Time-resolved vibrational spectroscopy detects protein-based intermediates in the photosynthetic oxygen-evolving cycle

In memoriam of Professor Bridgette Barry (March 1, 1957-January 20, 2021)

We remember our colleague and friend, Bridgette Barry. Bridgette's numerous seminal contributions to photosynthesis research, beginning with her discovery that two functionally differentiated, redox active tyrosines are involved with the action of the oxygen-evolving complex, continue to impact studies of Photosystem II in laboratories around the world. Dr. Barry was widely recognized for employing clever adaptations of spectroscopic tools to probe unique experimental systems. Taken as a whole, she developed an in depth understanding of how biological proton coupled electron transfer reactions are subject to exquisite control over direction and kinetics. Bridgette dedicated significant time and energy in service to the scientific community. She was committed to her lab members and cared deeply that they each achieve their goals. She was an important role model and promoted women in her department and beyond. Bridgette will be deeply missed by the international scientific community and by those of us who had the honor to know her.

Detection of Catalytically Linked Conformational Changes in Wild-Type Class Ia Ribonucleotide Reductase Using Reaction-Induced FTIR Spectroscopy

The enzyme, ribonucleotide reductase (RNR), is essential for DNA synthesis in all cells. The class Ia Escherichia coli RNR consists of two dimeric subunits, α₂ and β₂, which form an active but unstable heterodimer of dimers, α₂β₂. The structure of the wild-type form of the enzyme has been challenging to study due to the instability of the catalytic complex. A long-range proton-coupled electron-transfer (PCET) pathway facilitates radical migration from the Y122 radical-diiron cofactor in the β subunit to an active site cysteine, C439, in the α subunit to initiate the RNR chemistry. The PCET reactions and active site chemistry are spectroscopically masked by a rate-limiting, conformational gate. Here, we present a reaction-induced Fourier transform infrared (RIFTIR) spectroscopic method to monitor the mechanism of the active, wild-type RNR α₂β₂ complex. This method is employed to obtain new information about conformational changes accompanying RNR catalysis, including the role of carboxylate interactions, deprotonation, and oxidation of active site cysteines, and a detailed description of reversible secondary structural changes. Labeling of tyrosine revealed a conformationally active tyrosine in the β subunit, assigned to Y356β, which is part of the intersubunit PCET pathway. New insights into the roles of the inhibitors, azidoUDP and dATP, and the sensitivity of RIFTIR spectroscopy to detect subtle conformational motions arising from protein allostery are also presented.

Energetics and Kinetics of S-State Transitions Monitored by Delayed Chlorophyll Fluorescence

Understanding energetic and kinetic parameters of intermediates formed in the course of the reaction cycle (S-state cycle) of photosynthetic water oxidation is of high interest and could support the rationale designs of artificial systems for solar fuels. We use time-resolved measurements of the delayed chlorophyll fluorescence to estimate rate constants, activation energies, free energy differences, and to discriminate between the enthalpic and the entropic contributions to the decrease of the Gibbs free energy of the individual transitions. Using a joint-fit simulation approach, kinetic parameters are determined for the reaction intermediates in the S-state transitions in buffers with different pH in H₂O and in D₂O.

Tracking Reactive Water and Hydrogen-Bonding Networks in Photosynthetic Oxygen Evolution

In oxygenic photosynthesis, photosystem II (PSII) converts water to molecular oxygen through four photodriven oxidation events at a Mn₄CaO₅ cluster. A tyrosine, YZ (Y161 in the D1 polypeptide), transfers oxidizing equivalents from an oxidized, primary chlorophyll donor to the metal center. Calcium or its analogue, strontium, is required for activity. The Mn₄CaO₅ cluster and YZ are predicted to be hydrogen bonded in a water-containing network, which involves amide carbonyl groups, amino acid side chains, and water. This hydrogen-bonded network includes amino acid residues in intrinsic and extrinsic subunits. One of the extrinsic subunits, PsbO, is intrinsically disordered. This extensive (35 Å) network may be essential in facilitating proton release from substrate water. While it is known that some proteins employ internal water molecules to catalyze reactions, there are relatively few methods that can be used to study the role of water. In this Account, we review spectroscopic evidence from our group supporting the conclusion that the PSII hydrogen-bonding network is dynamic and that water in the network plays a direct role in catalysis. Two approaches, transient electron paramagnetic resonance (EPR) and reaction-induced FT-IR (RIFT-IR) spectroscopies, were used. The EPR experiments focused on the decay kinetics of YZ• via recombination at 190 K and the solvent isotope, pH, and calcium dependence of these kinetics. The RIFT-IR experiments focused on shifts in amide carbonyl frequencies, induced by photo-oxidation of the metal cluster, and on the isotope-based assignment of bands to internal, small protonated water clusters at 190, 263, and 283 K. To conduct these experiments, PSII was prepared in selected steps along the catalytic pathway, the S_n state cycle (n = 0-4). This cycle ultimately generates oxygen. In the EPR studies, S-state dependent changes were observed in the YZ• lifetime and in its solvent isotope effect. The YZ• lifetime depended on the presence of calcium at pH 7.5, but not at pH 6.0, suggesting a two-donor model for PCET. At pH 6.0 or 7.5, barium and ammonia both slowed the rate of YZ• recombination, consistent with disruption of the hydrogen-bonding network. In the RIFT-IR studies of the S state transitions, infrared bands associated with the transient protonation and deprotonation of internal waters were identified by D₂O and H₂¹⁸O labeling. The infrared bands of these protonated water clusters, W_n⁺ (or nH₂O(H₃O)⁺, n = 5-6), exhibited flash dependence and were produced during the S₁ to S₂ and S₃ to S₀ transitions. Calcium dependence was observed at pH 7.5, but not at pH 6.0. S-state induced shifts were observed in amide C═O frequencies during the S₁ to S₂ transition and attributed to alterations in hydrogen bonding, based on ammonia sensitivity. In addition, isotope editing of the extrinsic subunit, PsbO, established that amide vibrational bands of this lumenal subunit respond to the S state transitions and that PsbO is a structural template for the reaction center. Taken together, these spectroscopic results support the hypothesis that proton transfer networks, extending from YZ to PsbO, play a functional and dynamic role in photosynthetic oxygen evolution.

Time-resolved infrared spectroscopy in the study of photosynthetic systems

Time-resolved (TR) infrared (IR) spectroscopy in the nanosecond to second timescale has been extensively used, in the last 30 years, in the study of photosynthetic systems. Interesting results have also been obtained at lower time resolution (minutes or even hours). In this review, we first describe the used techniques-dispersive IR, laser diode IR, rapid-scan Fourier transform (FT)IR, step-scan FTIR-underlying the advantages and disadvantages of each of them. Then, the main TR-IR results obtained so far in the investigation of photosynthetic reactions (in reaction centers, in light-harvesting systems, but also in entire membranes or even in living organisms) are presented. Finally, after the general conclusions, the perspectives in the field of TR-IR applied to photosynthesis are described.

An intrinsically disordered photosystem II subunit, PsbO, provides a structural template and a sensor of the hydrogen-bonding network in photosynthetic water oxidation

Photosystem II (PSII) is a membrane-bound enzyme that utilizes solar energy to catalyze the photooxidation of water. Molecular oxygen is evolved after four sequential light-driven oxidation reactions at the Mn4CaO5 oxygen-evolving complex, producing five sequentially oxidized states, Sn. PSII is composed of 17 membrane-spanning subunits and three extrinsic subunits, PsbP, PsbQ, and PsbO. PsbO is intrinsically disordered and plays a role in facilitation of the water oxidizing cycle. Native PsbO can be removed and substituted with recombinant PsbO, thereby restoring steady-state activity. In this report, we used reaction-induced Fourier transform infrared spectroscopy to obtain information concerning the role of PsbP, PsbQ, and PsbO during the S state cycle. Light-minus-dark difference spectra were acquired, monitoring structural changes associated with each accessible flash-induced S state transition in a highly purified plant PSII preparation (Triton X-100, octylthioglucoside). A comparison of S2 minus S1 spectra revealed that removal of PsbP and PsbQ had no significant effect on the data, whereas amide frequency and intensity changes were associated with PsbO removal. These data suggest that PsbO acts as an organizational template for the PSII reaction center. To identify any coupled conformational changes arising directly from PsbO, global (13)C-PsbO isotope editing was employed. The reaction-induced Fourier transform infrared spectra of accessible S states provide evidence that PsbO spectral contributions are temperature (263 and 277 K) and S state dependent. These experiments show that PsbO undergoes catalytically relevant structural dynamics, which are coupled over long distance to hydrogen-bonding changes at the Mn4CaO5 cluster.

Radiolytic mapping of solvent-contact surfaces in Photosystem II of higher plants: experimental identification of putative water channels within the photosystem

Photosystem II uses water as an enzymatic substrate. It has been hypothesized that this water is vectored to the active site for water oxidation via water channels that lead from the surface of the protein complex to the Mn4O5Ca metal cluster. The radiolysis of water by synchrotron radiation produces amino acid residue-modifying OH(•) and is a powerful technique to identify regions of proteins that are in contact with water. In this study, we have used this technique to oxidatively modify buried amino acid residues in higher plant Photosystem II membranes. Fourier transform ion cyclotron resonance mass spectrometry was then used to identify these oxidized amino acid residues that were located in several core Photosystem II subunits (D1, D2, CP43, and CP47). While, as expected, the majority of the identified oxidized residues (≈75%) are located on the solvent-exposed surface of the complex, a number of buried residues on these proteins were also modified. These residues form groups which appear to lead from the surface of the complex to the Mn4O5Ca cluster. These residues may be in contact with putative water channels in the photosystem. These results are discussed within the context of a number of largely computational studies that have identified putative water channels in Photosystem II.

Detection of an intermediary, protonated water cluster in photosynthetic oxygen evolution

In photosynthesis, photosystem II evolves oxygen from water by the accumulation of photooxidizing equivalents at the oxygen-evolving complex (OEC). The OEC is a Mn4CaO5 cluster, and its sequentially oxidized states are termed the Sn states. The dark-stable state is S1, and oxygen is released during the transition from S3 to S0. In this study, a laser flash induces the S1 to S2 transition, which corresponds to the oxidation of Mn(III) to Mn(IV). A broad infrared band, at 2,880 cm(-1), is produced during this transition. Experiments using ammonia and (2)H2O assign this band to a cationic cluster of internal water molecules, termed "W5(+)." Observation of the W5(+) band is dependent on the presence of calcium, and flash dependence is observed. These data provide evidence that manganese oxidation during the S1 to S2 transition results in a coupled proton transfer to a substrate-containing, internal water cluster in the OEC hydrogen-bonded network.

Alternating electron and proton transfer steps in photosynthetic water oxidation

Water oxidation by cyanobacteria, algae, and plants is pivotal in oxygenic photosynthesis, the process that powers life on Earth, and is the paradigm for engineering solar fuel-production systems. Each complete reaction cycle of photosynthetic water oxidation requires the removal of four electrons and four protons from the catalytic site, a manganese-calcium complex and its protein environment in photosystem II. In time-resolved photothermal beam deflection experiments, we monitored apparent volume changes of the photosystem II protein associated with charge creation by light-induced electron transfer (contraction) and charge-compensating proton relocation (expansion). Two previously invisible proton removal steps were detected, thereby filling two gaps in the basic reaction-cycle model of photosynthetic water oxidation. In the S(2) → S(3) transition of the classical S-state cycle, an intermediate is formed by deprotonation clearly before electron transfer to the oxidant (Y Z OX). The rate-determining elementary step (τ, approximately 30 µs at 20 °C) in the long-distance proton relocation toward the protein-water interface is characterized by a high activation energy (E(a) = 0.46 ± 0.05 eV) and strong H/D kinetic isotope effect (approximately 6). The characteristics of a proton transfer step during the S(0) → S(1) transition are similar (τ, approximately 100 µs; E(a) = 0.34 ± 0.08 eV; kinetic isotope effect, approximately 3); however, the proton removal from the Mn complex proceeds after electron transfer to . By discovery of the transient formation of two further intermediate states in the reaction cycle of photosynthetic water oxidation, a temporal sequence of strictly alternating removal of electrons and protons from the catalytic site is established.

Extended protein/water H-bond networks in photosynthetic water oxidation

Oxidation of water molecules in the photosystem II (PSII) protein complex proceeds at the manganese-calcium complex, which is buried deeply in the lumenal part of PSII. Understanding the PSII function requires knowledge of the intricate coupling between the water-oxidation chemistry and the dynamic proton management by the PSII protein matrix. Here we assess the structural basis for long-distance proton transfer in the interior of PSII and for proton management at its surface. Using the recent high-resolution crystal structure of PSII, we investigate prominent hydrogen-bonded networks of the lumenal side of PSII. This analysis leads to the identification of clusters of polar groups and hydrogen-bonded networks consisting of amino acid residues and water molecules. We suggest that long-distance proton transfer and conformational coupling is facilitated by hydrogen-bonded networks that often involve more than one protein subunit. Proton-storing Asp/Glu dyads, such as the D1-E65/D2-E312 dyad connected to a complex water-wire network, may be particularly important for coupling protonation states to the protein conformation. Clusters of carboxylic amino acids could participate in proton management at the lumenal surface of PSII. We propose that rather than having a classical hydrophobic protein interior, the lumenal side of PSII resembles a complex polyelectrolyte with evolutionary optimized hydrogen-bonding networks. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

A hydrogen-bonding network plays a catalytic role in photosynthetic oxygen evolution

In photosystem II, oxygen evolution occurs by the accumulation of photo-induced oxidizing equivalents at the oxygen-evolving complex (OEC). The sequentially oxidized states are called the S(0)-S(4) states, and the dark stable state is S(1). Hydrogen bonds to water form a network around the OEC; this network is predicted to involve multiple peptide carbonyl groups. In this work, we tested the idea that a network of hydrogen bonded water molecules plays a catalytic role in water oxidation. As probes, we used OEC peptide carbonyl frequencies, the substrate-based inhibitor, ammonia, and the sugar, trehalose. Reaction-induced FT-IR spectroscopy was used to describe the protein dynamics associated with the S(1) to S(2) transition. A shift in an amide CO vibrational frequency (1664 (S(1)) to 1653 (S(2)) cm(-1)) was observed, consistent with an increase in hydrogen bond strength when the OEC is oxidized. Treatment with ammonia/ammonium altered these CO vibrational frequencies. The ammonia-induced spectral changes are attributed to alterations in hydrogen bonding, when ammonia/ammonium is incorporated into the OEC hydrogen bond network. The ammonia-induced changes in CO frequency were reversed or blocked when trehalose was substituted for sucrose. This trehalose effect is attributed to a displacement of ammonia molecules from the hydrogen bond network. These results imply that ammonia, and by extension water, participate in a catalytically essential hydrogen bond network, which involves OEC peptide CO groups. Comparison to the ammonia transporter, AmtB, reveals structural similarities with the bound water network in the OEC.

Proton coupled electron transfer and redox active tyrosines in Photosystem II

In this article, progress in understanding proton coupled electron transfer (PCET) in Photosystem II is reviewed. Changes in acidity/basicity may accompany oxidation/reduction reactions in biological catalysis. Alterations in the proton transfer pathway can then be used to alter the rates of the electron transfer reactions. Studies of the bioenergetic complexes have played a central role in advancing our understanding of PCET. Because oxidation of the tyrosine results in deprotonation of the phenolic oxygen, redox active tyrosines are involved in PCET reactions in several enzymes. This review focuses on PCET involving the redox active tyrosines in Photosystem II. Photosystem II catalyzes the light-driven oxidation of water and reduction of plastoquinone. Photosystem II provides a paradigm for the study of redox active tyrosines, because this photosynthetic reaction center contains two tyrosines with different roles in catalysis. The tyrosines, YZ and YD, exhibit differences in kinetics and midpoint potentials, and these differences may be due to noncovalent interactions with the protein environment. Here, studies of YD and YZ and relevant model compounds are described.

Latent manganese deficiency increases transpiration in barley (Hordeum vulgare)

To investigate if latent manganese (Mn) deficiency leads to increased transpiration, barley plants were grown for 10 weeks in hydroponics with daily additions of Mn in the low nM range. The Mn-starved plants did not exhibit visual leaf symptoms of Mn deficiency, but Chl a fluorescence measurements revealed that the quantum yield efficiency of PSII (F(v)/F(m)) was reduced from 0.83 in Mn-sufficient control plants to below 0.5 in Mn-starved plants. Leaf Mn concentrations declined from 30 to 7 microg Mn g(-1) dry weight in control and Mn-starved plants, respectively. Mn-starved plants had up to four-fold higher transpiration than control plants. Stomatal closure and opening upon light/dark transitions took place at the same rate in both Mn treatments, but the nocturnal leaf conductance for water vapour was still twice as high in Mn-starved plants compared with the control. The observed increase in transpiration was substantiated by (13)C-isotope discrimination analysis and gravimetric measurement of the water consumption, showing significantly lower water use efficiency in Mn-starved plants. The extractable wax content of leaves of Mn-starved plants was approximately 40% lower than that in control plants, and it is concluded that the increased leaf conductance and higher transpirational water loss are correlated with a reduction in the epicuticular wax layer under Mn deficiency.

Azide as a probe of proton transfer reactions in photosynthetic oxygen evolution

In oxygenic photosynthesis, photosystem II (PSII) is the multisubunit membrane protein responsible for the oxidation of water to O2 and the reduction of plastoquinone to plastoquinol. One electron charge separation in the PSII reaction center is coupled to sequential oxidation reactions at the oxygen-evolving complex (OEC), which is composed of four manganese ions and one calcium ion. The sequentially oxidized forms of the OEC are referred to as the S(n) states. S(1) is the dark-adapted state of the OEC. Flash-induced oxygen production oscillates with period four and occurs during the S(3) to S(0) transition. Chloride plays an important, but poorly understood role in photosynthetic water oxidation. Chloride removal is known to block manganese oxidation during the S(2) to S(3) transition. In this work, we have used azide as a probe of proton transfer reactions in PSII. PSII was sulfate-treated to deplete chloride and then treated with azide. Steady state oxygen evolution measurements demonstrate that azide inhibits oxygen evolution in a chloride-dependent manner and that azide is a mixed or noncompetitive inhibitor. This result is consistent with two azide binding sites, one at which azide competes with chloride and one at which azide and chloride do not compete. At pH 7.5, the K(i) for the competing site was estimated as 1 mM, and the K(i)' for the uncompetitive site was estimated as 8 mM. Vibrational spectroscopy was then used to monitor perturbations in the frequency and amplitude of the azide antisymmetric stretching band. These changes were induced by laser-induced charge separation in the PSII reaction center. The results suggest that azide is involved in proton transfer reactions, which occur before manganese oxidation, on the donor side of chloride-depleted PSII.

Light-induced dynamics in photosystem I electron transfer

Protein dynamics are likely to play important, regulatory roles in many aspects of photosynthetic electron transfer, but a detailed description of these coupled protein conformational changes has been unavailable. In oxygenic photosynthesis, photosystem I catalyzes the light-driven oxidation of plastocyanin or cytochrome c and the reduction of ferredoxin. A chlorophyll (chl) a/a' heterodimer, P(700), is the secondary electron donor, and the two P(700) chl, are designated P(A) and P(B). We used specific chl isotopic labeling and reaction-induced Fourier-transform infrared spectroscopy to assign chl keto vibrational bands to P(A) and P(B). In the cyanobacterium, Synechocystis sp. PCC 6803, the chl keto carbon was labeled from (13)C-labeled glutamate, and the chl keto oxygen was labeled from (18)O(2). These isotope-based assignments provide new information concerning the structure of P(A)(+), which is found to give rise to two chl keto vibrational bands, with frequencies at 1653 and 1687 cm(-1). In contrast, P(A) gives rise to one chl keto band at 1638 cm(-1). The observation of two P(A)(+) keto frequencies is consistent with a protein relaxation-induced distribution in P(A)(+) hydrogen bonding. These results suggest a light-induced conformational change in photosystem I, which may regulate the oxidation of soluble electron donors and other electron-transfer reactions. This study provides unique information concerning the role of protein dynamics in oxygenic photosynthesis.

Time-resolved X-ray spectroscopy leads to an extension of the classical S-state cycle model of photosynthetic oxygen evolution

In oxygenic photosynthesis, a complete water oxidation cycle requires absorption of four photons by the chlorophylls of photosystem II (PSII). The photons can be provided successively by applying short flashes of light. Already in 1970, Kok and coworkers [Photochem Photobiol 11:457-475, 1970] developed a basic model to explain the flash-number dependence of O2 formation. The third flash applied to dark-adapted PSII induces the S3-->S4-->S0 transition, which is coupled to dioxygen formation at a protein-bound Mn4Ca complex. The sequence of events leading to dioxygen formation and the role of Kok's enigmatic S4-state are only incompletely understood. Recently we have shown by time-resolved X-ray spectroscopy that in the S3-->S0 transition an interesting intermediate is formed, prior to the onset of O-O bond formation [Haumann et al. Science 310:1019-1021, 2005]. The experimental results of the time-resolved X-ray experiments are discussed. The identity of the reaction intermediate is considered and the question is addressed how the novel intermediate is related to the S4-state proposed in 1970 by Bessel Kok. This leads us to an extension of the classical S-state cycle towards a basic model which describes sequence and interplay of electron and proton abstraction events at the donor side of PSII [Dau and Haumann, Science 312:1471-1472, 2006].

Oxidative photosynthetic water splitting: energetics, kinetics and mechanism

This minireview is an attempt to summarize our current knowledge on oxidative water splitting in photosynthesis. Based on the extended Kok model (Kok, Forbush, McGloin (1970) Photochem Photobiol 11:457-476) as a framework, the energetics and kinetics of two different types of reactions comprising the overall process are discussed: (i) P680+* reduction by the redox active tyrosine YZ of polypeptide D1 and (ii) Yz (ox) induced oxidation of the four step sequence in the water oxidizing complex (WOC) leading to the formation of molecular oxygen. The mode of coupling between electron transport (ET) and proton transfer (PT) is of key mechanistic relevance for the redox turnover of YZ and the reactions within the WOC. The peculiar energetics of the oxidation steps in the WOC assure that redox state S1 is thermodynamically most stable. This is a general feature in all oxygen evolving photosynthetic organisms and assumed to be of physiological relevance. The reaction coordinate of oxidative water splitting is discussed on the basis of the available information about the Gibbs energy differences between the individual redox states Si+1 and Si and the data reported for the activation energies of the individual oxidation steps in the WOC. Finally, an attempt is made to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and on the active role of the protein in tuning the local proton activity that depends on time and redox state Si. The O-O linkage is assumed to take place at the level of a complexed peroxide.

Perturbations at the chloride site during the photosynthetic oxygen-evolving cycle

Photosystem II (PSII) catalyzes the oxidation of water to O2 at the manganese-containing, oxygen-evolving complex (OEC). Photoexcitation of PSII results in the oxidation of the OEC; four sequential oxidation reactions are required for the generation and release of molecular oxygen. Therefore, with flash illumination, the OEC cycles among five Sn states. Chloride depletion inhibits O2 evolution. However, the binding site of chloride in the OEC is not known, and the role of chloride in oxygen evolution has not as yet been elucidated. We have employed reaction-induced FT-IR spectroscopy and selective flash excitation, which cycles PSII samples through the S state transitions. On the time scale employed, these FT-IR difference spectra reflect long-lived structural changes in the OEC. Bromide substitution supports oxygen evolution and was used to identify vibrational bands arising from structural changes at the chloride-binding site. Contributions to the vibrational spectrum from bromide-sensitive bands were observed on each flash. Sulfate treatment led to an elimination of oxygen evolution activity and of the FT-IR spectra assigned to the S3 to S0 (third flash) and S0 to S1 transitions (fourth flash). However, sulfate treatment changed, but did not eliminate, the FT-IR spectra obtained with the first and second flashes. Solvent isotope exchange in chloride-exchanged samples suggests flash-dependent structural changes, which alter protein dynamics during the S state cycle.

Light-induced FTIR difference spectroscopy as a powerful tool toward understanding the molecular mechanism of photosynthetic oxygen evolution

The molecular mechanism of photosynthetic oxygen evolution remains a mystery in photosynthesis research. Although recent X-ray crystallographic studies of the photosystem II core complex at 3.0-3.5 A resolutions have revealed the structure of the oxygen-evolving center (OEC), with approximate positions of the Mn and Ca ions and the amino acid ligands, elucidation of its detailed structure and the reactions during the S-state cycle awaits further spectroscopic investigations. Light-induced Fourier transform infrared (FTIR) difference spectroscopy was first applied to the OEC in 1992 as detection of its structural changes upon the S(1)-->S(2) transition, and spectra during the S-state cycle induced by consecutive flashes were reported in 2001. These FTIR spectra provide extensive structural information on the amino acid side groups, polypeptide chains, metal core, and water molecules, which constitute the OEC and are involved in its reaction. FTIR spectroscopy is thus becoming a powerful tool in investigating the reaction mechanism of photosynthetic oxygen evolution. In this mini-review, the measurement method of light-induced FTIR spectra of OEC is introduced and the results obtained thus far using this technique are summarized.

Calcium exchange and structural changes during the photosynthetic oxygen evolving cycle

PSII catalyzes the oxidation of water and reduction of plastoquinone in oxygenic photosynthesis. PSII contains an oxygen-evolving complex, which is located on the lumenal side of the PSII reaction center and which contains manganese, calcium, and chloride. Four sequential photooxidation reactions are required to generate oxygen. This process produces five Sn-states, where n refers to the number of oxidizing equivalents stored. Calcium is required for oxygen production. Strontium is the only divalent cation that replaces calcium and maintains activity. In our previous FT-IR work, we assessed the effect of strontium substitution on substrate-limited PSII preparations, which were inhibited at the S3 to S0 transition. In this work, we report reaction-induced FT-IR studies of hydrated PSII preparations, which undergo the full S-state cycle. The observed difference FT-IR spectra reflect long-lived photoinduced conformational changes in the oxygen-evolving complex; strontium exchange identifies vibrational bands sensitive to substitutions at the calcium site. During the S1' to S2' transition, the data are consistent with an electrostatic or structural perturbation of the calcium site. During the S3' to S0' and S0' to S1' transitions, the data are consistent with a perturbation of a hydrogen bonding network, which contains calcium, water, and peptide carbonyl groups. To explain our data, persistent shifts in divalent cation coordination must occur when strontium is substituted for calcium. A modified S-state model is proposed to explain these results and results in the literature.