Spectral and photochemical properties of borohydride-treated D1-D2-cytochrome b-559 complex of photosystem II

Primary charge separation in native and plant pheophytin a-modified reaction centers of Chloroflexus aurantiacus: Ultrafast transient absorption measurements at low temperature

Ultrafast transient absorption (TA) spectroscopy was used to study electron transfer (ET) at 100 K in native (as isolated) reaction centers (RCs) of the green filamentous photosynthetic bacterium Chloroflexus (Cfl.) aurantiacus. The rise and decay of the 1028 nm anion absorption band of the monomeric bacteriochlorophyll a molecule at the B_A binding site were monitored as indicators of the formation and decay of the P⁺B_A^- state, respectively (P is the primary electron donor, a dimer of bacteriochlorophyll a molecules). Global analysis of the TA data indicated the presence of at least two populations of the P^⁎ excited state, which decay by distinct means, forming the state P⁺H_A^- (H_A is a photochemically active bacteriopheophytin a molecule). In one population (~65 %), P^⁎ decays in ~2 ps with the formation of P⁺H_A^- via a short-lived P⁺B_A^- intermediate in a two-step ET process P^⁎ → P⁺B_A^- → P⁺H_A^-. In another population (~35 %), P^⁎ decays in ~20 ps to form P⁺H_A^- via a superexchange mechanism without producing measurable amounts of P⁺B_A^-. Similar TA measurements performed on chemically modified RCs of Cfl. aurantiacus containing plant pheophytin a at the H_A binding site also showed the presence of two P^⁎ populations (~2 and ~20 ps), with P^⁎ decaying through P⁺B_A^- only in the ~2 ps population. At 100 K, the quantum yield of primary charge separation in native RCs is determined to be close to unity. The results are discussed in terms of involving a one-step P^⁎ → P⁺H_A^- superexchange process as an alternative highly efficient ET pathway in Cfl. aurantiacus RCs.

On the Mechanism of Selective Chemical Exchange of Bacteriopheophytins in the Reaction Centers of Rhodobacter sphaeroides R-26

To elucidate the mechanism of site-selective chemical replacement of chromophores in the reaction centers (RCs) of photosynthetic bacteria by external pigments, we investigated how the efficiency of incorporation of plant pheophytin a (Pheo) into the binding sites for bacteriopheophytin a molecules (BPheo) in the isolated Rhodobacter sphaeroides R-26 RCs depended on the incubation medium temperature, Pheo aggregation state, and the presence of organic solvent (acetone). When Pheo was in a form of monomers in free detergent micelles in a water-detergent incubation medium, the degree of selective replacement of photochemically inactive BPheo H_B molecules upon incubation of the RC/Pheo mixture at 5°C was ~15%. The exchange efficiency increased to 40% upon incubation at 25°C and reached 100% at the same temperature when 10% acetone was added to the incubation medium. At both 5 and 25°C, the degree of pigment exchange increased approximately twice, when a mixture of Pheo monomers and dimers in the presence of 10% acetone was used as the incubation medium. The removal of acetone from this medium with the preservation of pigment forms led to a significant decrease in the efficiency of Pheo incorporation. The effect of acetone on the pigment exchange was also observed at an elevated incubation temperature (43.5°C), when functionally active BPheo H_A molecules were partially replaced. The results are discussed in terms of the mechanism according to which (i) the temperature-dependent internal movements of the RC protein facilitate the release of the BPheo molecule from the binding site with simultaneous insertion of the Pheo molecule into the same site in a coupled process, (ii) the role of temperature largely depends on the steric accessibility of binding pockets in the RC protein, (iii) the incorporation of Pheo occurs from a pool of monomeric molecules included in the RC-detergent micelles, and (iv) the presence of acetone in the incubation medium facilitates the exchange of Pheo monomers between micelles in the solution and the detergent belt of the RC complex.

Characterization of the low-temperature triplet state of chlorophyll in photosystem II core complexes: Application of phosphorescence measurements and Fourier transform infrared spectroscopy

Phosphorescence measurements at 77 K and light-induced FTIR difference spectroscopy at 95 K were applied to study of the triplet state of chlorophyll a ((3)Chl) in photosystem II (PSII) core complexes isolated from spinach. Using both methods, (3)Chl was observed in the core preparations with doubly reduced primary quinone acceptor QA. The spectral parameters of Chl phosphorescence resemble those in the isolated PSII reaction centers (RCs). The main spectral maximum and the lifetime of the phosphorescence corresponded to 955±1 nm and of 1.65±0.05 ms respectively; in the excitation spectrum, the absorption maxima of all core complex pigments (Chl, pheophytin a (Pheo), and β-carotene) were observed. The differential signal at 1667(-)/1628(+)cm(-1) reflecting a downshift of the stretching frequency of the 13(1)-keto C=O group of Chl was found to dominate in the triplet-minus-singlet FTIR difference spectrum of core complexes. Based on FTIR results and literature data, it is proposed that (3)Chl is mostly localized on the accessory chlorophyll that is in triplet equilibrium with P680. Analysis of the data suggests that the Chl triplet state responsible for the phosphorescence and the FTIR difference spectrum is mainly generated due to charge recombination in the reaction center radical pair P680(+)PheoD1(-), and the energy and temporal parameters of this triplet state as well as the molecular environment and interactions of the triplet-bearing Chl molecule are similar in the PSII core complexes and isolated PSII RCs.

Dynamics of the Special Pair of Chlorophylls of Photosystem II

Cholophylls are at the basis of the photosynthetic energy conversion mechanisms in algae, plants, and cyanobacteria. In photosystem II, the photoproduced electrons leave a special pair of chlorophylls (namely, P(D1) and P(D2)) that becomes cationic. This oxidizing pair [P(D1),P(D2)](+), in turn, triggers a cascade of oxidative events, eventually leading to water splitting and oxygen evolution. In the present work, using quantum mechanics/molecular mechanics calculations, we investigate the electronic structure and the dynamics of the P(D1)P(D2) special pair in both its oxidized and reduced states. In agreement with previously reported static calculations, the symmetry between the two chlorophylls was found to be broken, the positive charge being preferentially located on P(D1). Nevertheless, this study reveals for the first time that large charge fluctuations occur along dynamics, temporarily inverting the charge preference for the two branches. Finally, a vibrational analysis pinpointed that such charge fluctuations are strongly coupled to specific modes of the special pair.

Primary electron transfer processes in photosynthetic reaction centers from oxygenic organisms

This minireview is written in honor of Vladimir A. Shuvalov, a pioneer in the area of primary photochemistry of both oxygenic and anoxygenic photosyntheses (See a News Report: Allakhverdiev et al. 2014). In the present paper, we describe the current state of the formation of the primary and secondary ion-radical pairs within photosystems (PS) II and I in oxygenic organisms. Spectral-kinetic studies of primary events in PS II and PS I, upon excitation by ~20 fs laser pulses, are now available and reviewed here; for PS II, excitation was centered at 710 nm, and for PS I, it was at 720 nm. In PS I, conditions were chosen to maximally increase the relative contribution of the direct excitation of the reaction center (RC) in order to separate the kinetics of the primary steps of charge separation in the RC from that of the excitation energy transfer in the antenna. Our results suggest that the sequence of the primary electron transfer reactions is P680 → ChlD1 → PheD1 → QA (PS II) and P700 → A 0A/A 0B → A 1A/A 1B (PS I). However, alternate routes of charge separation in PS II, under different excitation conditions, are not ruled out.

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.

Dynamic protein conformations preferentially drive energy transfer along the active chain of the photosystem II reaction centre

One longstanding puzzle concerning photosystem II, a core component of photosynthesis, is that only one of the two symmetric branches in its reaction centre is active in electron transfer. To investigate the effect of the photosystem II environment on the preferential selection of the energy transfer pathway (a prerequisite for electron transfer), we have constructed an exciton model via extensive molecular dynamics simulations and quantum mechanics/molecular mechanics calculations based on a recent X-ray structure. Our results suggest that it is essential to take into account an ensemble of protein conformations to accurately compute the site energies. We identify the cofactor CLA606 of active chain as the most probable site for the energy excitation. We further pinpoint a number of charged protein residues that collectively lower the CLA606 site energy. Our work provides insights into the understanding of molecular mechanisms of the core machinery of the green-plant photosynthesis.

Cofactor-mediated energy and electron transfer in photosystem II occurs preferentially through one branch of the reaction centre, despite there being a symmetric path available. Here, the authors use computational methods to determine the influence of protein conformation on this selectivity.

Formation and decay of P680 (P(D1)-P(D2))⁺PheoD1⁻ radical ion pair in photosystem II core complexes

Under physiological conditions (278 K) femtosecond pump-probe laser spectroscopy with 20-fs time resolution was applied to study primary charge separation in spinach photosystem II (PSII) core complexes excited at 710 nm. It was shown that initial formation of anion radical band of pheophytin molecule (Pheo⁻) at 460 nm is observed with rise time of ~11ps. The kinetics of the observed rise was ascribed to charge separation between Chl (chlorophyll a) dimer, primary electron donor in PSII (P680*) and Pheo located in D1 protein subunit (PheoD1) absorbing at 420 nm, 545 nm and 680 nm with formation of the ion-radical pair P680⁺PheoDI⁻. The subsequent electron transfer from Pheo(D1)⁻ to primary plastoquinone electron acceptor (Q(A)) was accompanied by relaxation of the 460-nm band and occurred within ~250 ps in good agreement with previous measurements in Photosystem II-enriched particles and bacterial reaction centers. The subtraction of the P680⁺ spectrum measured at 455 ps delay from the spectra at 23 ps or 44 ps delay reveals the spectrum of Pheo(DI)⁻, which is very similar to that measured earlier by accumulation method. The spectrum of Pheo(DI)⁻ formation includes a bleaching (or red shift) of the 670 nm band indicating that Chl-670 is close to Pheo(D1). According to previous measurements in the femtosecond-picosecond time range this Chl-670 was ascribed to Chl(D1) [Shelaev, Gostev, Vishnev, Shkuropatov, Ptushenko, Mamedov, Sarkisov, Nadtochenko, Semenov and Shuvalov, J. Photochemistry and Photobiology, B: Biology 104 (2011) 45-50]. Stimulated emission at 685 nm was found to have two decaying components with time constants of ~1ps and ~14ps. These components appear to reflect formation of P680⁺Chl(D1)⁻ and P680⁺Pheo(D1)⁻, respectively, as found earlier. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

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.

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).

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.

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.

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.

Reaction centers of photosystem II with a chemically-modified pigment composition: exchange of pheophytins with 13(1)-deoxo-13(1)-hydroxy-pheophytin a

Isolated reaction centers of photosystem II with an altered pigment content were obtained by chemical exchange of the native pheophytin a molecules with externally added 13(1)-deoxo-13(1)-hydroxy-pheophytin a. Judged from a comparison of the absorption spectra and photochemical activities of exchanged and control reaction centers, 70-80% of the pheophytin molecules active in charge separation are replaced by 13(1)-deoxo-13(1)-hydroxy-pheophytin a after double application of the exchange procedure. The new molecule at the active branch was not active photochemically. This appears to be the first stable preparation in which a redox active chromophore of the reaction center of photosystem II was modified by chemical substitution. The data are compatible with the presence of an active and inactive branch of cofactors, as in bacterial reaction centers. Possible applications of the 13(1)-deoxo-13(1)-hydroxy-pheophytin a-exchanged preparation to the spectral and functional analysis of native reaction centers of photosystem II are discussed.