A multimer model for P680, the primary electron donor of photosystem II

Spectral hole burning: examples from photosynthesis

The optical spectra of photosynthetic pigment-protein complexes usually show broad absorption bands, often consisting of a number of overlapping, "hidden" bands belonging to different species. Spectral hole burning is an ideal technique to unravel the optical and dynamic properties of such hidden species. Here, the principles of spectral hole burning (HB) and the experimental set-up used in its continuous wave (CW) and time-resolved versions are described. Examples from photosynthesis studied with hole burning, obtained in our laboratory, are then presented. These examples have been classified into three groups according to the parameters that were measured: (1) hole widths as a function of temperature, (2) hole widths as a function of delay time and (3) hole depths as a function of wavelength. Two examples from light-harvesting (LH) 2 complexes of purple bacteria are given within the first group: (a) the determination of energy-transfer times from the chromophores in the B800 ring to the B850 ring, and (b) optical dephasing in the B850 absorption band. One example from photosystem II (PSII) sub-core complexes of higher plants is given within the second group: it shows that the size of the complex determines the amount of spectral diffusion measured. Within the third group, two examples from (green) plants and purple bacteria have been chosen for: (a) the identification of "traps" for energy transfer in PSII sub-core complexes of green plants, and (b) the uncovering of the lowest k = 0 exciton-state distribution within the B850 band of LH2 complexes of purple bacteria. The results prove the potential of spectral hole burning measurements for getting quantitative insight into dynamic processes in photosynthetic systems at low temperature, in particular, when individual bands are hidden within broad absorption bands. Because of its high-resolution wavelength selectivity, HB is a technique that is complementary to ultrafast pump-probe methods. In this review, we have provided an extensive bibliography for the benefit of scientists who plan to make use of this valuable technique in their future research.

Proton-coupled electron transfer in biology: results from synergistic studies in natural and model systems

Proton-coupled electron transfer (PCET) underpins energy conversion in biology. PCET may occur with the unidirectional or bidirectional transfer of a proton and electron and may proceed synchronously or asynchronously. To illustrate the role of PCET in biology, this review presents complementary biological and model systems that explore PCET in electron transfer (ET) through hydrogen bonds [azurin as compared to donor-acceptor (D-A) hydrogen-bonded networks], the activation of C-H bonds [alcohol dehydrogenase and soybean lipoxygenase (SLO) as compared to Fe(III) metal complexes], and the generation and transport of amino acid radicals [photosystem II (PSII) and ribonucleotide reductase (RNR) as compared to tyrosine-modified photoactive Re(I) and Ru(II) complexes]. In providing these comparisons, the fundamental principles of PCET in biology are illustrated in a tangible way.

Room temperature photooxidation of beta-carotene and peripheral chlorophyll in photosystem II reaction centre

Differential kinetic absorption spectra were measured during actinic illumination of photosystem II reaction centres and core complexes in the presence of electron acceptors silicomolybdate and ferricyanide. The spectra of samples with ferricyanide differ from those with both ferricyanide and silicomolybdate. Near-infrared spectra show temporary beta-carotene and peripheral chlorophyll oxidation during room temperature actinic illumination. Peripheral chlorophyll is photooxidized even after decay of beta-carotene oxidation activity and significant reduction of beta-carotene content in both reaction centres and photosystem II core complexes. Besides, new carotenoid cation is observed after about 1 s of actinic illumination in the reaction centres when silicomolybdate is present. Similar result was observed in PSII core complexes. HPLC analyses of illuminated reaction centres reveal several novel carotenoids, whereas no new carotenoid species were observed in HPLC of illuminated core complexes. Our data support the proposal that pigments of inner antenna are a sink of cations originating in the photosystem II reaction centre.

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.

Photosystem II: The machinery of photosynthetic water splitting

This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair [Formula: see text] formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with P680+* as driving force and tyrosine Y(Z) as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special Q(B) binding site with Q(A)-* acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: [Formula: see text] where (1)(RC-PC)* denotes the excited singlet state (1)P680* of the reaction centre pigment complex. The structure of the catalytic Mn(4)O(X)Ca cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S(3), are discussed. The unravelling of the mode of O-O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S(3) and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the Q(B) site for PQ reduction to PQH(2) and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.

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.

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.

Crystal structure of the oxygen-evolving complex of photosystem II

The oxygen in our atmosphere is derived from and maintained by the water-splitting process of photosynthesis. The enzyme that facilitates this reaction and therefore underpins virtually all life on our planet is known as photosystem II (PSII). It is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae, and cyanobacteria. Powered by light, PSII catalyzes the chemically and thermodynamically demanding reaction of water splitting. In so doing, it releases molecular oxygen into the atmosphere and provides the reducing equivalents required for the conversion of carbon dioxide into the organic molecules of life. Recently, a fully refined structure of an isolated 700 kDa cyanobacterial dimeric PSII complex was elucidated by X-ray crystallography, which gave organizational and structural details of the 19 subunits (16 intrinsic and 3 extrinsic) that make up each monomer and provided information about the position and protein environments of the many different cofactors it binds. The water-splitting site was revealed as a cluster of four Mn ions and a Ca ion surrounded by amino acid side chains, of which six or seven form direct ligands to the metals. The metal cluster was originally modeled as a cubane-like structure composed of three Mn ions and the Ca (2+) linked by oxo bonds and the fourth Mn attached to the cubane via one of its O atoms. New data from X-ray diffraction and X-ray spectroscopy suggest some alternative arrangements. Nevertheless, all of the models are sufficiently similar to provide a basis for discussing the chemistry by which PSII splits water and makes oxygen.

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).

Primary charge separation in the photosystem II core from Synechocystis: a comparison of femtosecond visible/midinfrared pump-probe spectra of wild-type and two P680 mutants

It is now quite well accepted that charge separation in PS2 reaction centers starts predominantly from the accessory chlorophyll B(A) and not from the special pair P(680). To identify spectral signatures of B(A,) and to further clarify the process of primary charge separation, we compared the femtosecond-infrared pump-probe spectra of the wild-type (WT) PS2 core complex from the cyanobacterium Synechocystis sp. PCC 6803 with those of two mutants in which the histidine residue axially coordinated to P(B) (D2-His(197)) has been changed to Ala or Gln. By analogy with the structure of purple bacterial reaction centers, the mutated histidine is proposed to be indirectly H-bonded to the C(9)=O carbonyl of the putative primary donor B(A) through a water molecule. The constructed mutations are thus expected to perturb the vibrational properties of B(A) by modifying the hydrogen bond strength, possibly by displacing the H-bonded water molecule, and to modify the electronic properties and the charge localization of the oxidized donor P(680)(+). Analysis of steady-state light-induced Fourier transform infrared difference spectra of the WT and the D2-His(197)Ala mutant indeed shows that a modification of the axially coordinating ligand to P(B) induces a charge redistribution of P(680)(+). In addition, a comparison of the time-resolved visible/midinfrared spectra of the WT and mutants has allowed us to investigate the changes in the kinetics of primary charge separation induced by the mutations and to propose a band assignment identifying the characteristic vibrations of B(A).

Site-directed mutations at D1-Thr179 of photosystem II in Synechocystis sp. PCC 6803 modify the spectroscopic properties of the accessory chlorophyll in the D1-branch of the reaction center

D1-Thr179, which overlies the reaction center chlorophyll Chl D1 of Photosystem II was replaced with His and Glu through site-directed mutation in Synechocystis sp. PCC 6803. Spectroscopic characterization of the mutants indicates that, compared to wild type, the main bleaching in the triplet-minus-singlet absorbance difference spectrum and the electrochromic band shift in the (P680 (+)Q A (-)-P680Q A) absorbance difference spectrum are displaced to the red by approximately 2 nm in the D1-Thr179His mutant and to the blue by approximately 1 nm in the D1-Thr179Glu mutant. These difference spectra are compared with the absorbance difference spectra, measured on the same states in the D1-His198Gln mutant in which the axial ligand D1-His198 of the special pair chlorophyll, P D1, was replaced by glutamine. Together, these results give direct evidence that (a) the reaction center triplet state, produced upon charge recombination from (3)[P (+)Pheo (-)], is primarily localized on Chl D1; (b) the cation of the oxidized donor P (+) is predominantly localized on chlorophyll P D1 of the special pair; and (c) the Q Y band of the accessory chlorophyll Chl D1 is electrochromically shifted in response to charges on P (+) and Q A (-). Light-induced absorbance difference spectra (between 650 and 710 nm), associated with the oxidation of secondary donors and the reduction of Q A, exhibit a bleaching attributed to the oxidation of a Chl Z and strong electrochromic band shifts. On the basis of mutation-induced spectroscopic changes and of structure-based calculations, we conclude that the experimental spectra are best explained by a blue-shift of the Q Y band of the accessory chlorophyll Chl D1, arising from charges on Car D2 (+) and Chl ZD2 (+) and on reduced Q A.

Steady-state and transient polarized absorption spectroscopy of photosytem I complexes from the cyanobacteria Arthrospira platensis and Thermosynechococcus elongatus

Core antenna and reaction centre of photosystem I (PS I) complexes from the cyanobacteria Arthrospira platensis and Thermosynechococcus elongatus have been characterized by steady-state polarized absorption spectroscopy, including linear dichroism (LD) and circular dichroism (CD). CD spectra and the second derivatives of measured 77 K CD spectra reveal the spectral components found in the polarized absorption spectra indicating the excitonic origin of the spectral forms of chlorophyll in the PS I complexes. The CD bands at 669-670(+), 673(+), 680(-), 683-685(-), 696-697(-), and 711(-) nm are a common feature of used PSI complexes. The 77 K CD spectra of the trimeric PS I complexes exhibit also low amplitude components around 736 nm for A. platensis and 720 nm for T. elongatus attributed to red-most chlorophylls. The LD measurements indicate that the transition dipole moments of the red-most states are oriented parallel to the membrane plane. The formation of P700(+)A(1)(-) or (3)P700 was monitored by time-resolved difference absorbance and LD spectroscopy to elucidate the spectral properties of the PS I reaction centre. The difference spectra give strong evidence for the delocalization of the excited singlet states in the reaction centre. Therefore, P700 cannot be considered as a dimer but should be regarded as a multimer of the six nearly equally coupled reaction centre chlorophylls in accordance with structure-based calculations. On the basis of the results presented in this work and earlier work in the literature it is concluded that the triplet state is localized most likely on P(A), whereas the cation is localized most likely on P(B).

Both chlorophylls a and d are essential for the photochemistry in photosystem II of the cyanobacteria, Acaryochloris marina

We have measured the flash-induced absorbance difference spectrum attributed to the formation of the secondary radical pair, P(+)Q(-), between 270 nm and 1000 nm at 77 K in photosystem II of the chlorophyll d containing cyanobacterium, Acaryochloris marina. Despite the high level of chlorophyll d present, the flash-induced absorption difference spectrum of an approximately 2 ms decay component shows a number of features which are typical of the difference spectrum seen in oxygenic photosynthetic organisms containing no chlorophyll d. The spectral shape in the near-UV indicates that a plastoquinone is the secondary acceptor molecule (Q(A)). The strong C-550 change at 543 nm confirms previous reports that pheophytin a is the primary electron acceptor. The bleach at 435 nm and increase in absorption at 820 nm indicates that the positive charge is stabilized on a chlorophyll a molecule. In addition a strong electrochromic band shift, centred at 723 nm, has been observed. It is assigned to a shift of the Qy band of the neighbouring accessory chlorophyll d, Chl(D1). It seems highly likely that it accepts excitation energy from the chlorophyll d containing antenna. We therefore propose that primary charge separation is initiated from this chlorophyll d molecule and functions as the primary electron donor. Despite its lower excited state energy (0.1 V less), as compared to chlorophyll a, this chlorophyll d molecule is capable of driving the plastoquinone oxidoreductase activity of photosystem II. However, chlorophyll a is used to stabilize the positive charge and ultimately to drive water oxidation.

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.

Photosystem II: an enzyme of global significance

Photosystem II (PSII) is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Powered by light, this enzyme catalyses the chemically and thermodynamically demanding reaction of water splitting. In so doing, it releases dioxygen into the atmosphere and provides the reducing equivalents required for the conversion of CO2 into the organic molecules of life. Recently, a fully refined structure of a 700 kDa cyanobacterial dimeric PSII complex was elucidated by X-ray crystallography which gave organizational and structural details of the 19 subunits (16 intrinsic and three extrinsic) which make up each monomer and provided information about the position and protein environments of 57 different cofactors. The water-splitting site was revealed as a cluster of four Mn ions and a Ca2+ ion surrounded by amino acid side chains, of which six or seven form direct ligands to the metals. The metal cluster was modelled as a cubane-like structure composed of three Mn ions and the Ca2+ linked by oxo-bonds with the fourth Mn attached to the cubane via one of its oxygens. The overall structure of the catalytic site is providing a framework to develop a mechanistic scheme for the water-splitting process, knowledge which could have significant implications for mimicking the reaction in an artificial chemical system.

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.

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.

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.

Evidence for localisation of accumulated chlorophyll cation on the D1-accessory chlorophyll in the reaction centre of photosystem II

Absorption and circular dichroism spectra of Photosystem II (PS II) reaction centres (RC) were studied and compared with spectra calculated on the basis of point-dipole approximation. Chlorophyll cation was accumulated during a light treatment of PS II RC in the presence of artificial electron acceptor silicomolybdate. Light-induced difference spectra and their calculated counterparts revealed the location of accumulated cation at the accessory chlorophyll of the D1 protein subunit.

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.

Excitonic interactions in wild-type and mutant PSI reaction centers

Femtosecond excitation of the red edge of the chlorophyll a Q(Y) transition band in photosystem I (PSI), with light of wavelength > or = 700 nm, leads to wide transient (subpicosecond) absorbance changes: positive DeltaA between 635 and 665 nm, and four negative DeltaA bands at 667, 675, 683, and 695 nm. Here we compare the transient absorbance changes after excitation at 700, 705, and 710 nm at 20 K in several PSI preparations of Chlamydomonas reinhardtii where amino acid ligands of the primary donor, primary acceptor, or connecting chlorophylls have been mutated. Most of these mutations influence the spectrum of the absorbance changes. This supports the view that the chlorophylls of the electron transfer chain as well as the connecting chlorophylls are engaged in the observed absorbance changes. The wide absorption spectrum of the electron transfer chain revealed by the transient measurements may contribute to the high efficiency of energy trapping in photosystem 1. Exciton calculations, based on the recent PSI structure, allow an assignment of the DeltaA bands to particular chlorophylls: the bands at 675 and 695 nm to the dimers of primary acceptor and accessory chlorophyll and the band at 683 nm to the connecting chlorophylls. The subpicosecond transient absorption bands decay may reflect rapid charge separation in the PSI reaction center.

Water, water everywhere, and its remarkable chemistry

Photosystem II (PSII), the multisubunit pigment-protein complex localised in the thylakoid membranes of oxygenic photosynthetic organisms, uses light energy to drive a series of remarkable reactions leading to the oxidation of water. The products of this oxidation are dioxygen, which is released to the atmosphere, and reducing equivalents destined to reduce carbon dioxide to organic molecules. The water oxidation occurs at catalytic sites composed of four manganese atoms (Mn(4)-cluster) and powered by the redox potential of an oxidised chlorophyll a molecule (P680(*+)). Gerald T (Jerry) Babcock and colleagues showed that electron/proton transfer processes from substrate water to P680(*+) involved a tyrosine residue (Y(Z)) and proposed an attractive reaction mechanism for the direct involvement of Y(Z) in the chemistry of water oxidation. The 'hydrogen-atom abstract/metalloradical' mechanism he formulated is an expression of his genius and a highlight of his many other outstanding contributions to photosynthesis research. A structural basis for Jerry's model is now being revealed by X-ray crystallography.

Architecture of the photosynthetic oxygen-evolving center

Photosynthesis uses light energy to drive the oxidation of water at an oxygen-evolving catalytic site within photosystem II (PSII). We report the structure of PSII of the cyanobacterium Thermosynechococcus elongatus at 3.5 angstrom resolution. We have assigned most of the amino acid residues of this 650-kilodalton dimeric multisubunit complex and refined the structure to reveal its molecular architecture. Consequently, we are able to describe details of the binding sites for cofactors and propose a structure of the oxygen-evolving center (OEC). The data strongly suggest that the OEC contains a cubane-like Mn3CaO4 cluster linked to a fourth Mn by a mono-micro-oxo bridge. The details of the surrounding coordination sphere of the metal cluster and the implications for a possible oxygen-evolving mechanism are discussed.

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.

Engine of life and big bang of evolution: a personal perspective

Photosystem II (PS II) is the engine for essentially all life on our planet and its beginning 2.5 billion years ago was the 'big bang of evolution.' It produces reducing equivalents for making organic compounds on an enormous scale and at the same time provides us with an oxygenic atmosphere and protection against UV radiation (in the form of the ozone layer). In 1967, when I began my career in photosynthesis research, little was known about PS II. The Z-scheme had been formulated [Hill and Bendall (1960) Nature 186: 136-137] and Boardman and Anderson [(1964) Nature 203: 166-167] had isolated PS II as a discrete biochemical entity. PS II was known not only to be the source of oxygen but of variable chlorophyll fluorescence [Duysens and Sweers (1963) In: Studies on Microalgae and Photosynthetic Bacteria, pp. 353-372. University of Tokyo Press, Tokyo] and delayed chlorophyll fluorescence [Arnold and Davidson (1954) J Gen Physiol 37: 677-684]. P680 had just been discovered [Döring et al. (1967) Z Naturforsch 22b: 639-644]. No wonder the 'black box of PS II' was described at that time by Bessel Kok and George Cheniae [Current Topics in Bioenergetics 1: 1-47 (1966)] as the 'inner sanctum of photosynthesis.' What a change in our level of understanding of PS II since then! The contributions of many talented scientists have unraveled the mechanisms and structural basis of PS II function and we are now very close to revealing the molecular details of the remarkable and thermodynamically demanding reaction which it catalyzes, namely the splitting of water into its elemental constituents. It has been a privilege to be involved in this journey.

Steps on the Way to Building Blocks, Topologies, Crystals and X-ray Structural Analysis of Photosystems I and II of Water-oxidizing Photosynthesis

Basic structural elements of the two photosystems and their component electron donors, acceptors, and carriers were revealed by newly developed spectroscopic methods in the 1960s and subsequent years. The spatial organization of these constituents within the functional membrane was elucidated by electrochromic band shift analysis, whereby the membrane-spanning chlorophyll-quinone couple of Photosystem (PS) II emerged as reaction center and as a model relevant also to other photosystems. A further step ahead for improved structural information was realized with the use of thermophilic cyanobacteria instead of plants which led to isolation of supramolecular complexes of the photosystems and their identification as PS I trimers and PS II dimers. The preparation of crystals of the PS I trimer, started in the late 1980s. Genes encoding the 11 subunits of PS I from Synechococcus elongatus were isolated and the predicted sequences of amino acid residues formed a basis for the interpretation of X-ray structure analysis of the PS I crystals. The crystallization of PS I was optimized by introduction of the 'reverse of salting in' crystallization with water as precipitating agent. On this basis the PS I structure was successively established from 6 A resolution in the early 1990s up to a model at 2.5 A resolution in 2001. The first crystals of the PS II dimer, capable of water oxidation, were prepared in the late 1990s; a PS II model at 3.8-3.6 A resolution was presented in 2001. Implications of the PS II structure for the mechanism of transmembrane charge separation are discussed. With the availability of PS I and PS II crystals, new directional structural results became possible also by application of different magnetic resonance techniques through measurements on single crystals in different orientations.

Structure, function and assembly of Photosystem II and its light-harvesting proteins

Photosystem II (PSII) is a multisubunit chlorophyll-protein complex that drives electron transfer from water to plastoquinone using energy derived from light. In green plants, the native form of PSII is surrounded by the light-harvesting complex (LHCII complex) and thus it is called the PSII-LHCII supercomplex. Over the past several years, understanding of the structure, function, and assembly of PSII and LHCII complexes has increased considerably. The unicellular green alga Chlamydomonas reinhardtii has been an excellent model organism to study PSII and LHCII complexes, because this organism grows heterotrophically and photoautotrophically and it is amenable to biochemical, genetic, molecular biological and recombinant DNA methodology. Here, the genes encoding and regulating components of the C. reinhardtii PSII-LHCII supercomplex have been thoroughly catalogued: they include 15 chloroplast and 20 nuclear structural genes as well as 13 nuclear genes coding for regulatory factors. This review discusses these molecular genetic data and presents an overview of the structure, function and assembly of PSII and LHCII complexes.

The Native Reaction Centre of Photosystem II: A New Paradigm for P680

Low-temperature spectra of fully active (oxygen-evolving) Photosystem II (PSII) cores prepared from spinach exhibit well developed structure. Spectra of isolated sub-fragments of PSII cores establish that the native reaction centre is better structured and red-shifted compared to the isolated reaction centre. Laser illumination of PSII cores leads to efficient and deep spectral hole-burning. Measurements of homogeneous hole-widths establish excited-state lifetimes in the 40–300 ps range. The high hole-burning efficiency is attributed to charge separation of P680 in native PSII that follows reaction-centre excitation via ‘slow transfer’ states in the inner light-harvesting assemblies CP43 and CP47. The ‘slow transfer’ state in CP47 and that in CP43 can be distinguished in the hole-burning action spectrum and high-resolution hole-burning spectra. An important observation is that 685–700 nm illumination gives rise to efficient P680 charge separation, as established by QA− formation. This leads to a new paradigm for P680. The charge-separating state has surprisingly weak absorption and extends to 700 nm.