charge-transfer kinetics in the core complex.

Primary light-energy conversion in tetrameric chlorophyll structure of photosystem II and bacterial reaction centers: II. Femto- and picosecond charge separation in PSII D1/D2/Cyt b559 complex

In Part I of the article, a review of recent data on electron-transfer reactions in photosystem II (PSII) and bacterial reaction center (RC) has been presented. In Part II, transient absorption difference spectroscopy with 20-fs resolution was applied to study the primary charge separation in PSII RC (DI/DII/Cyt b 559 complex) excited at 700 nm at 278 K. It was shown that the initial electron-transfer reaction occurs within 0.9 ps with the formation of the charge-separated state P680(+)Chl(D1)(-), which relaxed within 14 ps as indicated by reversible bleaching of 670-nm band that was tentatively assigned to the Chl(D1) absorption. The subsequent electron transfer from Chl(D1)(-) within 14 ps was accompanied by a development of the radical anion band of Pheo(D1) at 445 nm, attributable to the formation of the secondary radical pair P680(+)Pheo(D1)(-). The key point of this model is that the most blue Q(y) transition of Chl(D1) in RC is allowing an effective stabilization of separated charges. Although an alternative mechanism of charge separation with Chl(D1)* as a primary electron donor and Pheo(D1) as a primary acceptor can not be ruled out, it is less consistent with the kinetics and spectra of absorbance changes induced in the PSII RC preparation by femtosecond excitation at 700 nm.

Spectral characteristics of PS II reaction centres: as isolated preparations and when integral to PS II core complexes

The discovery that the native PS II enzyme undergoes charge separation via an absorption extending to 730 nm has led us to re-examine the low-temperature absorption spectra of Nanba-Satoh PS II reaction centre preparations with particular focus on the long wavelength region. It is shown that these preparations do not exhibit absorption in the 700-730 nm region at 1.7 K. Absorption in the Nanba-Satoh type preparations analogous to the 'red tail' as observed in functional PS II core complexes is likely shifted to higher energy by >20 nm. Spectral changes associated with the stable reduction of pheo(a) in chemically treated reaction centre preparations are also revisited. Dithionite treatment of PS II preparations in the dark leads to changes of pigment-pigment and/or pigment-protein interactions, as evidenced by changes in absorption and CD spectra. Absorption and CD changes associated with stable Pheo(D1) photo-reduction in PS II core complexes and Nanba-Satoh preparations are compared. For Nanba-Satoh preparations, Q(y) bleaches are approximately 3x broader than in PS II core complexes and are blue-shifted by approximately 4 nm. These data are discussed in terms of current models of PS II, and suggest a need to consider protein-induced changes of some electronic properties of reaction centre pigments.

Primary light-energy conversion in tetrameric chlorophyll structure of photosystem II and bacterial reaction centers: I. A review

The purpose of the review is to show that the tetrameric (bacterio)chlorophyll ((B)Chl) structures in reaction centers of photosystem II (PSII) of green plants and in bacterial reaction centers (BRCs) are similar and play a key role in the primary charge separation. The Stark effect measurements on PSII reaction centers have revealed an increased dipole moment for the transition at approximately 730 nm (Frese et al., Biochemistry 42:9205-9213, 2003). It was found (Heber and Shuvalov, Photosynth Res 84:84-91, 2005) that two fluorescent bands at 685 and 720 nm are observed in different organisms. These two forms are registered in the action spectrum of Q(A) photoreduction. Similar results were obtained in core complexes of PSII at low temperature (Hughes et al., Biochim Biophys Acta 1757: 841-851, 2006). In all cases the far-red absorption and emission can be interpreted as indication of the state with charge transfer character in which the chlorophyll monomer plays a role of an electron donor. The role of bacteriochlorophyll monomers (B(A) and B(B)) in BRCs can be revealed by different mutations of axial ligand for Mg central atoms. RCs with substitution of histidine L153 by tyrosine or leucine and of histidine M182 by leucine (double mutant) are not stable in isolated state. They were studied in antennaless membrane by different kinds of spectroscopy including one with femtosecond time resolution. It was found that the single mutation (L153HY) was accompanied by disappearance of B(A) molecule absorption near 802 nm and by 14-fold decrease of photochemical activity measured with ms time resolution. The lifetime of P(870)* increased up to approximately 200 ps in agreement with very low rate of the electron transfer to A-branch. In the double mutant L153HY + M182HL, the B(A) appears to be lost and B(B) is replaced by bacteriopheophytin Phi(B) with the absence of any absorption near 800 nm. Femtosecond measurements have revealed the electron transfer to B-branch with a time constant of approximately 2 ps. These results are discussed in terms of obligatory role of B(A) and Phi(B) molecules located near P for efficient electron transfer from P*.

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.

Spectroscopic properties of reaction center pigments in photosystem II core complexes: revision of the multimer model

Absorbance difference spectra associated with the light-induced formation of functional states in photosystem II core complexes from Thermosynechococcus elongatus and Synechocystis sp. PCC 6803 (e.g., P(+)Pheo(-),P(+)Q(A)(-),(3)P) are described quantitatively in the framework of exciton theory. In addition, effects are analyzed of site-directed mutations of D1-His(198), the axial ligand of the special-pair chlorophyll P(D1), and D1-Thr(179), an amino-acid residue nearest to the accessory chlorophyll Chl(D1), on the spectral properties of the reaction center pigments. Using pigment transition energies (site energies) determined previously from independent experiments on D1-D2-cytb559 complexes, good agreement between calculated and experimental spectra is obtained. The only difference in site energies of the reaction center pigments in D1-D2-cytb559 and photosystem II core complexes concerns Chl(D1). Compared to isolated reaction centers, the site energy of Chl(D1) is red-shifted by 4 nm and less inhomogeneously distributed in core complexes. The site energies cause primary electron transfer at cryogenic temperatures to be initiated by an excited state that is strongly localized on Chl(D1) rather than from a delocalized state as assumed in the previously described multimer model. This result is consistent with earlier experimental data on special-pair mutants and with our previous calculations on D1-D2-cytb559 complexes. The calculations show that at 5 K the lowest excited state of the reaction center is lower by approximately 10 nm than the low-energy exciton state of the two special-pair chlorophylls P(D1) and P(D2) which form an excitonic dimer. The experimental temperature dependence of the wild-type difference spectra can only be understood in this model if temperature-dependent site energies are assumed for Chl(D1) and P(D1), reducing the above energy gap from 10 to 6 nm upon increasing the temperature from 5 to 300 K. At physiological temperature, there are considerable contributions from all pigments to the equilibrated excited state P*. The contribution of Chl(D1) is twice that of P(D1) at ambient temperature, making it likely that the primary charge separation will be initiated by Chl(D1) under these conditions. The calculations of absorbance difference spectra provide independent evidence that after primary electron transfer the hole stabilizes at P(D1), and that the physiologically dangerous charge recombination triplets, which may form under light stress, equilibrate between Chl(D1) and P(D1).

Crystal structure of cyanobacterial photosystem II at 3.0 A resolution: a closer look at the antenna system and the small membrane-intrinsic subunits

Photosystem II (PSII) is a homodimeric protein-cofactor complex embedded in the thylakoid membrane that catalyses light-driven charge separation accompanied by the water splitting reaction during oxygenic photosynthesis. In the first part of this review, we describe the current state of the crystal structure at 3.0 A resolution of cyanobacterial PSII from Thermosynechococcus elongatus [B. Loll et al., Towards complete cofactor arrangement in the 3.0 A resolution structure of photosystem II, Nature 438 (2005) 1040-1044] with emphasis on the core antenna subunits CP43 and CP47 and the small membrane-intrinsic subunits. The second part describes first the general theory of optical spectra and excitation energy transfer and how the parameters of the theory can be obtained from the structural data. Next, structure-function relationships are discussed that were identified from stationary and time-resolved experiments and simulations of optical spectra and energy transfer processes.

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.

Focus on the aggregation processes of Photosystem II complexes

In this work the effect of temperature and n-dodecyl-beta-d-maltoside (DM) on PSII complexes organization was investigated. An aggregation process of PSII monomers and dimers was documented at different temperatures and low DM concentration by steady-state fluorescence, absorption, circular dichroism, Rayleigh and dynamic light-scattering experiments. Measures of oxygen evolution enabled us to estimate the change in photoactivity of PSII during the aggregation. This process was found to be extensively reversed by increasing DM concentration as proved by means of steady-state fluorescence and dynamic light-scattering experiments.

Excitonic States in Photosystem II Reaction Center

The excited states of a structurally well-determined photosystem II (PSII) reaction center are obtained using an effective Hamiltonian for the interaction between the Q(y) states. The latter are calculated using the time-dependent density functional theory (DFT) method in DFT-optimized geometries, but with conserved side group orientations. Of particular importance is the orientation of the vinyl group of ring I. Couplings are calculated using actual transition charge distributions via the INDO/S model. Good agreement with experimental spectra is obtained. The lowest excited state is mainly located on the inactive B-side, but with a large component on P(A) too, making charge separation to H(A) possible at low temperature. The "trap state" and triplet state are localized on the inactive B-side. Since the spin singlet Q(y) states of the reaction center are all within a rather small energy range, the state with the highest component of B(A)*, on the blue side of the Q(y) absorption, has a rather high Boltzmann population at room temperature. The charge-transfer states, however, have a rather large spread and cannot be calculated accurately at present. The orientation of the phytyl chains is important and has as a consequence that the energy for the charge-separated B(A)+ H(A)- state is significantly lower than the corresponding state on the B-side. It follows that the B(A)* and P(A)* states are both possible origins for a fast charge separation in PSII.

Circular dichroism of the peripheral chlorophylls in photosystem II reaction centers revealed by electrochemical oxidation

Visible absorption spectra and circular dichroism (CD) of the red absorption band of isolated photosystem II reaction centers were measured at room temperature during progressive bleaching by electrochemical oxidation, in comparison with aerobic photochemical destruction, and with anaerobic photooxidation in the presence of the artificial electron acceptor silicomolybdate. Initially, selective bleaching of peripheral chlorophylls absorbing at 672 nm was obtained by electrochemical oxidation at +0.9 V, whereas little selectivity was observed at higher potentials. Illumination in the presence of silicomolybdate did not cause a bleaching but a spectral broadening of the 672-nm band was observed, apparently in response to the oxidation of carotene. The 672-nm absorption band is shown to exhibit a positive CD, which accounts for the 674-nm shoulder in CD spectra at low temperature. The origin of this CD is discussed in view of the observation that all CD disappears with the 680-nm absorption band during aerobic photodestruction.

Oxygen-evolving Photosystem II core complexes: a new paradigm based on the spectral identification of the charge-separating state, the primary acceptor and assignment of low-temperature fluorescence

We review our recent low-temperature absorption, circular dichroism (CD), magnetic CD (MCD), fluorescence and laser-selective measurements of oxygen-evolving Photosystem II (PSII) core complexes and their constituent CP 4 3, CP 47 and D1/D2/cytb(559) sub-assemblies. Quantitative comparisons reveal that neither absorption nor fluorescence spectra of core complexes are simple additive combinations of the spectra of the sub-assemblies. The absorption spectrum of the D1/D2/cytb(559) component embedded within the core complex appears significantly better structured and red-shifted compared to that of the isolated sub-assembly. A characteristic MCD reduction or 'deficit' is a useful signature for the central chlorins in the reaction centre. We note a congruence of the MCD deficit spectra of the isolated D1/D2/cytb(559) sub-assemblies to their laser-induced transient bleaches associated with P 680. A comparison of spectra of core complexes prepared from different organisms helps distinguish features due to inner light-harvesting assemblies and the central reaction-centre chlorins. Electrochromic spectral shifts in core complexes that occur following low-temperature illumination of active core complexes arise from efficient charge separation and subsequent plastoquinone anion (Q(A)(-)) formation. Such measurements allow determinations of both charge-separation efficiencies and spectral characteristics of the primary acceptor, Pheo(D1). Efficient charge separation occurs with excitation wavelengths as long as 700 nm despite the illuminations being performed at 1.7 K and with an extremely low level of incident power density. A weak, homogeneously broadened, charge-separating state of PSII lies obscured beneath the CP 47 state centered at 690 nm. We present new data in the 690-760 nm region, clearly identifying a band extending to 730 nm. Active core complexes show remarkably strong persistent spectral hole-burning activity in spectral regions attributable to CP 43 and CP 47. Measurements of homogeneous hole-widths have established that, at low temperatures, excitation transfer from these inner light-harvesting assemblies to the reaction centre occurs with approximately 70-270 ps(-1) rates, when the quinone acceptor is reduced. The rate is slower for lower-energy sub-populations of an inhomogeneously broadened antenna (trap) pigment. The complex low-temperature fluorescence behaviour seen in PSII is explicable in terms of slow excitation transfer from traps to the weak low-energy charge-separating state and transfer to the more intense reaction-centre excitations near 685 nm. The nature and origin of the charge-separating state in oxygen-evolving PSII preparations is briefly discussed.

Pathways and timescales of primary charge separation in the photosystem II reaction center as revealed by a simultaneous fit of time-resolved fluorescence and transient absorption

We model the dynamics of energy transfer and primary charge separation in isolated photosystem II (PSII) reaction centers. Different exciton models with specific site energies of the six core pigments and two peripheral chlorophylls (Chls) in combination with different charge transfer schemes have been compared using a simultaneous fit of the absorption, linear dichroism, circular dichroism, steady-state fluorescence, transient absorption upon different excitation wavelengths, and time-resolved fluorescence. To obtain a quantitative fit of the data we use the modified Redfield theory, with the experimental spectral density including coupling to low-frequency phonons and 48 high-frequency vibrations. The best fit has been obtained with a model implying that the final charge separation occurs via an intermediate state with charge separation within the special pair (RP(1)). This state is weakly dipole-allowed, due to mixing with the exciton states, and can be populated directly or via 100-fs energy transfer from the core-pigments. The RP(1) and next two radical pairs with the electron transfer to the accessory Chl (RP(2)) and to the pheophytin (RP(3)) are characterized by increased electron-phonon coupling and energetic disorder. In the RP(3) state, the hole is delocalized within the special pair, with a predominant localization at the inactive-branch Chl. The intrinsic time constants of electron transfer between the three radical pairs vary from subpicoseconds to several picoseconds (depending on the realization of the disorder). The equilibration between RP(1) and RP(2) is reached within 5 ps at room temperature. During the 5-100-ps period the equilibrated core pigments and radical pairs RP(1) and RP(2) are slowly populated from peripheral chlorophylls and depopulated due to the formation of the third radical pair, RP(3). The effective time constant of the RP(3) formation is 7.5 ps. The calculated dynamics of the pheophytin absorption at 545 nm displays an instantaneous bleach (30% of the total amplitude) followed by a slow increase of the bleaching amplitude with time constants of 15 and 12 ps for blue (662 nm) and red (695 nm) excitation, respectively.

Effect of phosphatidylglycerol on molecular organization of photosystem I

Phosphatidylglycerol (PG) is the only anionic phospholipid in photosynthetic membrane. In this study, photosystem I (PSI) particles obtained from plant spinach were reconstituted into PG liposomes at a relatively high concentration. The results from visible absorption, fluorescence emission, and circular dichroism (CD) spectra reveal an existence of the interactions of PSI with PG. PG effect causes blue-shift and intensity decrease of Chl a peak bands in the absorption and 77 K fluorescence emission. The visible CD spectra indicate that the excitonic interactions for Chl a and Chl b molecules were enhanced upon reconstitution. Furthermore, more or less blue- or red-shift of the peaks characterized by Chl a, Chl b, and carotenoid molecules are also occurred. Simultaneously, an increase in alpha-helix and a decrease particularly in the disordered conformations of protein secondary structures are observed. In addition, the same effect also leads to somewhat more tryptophan (Trp) residues exposed to the polar environment. These results demonstrate that some alteration of molecular organization occurs within both the external antenna LHCI and PSI core complex after PSI reconstitution.

Theory of optical spectra of photosystem II reaction centers: location of the triplet state and the identity of the primary electron donor

Based on the structural analysis of photosystem II of Thermosynechococcus elongatus, a detailed calculation of optical properties of reaction-center (D1-D2) complexes is presented applying a theory developed previously. The calculations of absorption, linear dichroism, circular dichroism, fluorescence spectra, all at 6 K, and the temperature-dependence of the absorption spectrum are used to extract the local optical transition energies of the reaction-center pigments, the so-called site energies, from experimental data. The site energies are verified by calculations and comparison with seven additional independent experiments. Exciton relaxation and primary electron transfer in the reaction center are studied using the site energies. The calculations are used to interpret transient optical data. Evidence is provided for the accessory chlorophyll of the D1-branch as being the primary electron donor and the location of the triplet state at low temperatures.

Energy and electron transfer in photosystem II reaction centers with modified pheophytin composition

Energy and electron transfer in Photosystem II reaction centers in which the photochemically inactive pheophytin had been replaced by 13(1)-deoxo-13(1)-hydroxy pheophytin were studied by femtosecond transient absorption-difference spectroscopy at 77 K and compared to the dynamics in untreated reaction center preparations. Spectral changes induced by 683-nm excitation were recorded both in the Q(Y) and in the Q(X) absorption regions. The data could be described by a biphasic charge separation. In untreated reaction centers the major component had a time constant of 3.1 ps and the minor component 33 ps. After exchange, time constants of 0.8 and 22 ps were observed. The acceleration of the fast phase is attributed in part to the redistribution of electronic transitions of the six central chlorin pigments induced by replacement of the inactive pheophytin. In the modified reaction centers, excitation of the lowest energy Q(Y) transition produces an excited state that appears to be localized mainly on the accessory chlorophyll in the active branch (B(A) in bacterial terms) and partially on the active pheophytin H(A). This state equilibrates in 0.8 ps with the radical pair. B(A) is proposed to act as the primary electron donor also in untreated reaction centers. The 22-ps (pheophytin-exchanged) or 33-ps (untreated) component may be due to equilibration with the secondary radical pair. Its acceleration by H(B) exchange is attributed to a faster reverse electron transfer from B(A) to. After exchange both and are nearly isoenergetic with the excited state.

Optical spectra of synechocystis and spinach photosystem II preparations at 1.7 K: identification of the D1-pheophytin energies and stark shifts

We report and compare highly resolved, simultaneously recorded absorption and CD spectra of active Photosystem II (PSII) samples in the range 440-750 nm. From an appropriately scaled comparison of spinach membrane fragment (BBY) and PSII core spectra, we show that key features of the core spectrum are quantitatively represented in the BBY data. PSII from the cyanobacterium Synechocystis 6803 display spectral features in the Qy region of comparable width (50-70 cm(-1) fwhm) to those seen in plant PSII but the energies of the resolved features are distinctly different. A comparison of spectra taken of PSII poised in the S1QA and S2QA(-) redox states reveals electrochromic shifts largely attributable to the influence of QA(-) on Pheo(D1). This allows accurate determinations of the Pheo(D1) Qy absorption positions to be at 685.0 nm for spinach cores, 685.8 nm for BBY particles, and 683.0 nm for Synechocystis. These are discussed in terms of earlier reports of the Pheo(D1) energies in PSII. The Qx transition of Pheo(D1) undergoes a blue shift upon Q(A) reduction, and we place a lower limit of 80 cm(-1) on this shift in plant material. By comparing the magnitude of the Stark shifts of the Qx and Qy bands of Pheo(D1), the directions of the transition-induced dipole moment changes, Deltamu(x) and Deltamu(y), for this functionally important pigment could be determined, assuming normal magnitudes of the Deltamu's. Consequently, Deltamu(x) and Deltamu(y) are determined to be approximately orthogonal to the directions expected for these transitions. Low-fluence illumination experiments at 1.7 K resulted in very efficient formation of QA(-). This was accompanied by cyt b(559) oxidation in BBYs and carotenoid oxidation in cores. No chlorophyll oxidation was observed. Our data allow us to estimate the quantum efficiency of PSII at this temperature to be of the order 0.1-1. No Stark shift associated with the S1-to-S2 transition of the Mn cluster is evident in our samples. The similarity of Stark data in plants and Synechocystis points to minimal interactions of Pheo(D1) with nearby chloropyll pigments in active PSII preparations. This appears to be at variance with interpretations of experiments performed with inactive solubilized reaction-center preparations.

What is beta-carotene doing in the photosystem II reaction centre?

During photosynthesis carotenoids normally serve as antenna pigments, transferring singlet excitation energy to chlorophyll, and preventing singlet oxygen production from chlorophyll triplet states, by rapid spin exchange and decay of the carotenoid triplet to the ground state. The presence of two beta-carotene molecules in the photosystem II reaction centre (RC) now seems well established, but they do not quench the triplet state of the primary electron-donor chlorophylls, which are known as P(680). The beta-carotenes cannot be close enough to P(680) for triplet quenching because that would also allow extremely fast electron transfer from beta-carotene to P(+)(680), preventing the oxidation of water. Their transfer of excitation energy to chlorophyll, though not very efficient, indicates close proximity to the chlorophylls ligated by histidine 118 towards the periphery of the two main RC polypeptides. The primary function of the beta-carotenes is probably the quenching of singlet oxygen produced after charge recombination to the triplet state of P(680). Only when electron donation from water is disturbed does beta-carotene become oxidized. One beta-carotene can mediate cyclic electron transfer via cytochrome b559. The other is probably destroyed upon oxidation, which might trigger a breakdown of the polypeptide that binds the cofactors that carry out charge separation.

Photosystem II reaction center with altered pigment-composition: reconstitution of a complex containing five chlorophyll a per two pheophytin a with modified chlorophylls

Pigment-depleted Photosystem II reaction centers (PS II-RCs) from a higher plant (pea) containing five chlorophyll a (Chl) per two pheophytin a (Phe), were treated with Chl and several derivatives under exchange conditions [FEBS Lett. 434 (1998) 88]. The resulting reconstituted complexes were compared to those obtained by pigment exchange of "conventional" PS II-RCs containing six Chl per two Phe. (1) The extraction of one Chl is fully reversible. (2) The site of extraction is the same as the one into which previously extraneous pigments have been exchanged, most likely the peripheral D1-H118. (3) Introducing an efficient quencher (Ni-Chl) into this site results in only 25% reduction of fluorescence, indicating incomplete energy equilibration among the "core" and peripheral chlorophylls.

Excitonic interactions in the reaction centre of photosystem II studied by using circular dichroism

Changes in excitonic interactions of photosystem II (PSII) reaction centre (RC) pigments upon light-induced oxidation of primary donor (P680) or reduction of primary acceptor (pheophytin (Pheo)) were analysed using circular dichroism (CD). The CD spectrum of PSII RC shows positive bands at 417, 435 and 681 and negative bands at 447 and 664 nm. Oxidation of the primary donor by illuminating the sample in the presence of silicomolybdate resulted in nearly symmetric decrease of CD amplitudes at 664 and 684 nm. In the Soret region, the maximum bleaching of CD signal was detected at 449 and 440 nm. Accumulation of reduced Pheo in the presence of dithionite brought about much lower changes in CD amplitudes than P680 oxidation. In this case, only a small asymmetric bleaching at 680 and 668 nm in the red region and a bleaching at 445, 435 and 416 nm in the Soret region has been detected. Therefore, we suppose that the contribution of the Pheo of the primary acceptor to the total CD signal of RC is negligible. In contrast to the oxidation of primary donor, the light-induced change in the CD spectrum upon primary acceptor reduction was strongly temperature-dependent. The reversible CD bleaching was completely inhibited below 200 K, although the reduced Pheo was accumulated even at a temperature of 77 K. Since the temperature does not influence the excitonic interaction, the temperature dependence of the CD changes upon Pheo reduction does not support the model of Pheo excitonically interacting with the other chlorophylls (Chl) of the RC. We propose that Pheo should not be considered as a part of a multimer model.

Structure, dynamics, and energetics of the primary photochemistry of photosystem II of oxygenic photosynthesis

Recent progress in two-dimensional and three-dimensional electron and X-ray crystallography of Photosystem II (PSII) core complexes has led to major advances in the structural definition of this integral membrane protein complex. Despite the overall structural and kinetic similarity of the PSII reaction centers to their purple non-sulfur photosynthetic bacterial homologues, the different cofactors and subtle differences in their spatial arrangement result in significant differences in the energetics and mechanism of primary charge separation. In this review we discuss some of the recent spectroscopic, structural, and mutagenic work on the primary and secondary electron transfer reactions in PSII, stressing what is experimentally novel, what new insights have appeared, and where questions of interpretation remain.