Mechanism of light induced water splitting in Photosystem II of oxygen evolving photosynthetic organisms

The S3 State of the Oxygen-Evolving Complex: Overview of Spectroscopy and XFEL Crystallography with a Critical Evaluation of Early-Onset Models for O–O Bond Formation

The catalytic cycle of the oxygen-evolving complex (OEC) of photosystem II (PSII) comprises five intermediate states Si (i = 0–4), from the most reduced S0 state to the most oxidized S4, which spontaneously evolves dioxygen. The precise geometric and electronic structure of the Si states, and hence the mechanism of O–O bond formation in the OEC, remain under investigation, particularly for the final steps of the catalytic cycle. Recent advances in protein crystallography based on X-ray free-electron lasers (XFELs) have produced new structural models for the S3 state, which indicate that two of the oxygen atoms of the inorganic Mn4CaO6 core of the OEC are in very close proximity. This has been interpreted as possible evidence for “early-onset” O–O bond formation in the S3 state, as opposed to the more widely accepted view that the O–O bond is formed in the final state of the cycle, S4. Peroxo or superoxo formation in S3 has received partial support from computational studies. Here, a brief overview is provided of spectroscopic information, recent crystallographic results, and computational models for the S3 state. Emphasis is placed on computational S3 models that involve O–O formation, which are discussed with respect to their agreement with structural information, experimental evidence from various spectroscopic studies, and substrate exchange kinetics. Despite seemingly better agreement with some of the available crystallographic interpretations for the S3 state, models that implicate early-onset O–O bond formation are hard to reconcile with the complete line of experimental evidence, especially with X-ray absorption, X-ray emission, and magnetic resonance spectroscopic observations. Specifically with respect to quantum chemical studies, the inconclusive energetics for the possible isoforms of S3 is an acute problem that is probably beyond the capabilities of standard density functional theory.

Analyzing both the fast and the slow phases of chlorophyll a fluorescence and P700 absorbance changes in dark-adapted and preilluminated pea leaves using a Thylakoid Membrane model

The dark-to-light transitions enable energization of the thylakoid membrane (TM), which is reflected in fast and slow (OJIPSMT or OABCDE) stages of fluorescence induction (FI) and P700 oxidoreduction changes (ΔA810). A Thylakoid Membrane model (T-M model), in which special emphasis has been placed on ferredoxin-NADP+-oxidoreductase (FNR) activation and energy-dependent qE quenching, was applied for quantifying the kinetics of FI and ΔA810. Pea leaves were kept in darkness for 15 min and then the FI and ΔA810 signals were measured upon actinic illumination, applied either directly or after a 10-s light pulse coupled with a subsequent 10-s dark interval. On the time scale from 40 µs to 30 s, the parallel T-M model fittings to both FI and ΔA810 signals were obtained. The parameters of FNR activation and the buildup of qE quenching were found to differ for dark-adapted and preilluminated leaves. At the onset of actinic light, photosystem II (PSII) acceptors were oxidized (neutral) after dark adaptation, while the redox states with closed and/or semiquinone QA(-)QB(-) forms were supposedly generated after preillumination, and did not relax within the 10 s dark interval. In qE simulations, a pH-dependent Hill relationship was used. The rate constant of heat losses in PSII antenna kD(t) was found to increase from the basic value kDconst, at the onset of illumination, to its maximal level kDvar due to lumenal acidification. In dark-adapted leaves, a low value of kDconst of ∼ 2 × 106 s-1 was found. Simulations on the microsecond to 30 s time scale revealed that the slow P-S-M-T phases of the fluorescence induction were sensitive to light-induced FNR activation and high-energy qE quenching. Thus, the corresponding time-dependent rate constants kD(t) and kFNR(t) change substantially upon the release of electron transport on the acceptor side of PSI and during the NPQ development. The transitions between the cyclic and linear electron transport modes have also been quantified in this paper.

Spin, Valence, and Structural Isomerism in the S3 State of the Oxygen-Evolving Complex of Photosystem II as a Manifestation of Multimetallic Cooperativity

Photosynthetic water oxidation is catalyzed by a Mn₄CaO₅-cluster in photosystem II through an S-state cycle. Understanding the roles of heterogeneity in each S-state, as identified recently by the EPR spectroscopy, is very important to gain a complete description of the catalytic mechanism. We performed herein hybrid DFT calculations within the broken-symmetry formalism and associated analyses of Heisenberg spin models to study the electronic and spin structures of various isomeric structural motifs (hydroxo-oxo, oxyl-oxo, peroxo, and superoxo species) in the S₃ state. Our extensive study reveals several factors that affect the spin ground state: (1) (formal) Mn oxidation state; (2) metal-ligand covalency; (3) coordination geometry; and (4) structural change of the Mn cluster induced by alternations in Mn···Mn distances. Some combination of these effects could selectively stabilize/destabilize some spin states. We found that the high spin state ( S_total = 6) of the oxyl-oxo species can be causative for catalytic function, which manifests through mixing of the metal-ligand character in magnetic orbitals at relatively short O5···O6 distances (<2.0 Å) and long Mn_A···O5 distances (>2.0 Å). These results will serve as a basis to conceptually identify and rationalize the physicochemical synergisms that can be evoked by the unique "distorted chair" topology of the cluster through cooperative Jahn-Teller effects on multimetallic centers.

Earth-Abundant Transition-Metal-Based Electrocatalysts for Water Electrolysis to Produce Renewable Hydrogen

Fundamentals of water electrolysis, and recent research progress and trends in the development of earth-abundant first-row transition-metal (Mn, Fe, Co, Ni, Cu)-based oxygen evolution reaction (OER) and hydrogen evolution (HER) electrocatalysts working in acidic, alkaline, or neutral conditions are reviewed. The HER catalysts include mainly metal chalcogenides, metal phosphides, metal nitrides, and metal carbides. As for the OER catalysts, the basic principles of the OER catalysts in alkaline, acidic, and neutral media are introduced, followed by the review and discussion of the Ni, Co, Fe, Mn, and perovskite-type OER catalysts developed so far. The different design principles of the OER catalysts in photoelectrocatalysis and photocatalysis systems are also presented. Finally, the future research directions of electrocatalysts for water splitting, and coupling of photovoltaic (PV) panel with a water electrolyzer, so called PV-E, are given as perspectives.

Evidence of O-O Bond Formation in the Final Metastable S3 State of Nature's Water Oxidizing Complex Implying a Novel Mechanism of Water Oxidation

A novel mechanism for the final stages of Nature's photosynthetic water oxidation to molecular oxygen is proposed. This is based on a comparison of experimental and broken symmetry density functional theory (BS-DFT) calculated geometries and magnetic resonance properties of water oxidizing complex models in the final metastable oxidation state, S₃. We show that peroxo models of the S₃ state are in vastly superior agreement with the current experimental structural determinations compared with oxo-hydroxo models. Comparison of experimental and BS-DFT calculated ⁵⁵Mn hyperfine couplings for the electron paramagnetic resonance (EPR) visible form shows better agreement for the oxo-hydroxo model. An equilibrium between oxo-hydroxo and peroxo models is proposed for the S₃ state and the major implications for the final steps in the water oxidation mechanism are analyzed and discussed.

Electrogenic reactions in Mn-depleted photosystem II core particles in the presence of synthetic binuclear Mn complexes

An electrometrical technique was used to investigate electron transfer between synthetic binuclear manganese (Mn) complexes, designated M - 2 and M - 3, and the redox-active neutral tyrosine radical (Y_Z^•) in proteoliposomes containing Mn-depleted photosystem II (PS II) core particles in response to single laser flashes. In the absence of Mn-containing compounds, the observed flash-induced membrane potential (ΔΨ) decay was mainly due to charge recombination between the reduced primary quinone acceptor Q_A^- and the oxidized Y_Z^•. More significant slowing down of the ΔΨ decay in the presence of lower concentrations of M - 2 and M - 3 associated with electron donation from Mn in the Mn-binding site to Y_Z^• indicates that these synthetic compounds are more effective electron donors than MnCl₂. The exponential fitting of the kinetics of additional electrogenic components of ΔΨ rise in the presence of Mn-containing compounds revealed the following relative amplitudes (A) and lifetimes (τ): for MnCl₂ - A∼ 3.5, τ∼150 μs, for M - 2 - A∼5%, τ∼1.4 ms, and for M - 3 - A∼5.5%, τ∼150 μs. This suggests that the efficiency of the manganese complexes in electron donation depends on the chemical nature of ligands. The experiments with EDTA-treated samples indicated that the ligands for M - 2 and M - 3 are required for their tight binding with the PS II reaction center. The obtained results demonstrate the importance of understanding the molecular mechanism(s) of flash-induced electrogenic reduction of the tyrosine radical Y_Z^• by synthetic Mn complexes capable of splitting water into oxygen and reducing equivalents.

High-Oxidation-State 3d Metal (Ti-Cu) Complexes with N-Heterocyclic Carbene Ligation

High-oxidation-state 3d metal species have found a wide range of applications in modern synthetic chemistry and materials science. They are also implicated as key reactive species in biological reactions. These applications have thus prompted explorations of their formation, structure, and properties. While the traditional wisdom regarding these species was gained mainly from complexes supported by nitrogen- and oxygen-donor ligands, recent studies with N-heterocyclic carbenes (NHCs), which are widely used for the preparation of low-oxidation-state transition metal complexes in organometallic chemistry, have led to the preparation of a large variety of isolable high-oxidation-state 3d metal complexes with NHC ligation. Since the first report in this area in the 1990s, isolable complexes of this type have been reported for titanium(IV), vanadium(IV,V), chromium(IV,V), manganese(IV,V), iron(III,IV,V), cobalt(III,IV,V), nickel(IV), and copper(II). With the aim of providing an overview of this intriguing field, this Review summarizes our current understanding of the synthetic methods, structure and spectroscopic features, reactivity, and catalytic applications of high-oxidation-state 3d metal NHC complexes of titanium to copper. In addition to this progress, factors affecting the stability and reactivity of high-oxidation-state 3d metal NHC species are also presented, as well as perspectives on future efforts.

Model of the Oxygen Evolving Complex Which Is Highly Predisposed to O-O Bond Formation

Light-driven water oxidation is a fundamental reaction in the biosphere. The Mn₄Ca cluster of photosystem II cycles through five redox states termed S₀-S₄, after which oxygen is evolved. Critically, the timing of O-O bond formation within the Kok cycle remains unknown. By combining recent crystallographic, spectroscopic, and DFT results, we demonstrate an atomistic S₃ state model with the possibility of a low barrier to O-O bond formation prior to the final oxidation step. Furthermore, the associated one electron oxidized S₄ state does not provide more advantages in terms of spin alignment or the energy of O-O bond formation. We propose that a high energy peroxide isoform of the S₃ state can preferentially be oxidized by Tyr _z^ox in the course of final electron transfer leading to O₂ evolution. Such a mechanism may explain the peculiar kinetic behavior of O₂ evolution as well as serve as an evolutionary adaptation to avoid release of the harmful peroxides.

Formation of tyrosine radicals in photosystem II under far-red illumination

Photosystem II (PS II) contains two redox-active tyrosine residues on the donor side at symmetrical positions to the primary donor, P680. TyrZ, part of the water-oxidizing complex, is a preferential fast electron donor while TyrD is a slow auxiliary donor to P680+. We used PS II membranes from spinach which were depleted of the water oxidation complex (Mn-depleted PS II) to study electron donation from both tyrosines by time-resolved EPR spectroscopy under visible and far-red continuous light and laser flash illumination. Our results show that under both illumination regimes, oxidation of TyrD occurs via equilibrium with TyrZ• at pH 4.7 and 6.3. At pH 8.5 direct TyrD oxidation by P680+ occurs in the majority of the PS II centers. Under continuous far-red light illumination these reactions were less effective but still possible. Different photochemical steps were considered to explain the far-red light-induced electron donation from tyrosines and localization of the primary electron hole (P680+) on the ChlD1 in Mn-depleted PS II after the far-red light-induced charge separation at room temperature is suggested.

O2 evolution and recovery of the water-oxidizing enzyme

In photosystem II, light-induced water oxidation occurs at the Mn₄CaO₅ cluster. Here we demonstrate proton releases, dioxygen formation, and substrate water incorporation in response to Mn₄CaO₅ oxidation in the protein environment, using a quantum mechanical/molecular mechanical approach and molecular dynamics simulations. In S₂, H₂O at the W1 site forms a low-barrier H-bond with D1-Asp61. In the S₂-to-S₃ transition, oxidation of O_W1H^– to O_W1^•–, concerted proton transfer from O_W1H^– to D1-Asp61, and binding of a water molecule W_n-W1 at O_W1^•– are observed. In S₄, W_n-W1 facilitates oxo-oxyl radical coupling between O_W1^•– and corner μ-oxo O4. Deprotonation via D1-Asp61 leads to formation of O_W1=O4. As O_W1=O4 moves away from Mn, H₂O at W539 is incorporated into the vacant O4 site of the O₂-evolved Mn₄CaO₄ cluster, forming a μ-oxo bridge (Mn3–O_W539–Mn4) in an exergonic process. Simultaneously, W_n-W1 is incorporated as W1, recovering the Mn₄CaO₅ cluster.

Water splitting during photosynthesis results in the combination of two oxygen atoms to form O₂. Here, based on computational simulations, the authors develop a possible mechanism for this reaction, which is different from the mechanisms previous studies have suggested.

A Comparison of Experimental and Broken Symmetry Density Functional Theory (BS-DFT) Calculated Electron Paramagnetic Resonance (EPR) Parameters for Intermediates Involved in the S2 to S3 State Transition of Nature's Oxygen Evolving Complex

A broken symmetry density functional theory (BS-DFT) magnetic analysis of the S₂, S₂Y_Z^•, and S₃ states of Nature's oxygen evolving complex is performed for both the native Ca and Sr substituted forms. Good agreement with experiment is observed between the tyrosyl calculated g-tensor and ¹H hyperfine couplings for the native Ca form. Changes in the hydrogen bonding environment of the tyrosyl radical in S₂Y_Z^• caused by Sr substitution lead to notable changes in the calculated g-tensor of the tyrosyl radical. Comparison of calculated and experimental ⁵⁵Mn hyperfine couplings for the S₃ state presently favors an open cubane form of the complex with an additional OH ligand coordinating to Mn_D. In Ca models, this additional ligation can arise by closed-cubane form deprotonation of the Ca ligand W3 in the S₂Y_Z^• state accompanied by spontaneous movement to the vacant Mn coordination site or by addition of an external OH group. For the Sr form, no spontaneous movement of W3 to the vacant Mn coordination site is observed in contrast to the native Ca form, a difference which may lead to the reduced catalytic activity of the Sr substituted form. BS-DFT studies on peroxo models of S₃ as indicated by a recent X-ray free electron laser (XFEL) crystallography study give rise to a structural model compatible with experimental data and an S = 3 ground state compatible with EPR studies.

Links between peptides and Mn oxide: nano-sized manganese oxide embedded in a peptide matrix

We report on a poly-peptide/Mn oxide nanocomposite as a model for the water-oxidizing catalyst in Photosystem II.

Natural and Artificial Mn4 Ca Cluster for the Water Splitting Reaction

The oxygen-evolving center (OEC) in photosystem II (PSII) is a unique biological catalyst that splits water into electrons, protons, and O₂ by using solar energy. Recent crystallographic studies have revealed that the structure of the OEC is an asymmetric Mn₄ Ca cluster, which provides a blueprint to develop man-made water-splitting catalysts for artificial photosynthesis. Although it is a great challenge to mimic the whole structure and function of the OEC in the laboratory, significant advances have recently been achieved. In this Minireview, recent progress on mimicking the natural OEC is discussed. New strategies are suggested to construct more stable and efficient new generation of catalytic materials for the water splitting reaction based on the artificial Mn₄ Ca cluster in the future.

D1-Asn-298 in photosystem II is involved in a hydrogen-bond network near the redox-active tyrosine YZ for proton exit during water oxidation

In photosynthetic water oxidation, two water molecules are converted into one oxygen molecule and four protons at the Mn₄CaO₅ cluster in photosystem II (PSII) via the S-state cycle. Efficient proton exit from the catalytic site to the lumen is essential for this process. However, the exit pathways of individual protons through the PSII proteins remain to be identified. In this study, we examined the involvement of a hydrogen-bond network near the redox-active tyrosine Y_Z in proton transfer during the S-state cycle. We focused on spectroscopic analyses of a site-directed variant of D1-Asn-298, a residue involved in a hydrogen-bond network near Y_Z We found that the D1-N298A mutant of Synechocystis sp. PCC 6803 exhibits an O₂ evolution activity of ∼10% of the wild-type. D1-N298A and the wild-type D1 had very similar features of thermoluminescence glow curves and of an FTIR difference spectrum upon Y_Z oxidation, suggesting that the hydrogen-bonded structure of Y_Z and electron transfer from the Mn₄CaO₅ cluster to Y_Z were little affected by substitution. In the D1-N298A mutant, however, the flash-number dependence of delayed luminescence showed a monotonic increase without oscillation, and FTIR difference spectra of the S-state cycle indicated partial and significant inhibition of the S₂ → S₃ and S₃ → S₀ transitions, respectively. These results suggest that the D1-N298A substitution inhibits the proton transfer processes in the S₂ → S₃ and S₃ → S₀ transitions. This in turn indicates that the hydrogen-bond network near Y_Z can be functional as a proton transfer pathway during photosynthetic water oxidation.

Large-scale QM/MM calculations of the CaMn4O5 cluster in the S3 state of the oxygen evolving complex of photosystem II. Comparison between water-inserted and no water-inserted structures

Large-scale QM/MM calculations were performed to elucidate an optimized geometrical structure of a CaMn₄O₅ cluster with and without water insertion in the S₃ state of the oxygen evolving complex (OEC) of photosystem II (PSII). The left (L)-opened structure was found to be stable under the assumption of no hydroxide anion insertion in the S₃ state, whereas the right (R)-opened structure became more stable if one water molecule is inserted to the Mn₄Ca cluster. The optimized Mn_a(4)–Mn_d(1) distance determined by QM/MM was about 5.0 Å for the S₃ structure without an inserted hydroxide anion, but this is elongated by 0.2–0.3 Å after insertion. These computational results are discussed in relation to the possible mechanisms of O–O bond formation in water oxidation by the OEC of PSII.

Thylakoid membrane model of the Chl a fluorescence transient and P700 induction kinetics in plant leaves

A new Thylakoid model is presented, which describes in detail the electron/proton transfer reactions between membrane protein complexes including photosystems II and I (PSII, PSI), cytochrome (Cyt) b 6 f, mobile plastoquinone PQ pool in the thylakoid membrane, plastocyanin in lumen and ferredoxin in stroma, reduction of NADP via FNR and cyclic electron transfer. The Thylakoid model parameters were fitted both to Chl fluorescence induction data (FI) and oxido-reductions of P700 (ΔA 810) measured from 20 μs up to 20 s in pea leaves. The two-wave kinetics of FI and ΔA 810 (O(JI)PSM and OABCDE) were described quantitatively, provided that the values of membrane electrochemical potential components ΔΨ(t), pHL(t)/pHS(t) are in physiologically relevant ranges. The time courses on the time scale from nanoseconds to tens of seconds of oxido-reduction changes of ET components as well as concentrations of proton/ions (K+, Cl-) were calculated. We assume a low constant FNR activity over this period. Charge movements across the thylakoid membrane by passive leakage and active ATPase transport and proton buffer reactions are simulated. The dynamics of charge fluxes during photosynthetic induction under low light (PFD 200 μmol photons m-2 s-1) were analyzed. The initial wave of P700 oxidation within 20 ms during independent operation of PSI and PSII was followed after 50 ms by PSI donor-side reduction from reduced PQ pool via Cyt b 6 f site. The Cyt b 6 f reactions contribute to the stabilization of fluxes in the time range 1 s < t < 10 s. The detailed analysis of Chl a fluorescence at the PSM stage (t > 10 s) would need the investigation of FNR activation effect in order to explain the transitions between cyclic and linear electron transport.

Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells

The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO₂ by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO₂ reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO₂ reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.

Gernot Renger (1937-2013): his life, Max-Volmer Laboratory, and photosynthesis research

Gernot Renger (October 23, 1937-January 12, 2013), one of the leading biophysicists in the field of photosynthesis research, studied and worked at the Max-Volmer-Institute (MVI) of the Technische Universität Berlin, Germany, for more than 50 years, and thus witnessed the rise and decline of photosynthesis research at this institute, which at its prime was one of the leading centers in this field. We present a tribute to Gernot Renger's work and life in the context of the history of photosynthesis research of that period, with special focus on the MVI. Gernot will be remembered for his thought-provoking questions and his boundless enthusiasm for science.

Synthesis and characterization of chlorophyll a enol derivatives: Chlorophyll a tert-butyldimethylsilyl-enol ether and 131-deoxo-131, 132-didehydro-chlorophyll a

Using the sterically hindered base, 1,8-diazabicyclo[5.4.0]undec-7-ene, for enolization and tert-butyldimethylsilyl-trifluoromethanesulfonate for silylation, chlorophyll (Chl) [Formula: see text] produced after 15 min at 0 [Formula: see text]C in deaerated pyridine solution under argon, after work-up and chromatographic purification on a sucrose column, tert-butyldimethylsilyl-enol ether of Chl [Formula: see text] in a yield of 77%. The 131-deoxo-131,132-didehydro-chlorophyll [Formula: see text], was obtained in a yield of 23% through a reaction sequence, where Chl [Formula: see text] was first reduced with sodium borohydride to 13[Formula: see text]-hydroxy-Chl [Formula: see text], which via demetalation yielded 13[Formula: see text]-hydroxypheophytin [Formula: see text]. In the presence of the sterically hindered base, 1,8-bis(dimethylamino)naphthalene, trifluoroacetylimidazole dehydrated 13[Formula: see text]-hydroxypheophytin [Formula: see text] to 131-deoxo-131,132-didehydro-pheophytin [Formula: see text], which after metalation yielded 131-deoxo-131,132-didehydro-Chl [Formula: see text]. Using 1,8-bis(dimethylamino)naphthalene and trifluoroacetylimidazole, the straight conversion of 13[Formula: see text]-hydroxy-Chl [Formula: see text] to 131-deoxo-131,132-didehydro-Chl [Formula: see text] was found unsuccessful. The major products were characterized by electronic absorption spectra (UV-vis) and practically completely assigned 1H and [Formula: see text]C NMR spectra. Some intermediates of the syntheses were also characterized by ESI-TOF mass spectra. Compared with Chl [Formula: see text], the macrocyclic ring-current in the synthesized Chl [Formula: see text] enol derivatives was found weakened by the expansion of the [Formula: see text]-system to include the isocyclic ring E. Nevertheless, these enol derivatives were still considered to be diamagnetic and aromatic. The possibility of the functional role of the enol derivatives of chlorophyll in photosynthesis is discussed.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Systems approach to excitation-energy and electron transfer reaction networks in photosystem II complex: model studies for chlorophyll a fluorescence induction kinetics

Photosystem II (PS II) is a protein complex which evolves oxygen and drives charge separation for photosynthesis employing electron and excitation-energy transfer processes over a wide timescale range from picoseconds to milliseconds. While the fluorescence emitted by the antenna pigments of this complex is known as an important indicator of the activity of photosynthesis, its interpretation was difficult because of the complexity of PS II. In this study, an extensive kinetic model which describes the complex and multi-timescale characteristics of PS II is analyzed through the use of the hierarchical coarse-graining method proposed in the authors׳ earlier work. In this coarse-grained analysis, the reaction center (RC) is described by two states, open and closed RCs, both of which consist of oxidized and neutral special pairs being in quasi-equilibrium states. Besides, the PS II model at millisecond scale with three-state RC, which was studied previously, could be derived by suitably adjusting the kinetic parameters of electron transfer between tyrosine and RC. Our novel coarse-grained model of PS II can appropriately explain the light-intensity dependent change of the characteristic patterns of fluorescence induction kinetics from O-J-I-P, which shows two inflection points, J and I, between initial point O and peak point P, to O-J-D-I-P, which shows a dip D between J and I inflection points.

Modeling of the redox state dynamics in photosystem II of Chlorella pyrenoidosa Chick cells and leaves of spinach and Arabidopsis thaliana from single flash-induced fluorescence quantum yield changes on the 100 ns-10 s time scale

The time courses of the photosystem II (PSII) redox states were analyzed with a model scheme supposing a fraction of 11-25 % semiquinone (with reduced [Formula: see text]) RCs in the dark. Patterns of single flash-induced transient fluorescence yield (SFITFY) measured for leaves (spinach and Arabidopsis (A.) thaliana) and the thermophilic alga Chlorella (C.) pyrenoidosa Chick (Steffen et al. Biochemistry 44:3123-3132, 2005; Belyaeva et al. Photosynth Res 98:105-119, 2008, Plant Physiol Biochem 77:49-59, 2014) were fitted with the PSII model. The simulations show that at high-light conditions the flash generated triplet carotenoid (3)Car(t) population is the main NPQ regulator decaying in the time interval of 6-8 μs. So the SFITFY increase up to the maximum level [Formula: see text]/F 0 (at ~50 μs) depends mainly on the flash energy. Transient electron redistributions on the RC redox cofactors were displayed to explain the SFITFY measured by weak light pulses during the PSII relaxation by electron transfer (ET) steps and coupled proton transfer on both the donor and the acceptor side of the PSII. The contribution of non-radiative charge recombination was taken into account. Analytical expressions for the laser flash, the (3)Car(t) decay and the work of the water-oxidizing complex (WOC) were used to improve the modeled P680(+) reduction by YZ in the state S 1 of the WOC. All parameter values were compared between spinach, A. thaliana leaves and C. pyrenoidosa alga cells and at different laser flash energies. ET from [Formula: see text] slower in alga as compared to leaf samples was elucidated by the dynamics of [Formula: see text] fractions to fit SFITFY data. Low membrane energization after the 10 ns single turnover flash was modeled: the ∆Ψ(t) amplitude (20 mV) is found to be about 5-fold smaller than under the continuous light induction; the time-independent lumen pHL, stroma pHS are fitted close to dark estimates. Depending on the flash energy used at 1.4, 4, 100 % the pHS in stroma is fitted to 7.3, 7.4, and 7.7, respectively. The biggest ∆pH difference between stroma and lumen was found to be 1.2, thus pH- dependent NPQ was not considered.

Correlation between pH dependence of O2 evolution and sensitivity of Mn cations in the oxygen-evolving complex to exogenous reductants

Effects of pH, Ca(2+), and Cl(-) ions on the extraction of Mn cations from oxygen-evolving complex (OEC) in Ca-depleted photosystem II (PSII(-Ca)) by exogenous reductants hydroquinone (H2Q) and H2O2 were studied. Two of 4 Mn cations are released by H2Q and H2O2 at pHs 5.7, 6.5, and 7.5, and their extraction does not depend on the presence of Ca(2+) and Cl(-) ions. One of Mn cations ("resistant" Mn cation) cannot be extracted by H2Q and H2O2 at any pH. Extraction of 4th Mn ion ("flexible" Mn cation) is sensitive to pH, Ca(2+), and Cl(-). This Mn cation is released by reductants at pH 6.5 but not at pHs 5.7 and 7.5. A pH dependence curve of the oxygen-evolving activity in PSII(-Ca) membranes (in the presence of exogenous Ca(2+)) has a bell-shaped form with the maximum at pH 6.5. Thus, the increase in the resistance of flexible Mn cation in OEC to the action of reductants at acidic and alkaline pHs coincides with the decrease in oxygen evolution activity at these pHs. Exogenous Ca(2+) protects the extraction of flexible Mn cation at pH 6.5. High concentration of Cl(-) anions (100 mM) shifts the pH optimum of oxygen evolution to alkaline region (around pH 7.5), while the pH of flexible Mn extraction is also shifted to alkaline pH. This result suggests that flexible Mn cation plays a key role in the water-splitting reaction. The obtained results also demonstrate that only one Mn cation in Mn4 cluster is under strong control of calcium. The change in the flexible Mn cation resistance to exogenous reductants in the presence of Ca(2+) suggests that Ca(2+) can control the redox potential of this cation.

Crystallization of Photosystem II for Time-Resolved Structural Studies Using an X-ray Free Electron Laser

Photosystem II (PSII) is a membrane protein supercomplex that executes the initial reaction of photosynthesis in higher plants, algae, and cyanobacteria. It captures the light from the sun to catalyze a transmembrane charge separation. In a series of four charge separation events, utilizing the energy from four photons, PSII oxidizes two water molecules to obtain dioxygen, four protons, and four electrons. The light reactions of photosystems I and II (PSI and PSII) result in the formation of an electrochemical transmembrane proton gradient that is used for the production of ATP. Electrons that are subsequently transferred from PSI via the soluble protein ferredoxin to ferredoxin-NADP(+) reductase that reduces NADP(+) to NADPH. The products of photosynthesis and the elemental oxygen evolved sustain all higher life on Earth. All oxygen in the atmosphere is produced by the oxygen-evolving complex in PSII, a process that changed our planet from an anoxygenic to an oxygenic atmosphere 2.5 billion years ago. In this chapter, we provide recent insight into the mechanisms of this process and methods used in probing this question.

Microcrystallization techniques for serial femtosecond crystallography using photosystem II from Thermosynechococcus elongatus as a model system

Serial femtosecond crystallography (SFX) is a new emerging method, where X-ray diffraction data are collected from a fully hydrated stream of nano- or microcrystals of biomolecules in their mother liquor using high-energy, X-ray free-electron lasers. The success of SFX experiments strongly depends on the ability to grow large amounts of well-ordered nano/microcrystals of homogeneous size distribution. While methods to grow large single crystals have been extensively explored in the past, method developments to grow nano/microcrystals in sufficient amounts for SFX experiments are still in their infancy. Here, we describe and compare three methods (batch, free interface diffusion (FID) and FID centrifugation) for growth of nano/microcrystals for time-resolved SFX experiments using the large membrane protein complex photosystem II as a model system.

FTIR studies of metal ligands, networks of hydrogen bonds, and water molecules near the active site Mn₄CaO₅ cluster in Photosystem II

The photosynthetic conversion of water to molecular oxygen is catalyzed by the Mn₄CaO₅ cluster in Photosystem II and provides nearly our entire supply of atmospheric oxygen. The Mn₄CaO₅ cluster accumulates oxidizing equivalents in response to light-driven photochemical events within Photosystem II and then oxidizes two molecules of water to oxygen. The Mn₄CaO₅ cluster converts water to oxygen much more efficiently than any synthetic catalyst because its protein environment carefully controls the cluster's reactivity at each step in its catalytic cycle. This control is achieved by precise choreography of the proton and electron transfer reactions associated with water oxidation and by careful management of substrate (water) access and proton egress. This review describes the FTIR studies undertaken over the past two decades to identify the amino acid residues that are responsible for this control and to determine the role of each. In particular, this review describes the FTIR studies undertaken to characterize the influence of the cluster's metal ligands on its activity, to delineate the proton egress pathways that link the Mn₄CaO₅ cluster with the thylakoid lumen, and to characterize the influence of specific residues on the water molecules that serve as substrate or as participants in the networks of hydrogen bonds that make up the water access and proton egress pathways. This information will improve our understanding of water oxidation by the Mn₄CaO₅ catalyst in Photosystem II and will provide insight into the design of new generations of synthetic catalysts that convert sunlight into useful forms of storable energy. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Serial time-resolved crystallography of photosystem II using a femtosecond X-ray laser

Photosynthesis, a process catalysed by plants, algae and cyanobacteria converts sunlight to energy thus sustaining all higher life on Earth. Two large membrane protein complexes, photosystem I and II (PSI and PSII), act in series to catalyse the light-driven reactions in photosynthesis. PSII catalyses the light-driven water splitting process, which maintains the Earth's oxygenic atmosphere. In this process, the oxygen-evolving complex (OEC) of PSII cycles through five states, S0 to S4, in which four electrons are sequentially extracted from the OEC in four light-driven charge-separation events. Here we describe time resolved experiments on PSII nano/microcrystals from Thermosynechococcus elongatus performed with the recently developed technique of serial femtosecond crystallography. Structures have been determined from PSII in the dark S1 state and after double laser excitation (putative S3 state) at 5 and 5.5 Å resolution, respectively. The results provide evidence that PSII undergoes significant conformational changes at the electron acceptor side and at the Mn4CaO5 core of the OEC. These include an elongation of the metal cluster, accompanied by changes in the protein environment, which could allow for binding of the second substrate water molecule between the more distant protruding Mn (referred to as the 'dangler' Mn) and the Mn3CaOx cubane in the S2 to S3 transition, as predicted by spectroscopic and computational studies. This work shows the great potential for time-resolved serial femtosecond crystallography for investigation of catalytic processes in biomolecules.

Fourier transform infrared difference and time-resolved infrared detection of the electron and proton transfer dynamics in photosynthetic water oxidation

Photosynthetic water oxidation, which provides the electrons necessary for CO₂ reduction and releases O₂ and protons, is performed at the Mn₄CaO₅ cluster in photosystem II (PSII). In this review, studies that assessed the mechanism of water oxidation using infrared spectroscopy are summarized focusing on electron and proton transfer dynamics. Structural changes in proteins and water molecules between intermediates known as Si states (i=0-3) were detected using flash-induced Fourier transform infrared (FTIR) difference spectroscopy. Electron flow in PSII and proton release from substrate water were monitored using the infrared changes in ferricyanide as an exogenous electron acceptor and Mes buffer as a proton acceptor. Time-resolved infrared (TRIR) spectroscopy provided information on the dynamics of proton-coupled electron transfer during the S-state transitions. In particular, a drastic proton movement during the lag phase (~200μs) before electron transfer in the S3→S0 transition was detected directly by monitoring the infrared absorption of a polarizable proton in a hydrogen bond network. Furthermore, the proton release pathways in the PSII proteins were analyzed by FTIR difference measurements in combination with site-directed mutagenesis, isotopic substitutions, and quantum chemical calculations. Therefore, infrared spectroscopy is a powerful tool for understanding the molecular mechanism of photosynthetic water oxidation. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Fourier transform infrared detection of a polarizable proton trapped between photooxidized tyrosine YZ and a coupled histidine in photosystem II: relevance to the proton transfer mechanism of water oxidation

The redox-active tyrosine YZ (D1-Tyr161) in photosystem II (PSII) functions as an immediate electron acceptor of the Mn4Ca cluster, which is the catalytic center of photosynthetic water oxidation. YZ is also located in the hydrogen bond network that connects the Mn4Ca cluster to the lumen and hence is possibly related to the proton transfer process during water oxidation. To understand the role of YZ in the water oxidation mechanism, we have studied the hydrogen bonding interactions of YZ and its photooxidized neutral radical YZ(•) together with the interaction of the coupled His residue, D1-His190, using light-induced Fourier transform infrared (FTIR) difference spectroscopy. The YZ(•)-minus-YZ FTIR difference spectrum of Mn-depleted PSII core complexes exhibited a broad positive feature around 2800 cm(-1), which was absent in the corresponding spectrum of another redox-active tyrosine YD (D2-Tyr160). Analyses by (15)N and H/D substitutions, examination of the pH dependence, and density functional theory and quantum mechanics/molecular mechanics (QM/MM) calculations showed that this band arises from the N-H stretching vibration of the protonated cation of D1-His190 forming a charge-assisted strong hydrogen bond with YZ(•). This result provides strong evidence that the proton released from YZ upon its oxidation is trapped in D1-His190 and a positive charge remains on this His. The broad feature of the ~2800 cm(-1) band reflects a large proton polarizability in the hydrogen bond between YZ(•) and HisH(+). QM/MM calculations further showed that upon YZ oxidation the hydrogen bond network is rearranged and one water molecule moves toward D1-His190. From these data, a novel proton transfer mechanism via YZ(•)-HisH(+) is proposed, in which hopping of the polarizable proton of HisH(+) to this water triggers the transfer of the proton from substrate water to the luminal side. This proton transfer mechanism could be functional in the S2 → S3 transition, which requires proton release before electron transfer because of an excess positive charge on the Mn4Ca cluster.