Oxygen-evolving complex of photosystem II: correlating structure with spectroscopy.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 11812

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Oxygen-evolving complex of photosystem II: correlating structure with spectroscopy Ravi Pokhrel and Gary W. Brudvig* Water oxidation at the oxygen-evolving complex (OEC) of photosystem II (PSII) involves multiple redox states called Sn states (n = 0–4). The S1 - S2 redox transition of the OEC has been studied extensively using various forms of spectroscopy, including electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopy. In the S2 state, two isomers of the OEC are observed by EPR: a ST = 1/2 form and a ST = 5/2 form. DFT-based structural models of the OEC have been proposed for the two spin isomers in the S2 state, but the factors that determine the stability of one form or the other are not known. Using structural information on the OEC and its surroundings, in conjunction with spectroscopic information available on the S1 - S2 transition for a variety of site-directed mutations, Ca2+ and Cl substitutions, and small molecule inhibitors, we propose that the hydrogen-bonding network encompassing D1-D61 and the OEC-bound waters plays an important role in stabilizing one spin isomer over the other. In the presence of ammonia, PSII centers can be trapped in either the ST = 5/2

Received 2nd February 2014, Accepted 27th March 2014

form after a 200 K illumination procedure or an ammonia-altered ST = 1/2 form upon annealing at

DOI: 10.1039/c4cp00493k

the hydrogen-binding requirements for ammonia binding and the specificity for binding of ammonia but

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analogous to the spin isomers of the S2 state, is also presented.

273 K. We propose a mechanism for ammonia binding to the OEC in the S2 state that takes into account not methylamine. A discussion regarding the possibility of spin isomers of the OEC in the S1 state,

Introduction Water oxidation in oxygenic photosynthetic organisms takes place in a protein–pigment complex called photosystem II (PSII). The active site for water oxidation is the oxygen-evolving complex (OEC), a Mn4CaO5 inorganic cluster coordinated by several amino-acid residues.1 During light-driven water-oxidation catalysis, the OEC cycles through five redox states known as the Sn states (n = 0–4). The S0 state is the most reduced state of the catalytic cycle and the S4 state is the most oxidized state. The S1 state is the dark-stable state; if PSII is incubated in the dark, the majority of centers will convert to the S1 state. In this paper, we focus on the S1 - S2 transition. Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) spectroscopy have been extensively used to study changes arising from the individual S-state transitions in PSII.2–8 These two techniques are complementary in the following ways: (1) the S1 state of the OEC is EPR inactive while the S2 state is EPR active when probed using perpendicular-mode EPR. The EPR signals from the OEC observed upon the S1 - S2 transition reveal structural and magnetic properties of the OEC in the S2 state. However, the S1 state of the OEC can also be probed using Department of Chemistry, Yale University, New Haven, CT 06520-8107, USA. E-mail: [email protected]

11812 | Phys. Chem. Chem. Phys., 2014, 16, 11812--11821

parallel-mode EPR, although this is less common. (2) FTIR spectroscopy yields information about the entire protein, including the OEC with its metal-bound oxo groups, water molecules and amino-acid ligands, along with surrounding waters and amino-acid residues. The difference spectrum of the S1 - S2 transition reveals changes in the protein backbone, in free and bound carboxylates, in the metal–oxo bonds in the OEC, and in the bound waters between the S2 and S1 states. The S1 and S2 states of the OEC are the most well characterized S states. The S1 state is dark stable and the S2 state can be trapped in good yield by a simple 200 K illumination procedure.9 Long before the crystal structures of PSII were able to identify residues ligating the OEC, FTIR and EPR studies on wild-type and sitedirected mutated PSII generated a large body of information about the OEC and its surroundings.2 High-resolution crystal structures of PSII have now been published (Fig. 1),10,11 although the structure of the OEC is in a more reduced state than the S1 state due to generation of electrons by the ionizing X-rays.12 Computational refinement of the crystal structure to match the S1-state EXAFS of the OEC has been published as a model for the S1 state of the OEC.13 Similarly, DFT models of the S2 state have also been published.14–19 There is a large amount of spectroscopic information on the S1 - S2 transition of the OEC, but solid structural information on the OEC and its surrounding environment was lacking until

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Fig. 1 The structure of the OEC and its ligands as modeled in the 1.9 Å resolution crystal structure.10 Mn ions are shown in purple and numbered in white, oxo–hydroxo bridges are shown in red and numbered in yellow, the Ca ion is shown in green, OEC-bound water molecules are shown in orange and labeled, and all the amino-acid residues ligating the metals in OEC are shown and labeled.

recently. In this paper, we review the structural information on the OEC and its surrounding environment from X-ray crystallography, together with spectroscopic information on the S1 - S2 transition of the OEC, to develop a structural model for the effect of site-directed mutations, Ca2+ and Cl substitutions, and small molecule inhibitors on the magnetic properties of the S2 state. We propose a role for a hydrogen-bonding network in stabilizing the ST = 1/2 spin isomer of the OEC in the S2 state. In the absence of this stabilization, we propose the formation of the ST = 5/2 spin isomer of the OEC in the S2 state. Because ammonia binding to PSII stabilizes different spin isomer forms under different conditions, we propose a mechanism for ammonia binding to the OEC that relates to the ST = 1/2 and ST = 5/2 spin isomers of the OEC in the S2 state. We conclude with a discussion of the possibility spin isomers of the OEC in the S1 state, analogous to the spin isomers of the S2 state.

EPR spectroscopy of the S2 state Under native conditions, the S2 state gives rise to a g = 2 EPR signal from a ST = 1/2 form of the OEC.20,21 However, under many other conditions, a g = 4 EPR signal from a ST = 5/2 form of the OEC is also observed in equilibrium with the g = 2 EPR signal (Table 1 and Fig. 2).22–25 In untreated PSII, the g = 2 form is the predominant form of the S2 state in spinach PSII and it is the only form observed in cyanobacterial PSII (Table 1). The species-dependent differences in the equilibrium between the g = 2 and g = 4 signals may arise from structural differences between spinach PSII and cyanobacterial PSII. Although the OEC, nearby residues, and the core subunits are conserved between higher-plant PSII and cyanobacterial PSII, there are


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several differences in the extrinsic subunits. In plant PSII, three extrinsic subunits, PsbO, PsbP, and PsbQ, are present and are required for maximal rates of oxygen evolution.26–28 However, cyanobacterial PSII lacks the PsbP and PsbQ extrinsic subunits, but has the PsbU, PsbV (cyt c550), CyanoQ, and CyanoP extrinsic subunits.26–29 In cyanobacterial PSII, there is structural evidence of one PsbO per PSII monomer,10 whereas in plant PSII, there is biochemical evidence for two copies of PsbO per PSII monomer.30–32 Even though the functional aspects for all the extrinsic subunits are not entirely clear, it is important to note the differences in the extrinsic subunit composition between higher plants and cyanobacteria, which may translate into some structural differences near the OEC. In wild-type cyanobacterial PSII, the g = 4 signal has only been observed upon substitution of calcium in the OEC with strontium33 and upon I substitution of Cl .34 Interestingly, the g = 4 signal has also been observed upon several mutations of the outer-sphere residues of the OEC.35,36 The g = 4 signal in cyanobacteria, however, has not been observed upon any mutations of the direct ligands to the OEC (Table 1 and Fig. 4), although some of the g = 2 multiline signals observed in the mutants are different from the g = 2 multiline signal observed in wild type.37,38 Nonetheless, altered g = 2 signals also arise from a ST = 1/2 form of the OEC. In spinach PSII, substitution of Ca2+ with Sr2+,39 depletion of Cl ,24,40 and substitution of Cl with other monovalent anions41–44 stabilize the ST = 5/2 form of the OEC in the S2 state, giving rise to the g = 4 signal. In contrast, the presence of methanol, ethanol, ethylene glycol, and glycerol stabilize the ST = 1/2 form of the OEC in the S2 state.45,46 In Cl -depleted centers that display the ST = 5/2 form, conversion to the ST = 1/2 form occurs when Cl is replenished in the dark.40 It has also been observed that near-IR illumination at 150 K converts the ST = 1/2 form to the ST = 5/2 form.25 Upon annealing the NIR illuminated sample at 200 K in the dark, the ST = 5/2 form converts back to the ST = 1/2 form.25 Thus, there are many treatments that affect the equilibrium between the g = 2 and the g = 4 forms in the S2 state of the OEC, but it is unclear how one form is being stabilized over another under these conditions. Structural models of the OEC from EPR studies Structural models of the spin isomers of the OEC in the S2 state, giving rise to the g = 2 and the g = 4 signals, have been proposed using DFT calculations.14 The proposed structure for the ST = 1/2 form involves the oxidation of Mn4, also called the ‘‘dangler’’ Mn from Mn(III) to Mn(IV) upon the S1 - S2 transition with the O5 oxo remaining a ligand to the ‘‘dangler’’ Mn (Fig. 3).14 In this form, the His332-bound Mn1 remains pentacoordinate and in the Mn(III) oxidation state. However, the proposed model for the ST = 5/2 form involves oxidation of the His332-bound Mn1 from Mn(III) to Mn(IV) upon the S1 - S2 transition with the O5 oxo ligating the His332-bound Mn1 rather than the ‘‘dangler’’ Mn (Fig. 3).14 EXAFS experiments on the g = 2 multiline form of the S2 state are consistent with three Mn–Mn distances of 2.7–2.8 Å and one Mn–Mn distance of 3.3 Å.47 In model complexes, a Mn–Mn distance B2.7 Å is characteristic of a Mn–(m-O)2–Mn unit

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Table 1 A list of measurement conditions under which the S2-state g = 2 and the g = 4 signals are observed by EPR. O denotes observation of the signal and X denotes absence of the signal. Treatments and/or perturbations that push the equilibrium to favor/disfavor the g = 2 or the g = 4 form are denoted by + and , respectively


Measurement conditions 2+

46

(+Ca /+Cl ) (+Sr2+/+Cl )39 (+Ca2+/ Cl )24 (+Ca2+/+I , NO3 , CH3COO , NH3, F , or NO2 )42–44,51

g=2

g=4

O

O + + + O

Calcium/chloride treatments (spinach)

WT WT WT WT

Effects of cryoprotectants (spinach)

Sucrose46 Glycerol45 Ethylene glycol45,46

O + +

Effects of small molecules (spinach)

Methanol, ethanol45

+

Other treatments (spinach)

2+

WT (+Ca / Cl ): advanced to the S2 state (predominantly g = 4) and subsequently added Cl in the dark40 WT (+Ca2+/+Cl ): advanced to the S2 state (predominantly g = 2) and illuminated with near-IR light at 150 K25 WT (+Ca2+/+Cl ): near IR-illuminated sample at 150 K (predominantly g = 4) warmed to 200 K in the dark25

+

Calcium/chloride treatments in WT (cyanobacteria)

WT (+Ca2+/+Cl )33 WT (+Sr2+/+Cl )33 WT (+Ca2+/+I )34

O

X + +

Mutations of outer-sphere residues (cyanobacteria); in the presence of Ca2+ and Cl

D2-K317A36 D2-K317R36 D1-A344G, D, N35

O O O

X O O

Mutations of ligands to the OEC (cyanobacteria); in the presence of Ca2+ and Cl

D1-H332Q, S, E37,52 CP43-E354Q38 D1-D170H53 D1-D342N54

O O O O

X X X X

+ +

Fig. 3 Models for the g = 2 form and the g = 4 forms of the OEC in the S2 state proposed by Pantazis et al.14 The Mn oxidized from Mn(III) to Mn(IV) in the S1 - S2 transition is shown in blue. Mn–Mn distances are included to correlate with EXAFS experiments.

Fig. 2 The S2-state g = 2 multiline and the S2-state g = 4 EPR signals obtained from spinach PSII membranes in sucrose-containing buffer.

whereas a Mn–Mn distance B3.3 Å is characteristic of a Mn–(m-O)– Mn unit.48,49 Differences in the Mn–Mn distances of the OEC between the S2-state g = 2 multiline form and the S2-state g = 4 form have also been studied by EXAFS.50 In the g = 4 form, one of the Mn–Mn distances is slightly elongated to 2.85 Å from 2.73 Å in the g = 2 form.50 These distances appear to be consistent with the


proposed models for the g = 2 form and the g = 4 form of the OEC in the S2 state. However, it should be noted that the Mn–Mn unit bridged by a single m-oxo changes from Mn1–Mn3 in the g = 2 form to Mn3–Mn4 in the g = 4 form (Fig. 3). Bovi et al. have studied the interconversion of the two S2 state model structures (the g = 2 model and the g = 4 model proposed by Pantazis et al.14) using ab initio molecular dynamics simulations performed within a QM/MM framework.19 In their study, they found that the conversion from the g = 2 form


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Fig. 4 (A) The S2-minus-S1 difference EPR spectra of PSII core complexes isolated from wild-type, and D2-K317R and D2-K317A mutated Synechocystis PCC 6803. (B) Enlarged view of the g = 4 signals observed in D2-K317R mutated Synechocystis PCC 6803 PSII. (C) Expanded version of the g = 2 multiline signals shown in (A).

to the g = 4 form was slightly endergonic with an activation barrier of DG# = 10.6 kcal mol 1.19 Based on this activation barrier, they calculate that the interconversion between these two states is thermally activated at room temperature.19 However, at low temperatures, at which the S2-state EPR spectra are collected, the interconversion is very slow and non-existent in the timescale of the experiments.19 Although models for the g = 2 and the g = 4 forms of the OEC have been proposed, the underlying causes for stabilizing the ST = 1/2 form versus the ST = 5/2 form of the OEC in the S2 state upon different treatments and mutations are not understood. In this paper, with careful analysis of the X-ray crystal structures, EPR data, and FTIR data, we propose that the hydrogen bonding on the active face of the OEC, the face with water molecules attached to Mn4 and Ca, determines the stabilization of the g = 2 or the g = 4 form of the OEC in the S2 state.

S1 - S2 transition probed by FTIR Mn–O–Mn vibrational mode of the OEC Upon the S1 - S2 transition of the OEC, a Mn–O–Mn vibrational mode, the 606 cm 1 (+) band in the S2-minus-S1 difference spectrum, was identified in the S2 state of the OEC.55 The corresponding Mn–O–Mn vibrational mode in the S1 state of the OEC was identified as the 625 cm 1 ( ) band in the difference spectrum.55 The 606 cm 1 (+) band in the S2 state was downshifted to 596 cm 1 (+) in the presence of 18O water, thereby indicating exchange of a bridging oxo in the S1 state within 30 minutes.55 The bands in the low-frequency region that are sensitive to 18O water substitution but not to D2O substitution, which include the 606 cm 1 band, have also been followed throughout the entire S-state cycle.56 In the S3-minus-S2 difference spectrum, a 602 cm 1 ( ) band appeared along with a few other


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positive and negative bands,56 thereby indicating that the oxo in the Mn–O–Mn moiety responsible for the 606 cm 1 (+) band in the S2 state undergoes changes in the S2 - S3 transition. Substitution of Ca2+ with Sr2+ upshifted the 606 cm 1 (+) band to 618 cm 1 (+), also indicating interaction of this oxo with Ca2+/Sr2+ in the OEC.55 However, the 606 cm 1 (+) band did not disappear or shift upon substitution of the 44Ca isotope with 40 Ca or upon Cl depletion.55,57 Furthermore, the D1-D170H mutation upshifted the 606 cm 1 (+) band to 612 cm 1 (+);58 D1-D170 is a bridging ligand to Ca and the ‘‘dangler’’ Mn4 (Fig. 1). Annealing the S2 state with NH3 caused almost a complete disappearance of the 606 cm 1 (+) band,64 suggesting that an oxo bridge associated with the Mn–O–Mn vibration is substituted by NH3, possibly as an imido or a nitrido bridge. These results are also consistent with the electron–electron double resonance (ELDOR) detected NMR and Davies/Mims electron-nuclear double resonance (ENDOR) measurements of the S2 state of the OEC carried out with 17O water.6,59 The narrowing of the 17O signal envelope in the presence of NH3 indicates that an oxo has exchanged with NH3 in the S2 state.6 These results are consistent with O5 in Fig. 1 being the putative exchangeable oxo in the S1 state as well as the oxo that can exchange with NH3, possibly in the form of an imido or a nitrido bridge, in the S2 state. These results, however, are not sufficient to identify the O5 oxo as a substrate water because the kinetics of exchange have not been measured on a fast enough timescale to compare directly to the kinetics of substrate exchange measured by mass spectrometry.60–62 OH vibrational mode of a weakly H-bonded water molecule A critical piece of information regarding Ca- or Mn-bound water molecules in the S1 - S2 transition also comes from FTIR. Upon the S1 - S2 transition, 3618 cm 1 (+) and 3585 cm 1 ( ) bands were observed in the S2-minus-S1 difference spectrum.63 Both bands were upshifted by about 12 cm 1 when 18O water was used rather than 16O water.63 Furthermore, when the experiments were done in the presence of D2O, these bands shifted to 2681 cm 1 (+) and 2652 cm 1 ( ).63 These observations indicate that the O–H vibrational mode comes from a Ca- or Mn-bound water molecule that is weakly hydrogen bonded in the S1 and S2 states. An upshift in the OH mode upon the S1 - S2 transition also indicates that the weak hydrogen bond becomes weaker in the S2 state.63 Decoupling experiments using H2O : D2O in a 1 : 1 mixture further indicated a significant asymmetric structure of the observed water molecule; one of the O–H groups is strongly hydrogen bonded, whereas the other O–H group is weakly hydrogen bonded.63 The asymmetry in the hydrogen-bonding pattern of the observed water increased upon the S1 - S2 transition; the weak hydrogen bond became weaker and the strong hydrogen bond became stronger.63 In the presence of NH3, the 3618 cm 1 (+) and 3585 cm 1 ( ) bands did not disappear, strongly suggesting that NH3 did not replace this water molecule.64 However, the weakly hydrogen-bonded O–H band was slightly shifted, around 2–3 cm 1, in the presence of NH3.64 These results are consistent with NH3 substitution of an oxo, but not the Mn-bound water, as discussed earlier.


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QO) of protonated carboxylic acids n(CQ The (CQO) stretching mode of protonated carboxylic acids appears in the 1700–1750 cm 1 region, a region that is free of overlapping signals. Several bands appear in this region in the S2-minus-S1 difference spectrum of wild-type Synechocystis PCC 6803. However, only the 1747 cm 1 ( ) band shifts in the presence of D2O.65 Therefore, this band is assigned to a protonated carboxylate residue whose pKa decreases upon the S1 - S2 transition. This band disappears upon the D1-E65A, D2-E312A, and D1-E329Q mutations, but remains upon the D1-D61A mutation.65 Therefore, the carboxylic acid giving rise to the 1747 cm 1 ( ) band is not D1-D61.65 Other treatments, such as replacement of Ca2+ with Sr2+, also result in the disappearance of this band.33

Differences in the H-bonding network between Ca-PSII and Sr-PSII as observed by X-ray crystallography: insight into the S1 - S2 transition Among the treatments and mutations listed in Table 1, the only treatment that has been studied by X-ray crystallography is Sr substitution.11 Only the S2-state g = 2 multiline EPR signal is observed in Ca-PSII from cyanobacteria, whereas the g = 4 signal is also observed in Sr-substituted PSII.33 Therefore, it is interesting to contrast the two available structures to understand the origin of the g = 2 and the g = 4 forms of the OEC. In addition to minor differences between the Mn4CaO5 and Mn4SrO5 cores, there are some significant differences in hydrogen bonding on the top face of the OEC between these two structures (Fig. 5). However, there are no significant changes in the position of the ligands when Ca2+ is substituted by Sr2+.11 Major differences between the two structures are localized around W1, W2, surrounding water molecules, and D1-D61 (Fig. 5 and 6). In comparison to Ca-PSII, the W2–W3 distance is elongated while the W2–W1 distance is shortened in Sr-PSII (Fig. 5 and 6). Furthermore, the W1–D61 distance is slightly elongated in Sr-PSII (Fig. 5 and 6). Ca-PSII and Sr-PSII have significantly different hydrogen-bonding environments for both W1 and W2 (Fig. 5 and 6). It should be noted, though, that the standard errors for bond lengths in the 1.9 Å Ca-PSII structure and the 2.1 Å Sr-PSII structure are 0.16 Å and 0.21 Å, respectively.10,11 However, when comparing the hydrogen-bond distances between the Ca-PSII structure and the Sr-PSII structure, there are several hydrogen-bond distances that are more than 0.2 Å different from each other, mostly localized around W1, W2, and D1-D61 region. Another caveat when comparing the XRD structures of the OEC is that the structure of the OEC does not represent the initial S1 state structure due reduction of the Mn ions by X-ray exposure.13 However, the X-ray dosages were comparable when obtaining both Ca-PSII and Sr-PSII XRD data, therefore allowing a general comparison to be made between these two structures.10,11


Fig. 5 Comparison of the OEC and its surroundings in the 1.9 Å structure of Ca-PSII (top) and the 2.1 Å structure of Sr-PSII (bottom). Figures were created using PDB files 3ARC and 4IL6. Mn ions are shown in purple, oxo bridges are shown in red, calcium is shown in green, Ca- and Mn-bound waters are shown in orange, other water molecules near the OEC are shown in red, and Cl is shown in green. All labeled distances are between two O atoms. In the bottom panel, distances for Sr-PSII shown in red are distances that vary from the corresponding distances in Ca-PSII by 0.2 Å or greater. The standard errors for bond lengths in the 1.9 Å Ca-PSII structure and the 2.1 Å Sr-PSII structure are 0.16 Å and 0.21 Å, respectively.

Proposal: hydrogen bonding of W1 and W2 determines the equilibrium between the S2 state g = 2 and g = 4 forms of the OEC The hydrogen-bonding network including W1, W2, and D1-D61 may be a critical factor in the interconversion of the g = 2 and g = 4 forms of the OEC in the S2 state. A list of treatments and point mutations that shift the equilibrium either towards the g = 4 form or towards the g = 2 form is given in Table 1. A particularly interesting observation is that the g = 4 signal has not been observed upon any of the mutations of the ligands to the OEC, whereas it has been observed upon several mutations of the outer-sphere residues in Synechocystis PCC 6803 PSII (Table 1). Observation of the g = 2 multiline EPR signal in the D1-D170H mutant may seem counterintuitive. However,


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Fig. 6 Comparison of the hydrogen-bonding network of W1 and W2 in the 1.9 Å structure of Ca-PSII (top) and the 2.1 Å structure of Sr-PSII (bottom). This figure zooms in on the hydrogen-bonding network of W1 and W2 shown in Fig. 5; the coloring of atoms and distances is the same as in Fig. 5.

when ligands to the OEC are mutated, it is not necessarily the case that the substituted residue acts as a ligand to Mn. It is possible that a hydroxide ion can ligate in place of the carboxylate D1-D170 residue that has been mutated, thereby stabilizing the g = 2 form. Another indication that the hydrogen-bonding network plays an important role in stabilizing the g = 2 or the g = 4 form of the OEC comes from studies probing the effect of Cl replenishment in Cl -depleted PSII. When Cl is introduced in the dark to the centers that have the g = 4 form of the S2 state (in Cl -depleted PSII), the centers convert to the g = 2 form in the dark.40 Therefore, subtle changes in the hydrogen-bonding


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network can stabilize the g = 4 form over the g = 2 form. Other conditions in which the g = 4 form is stabilized are substitution of Ca2+ with Sr2+,33,39 NH3 treatment,51 and depletion of Cl or substitution of Cl with other monovalent anions.24,42,44 These treatments are expected to perturb the hydrogen-bonding network around the OEC. Based on the mutation and treatment results and a comparison of the Sr-PSII and Ca-PSII crystal structures, we suggest that the hydrogen-bonding network plays a key role in the stabilization of the g = 2 and g = 4 forms of the S2 state. According to our proposal, the hydrogen-bonding environment of W1 and W2 determines the formation of the g = 2 form or the g = 4 form of the S2 state. Specifically, the hydrogen-bonding network may modulate the electron donor properties of these terminal water ligands to Mn4, thus favoring or disfavoring oxidation of Mn4 in the S1 - S2 transition. FTIR data imply that there is a Mn-bound water molecule with considerable asymmetry in hydrogen bonding between its two O–H groups.63 This water molecule could be either W1 or W2. Oxidation of the ‘‘dangler’’ Mn4 would be assisted by a weakening of the weak hydrogen bond in conjunction with strengthening of the strong hydrogen bond. A ‘‘hydroxide-type’’ water in the S2 state would help stabilize the Mn(IV) oxidation state of the ‘‘dangler’’ Mn and the g = 2 form of the S2 state. However, under circumstances where this strong hydrogen-bonding partner is not available for formation of the ‘‘hydroxide-type’’ W1 or W2, such as in the Sr-substituted OEC (Fig. 5 and 6), the His332-bound Mn1 is oxidized in the S2 state generating the g = 4 form of the S2 state. The oxidation of the His332-bound Mn1 is assisted by the movement of the O5 oxo towards Mn1, allowing for a hexacoordinate Mn(IV), as shown in Fig. 3. This proposal is consistent with the differences in the distances between W1, W2, surrounding water molecules, CQO of D1-S169 amide backbone, and D1-D61 (Fig. 5) between the Ca-PSII and the Sr-PSII structures;11 Sr-substituted PSII centers favor the formation of the g = 4 form in the S2 state.33 In the Sr-PSII structure, the distances between several hydrogen-bonding partners of W1 and W2 are elongated (Fig. 5). The absence of a strong hydrogen-bonding partner for either W1 or W2 does not allow for a strong donor ‘‘hydroxide-type’’ water ligand to stabilize the oxidation of the ‘‘dangler’’ Mn4 from Mn(III) to Mn(IV), which would, according to our proposal, favor the g = 4 form. Our proposal is also consistent with stabilizing the g = 4 form under Cl -depleted conditions. Under Cl -depleted conditions, D1-D61 may form a salt bridge with D2-K317.66 The formation of a salt bridge between these two residues will affect the hydrogen-bonding environment of W1 and W2, which may stabilize the g = 4 form in the Cl -depleted centers. It is interesting to note that most treatments that shift the S2-state equilibrium towards the g = 4 form involve changes on the upper face of the OEC, which potentially change the hydrogen-bonding network of W1 and W2. Replacing W1 or W2 with methanol or ethanol would favor the oxidation of the ‘‘dangler’’ Mn4 because these alcohols are stronger electron donors than water, thereby stabilizing the Mn(IV) form of Mn4. Substitution of other monovalent anions


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for Cl in the D2-K317 site would also change the hydrogenbonding network encompassing W1 and W2; disruption of this fine-tuned hydrogen-bonding pattern could again favor the formation of the g = 4 form. Thus, our proposal unifies many observations regarding the equilibrium between the S2-state g = 2 and the g = 4 forms of the OEC.

Proposal: mechanism for NH3 substitution of O5 in the S2 state It is known that there are two ammonia-binding sites in the OEC. NH3, but not substituted amines, binds to the Mn ions in the OEC in the S2 state but not in the S1 state (site A).64,67–69 A second chloride-competitive ammonia-binding site also exists near the OEC that can accommodate NH3 and small primary amines such as methylamine in both the S1 and S2 states (site B).51,68 Variation of the concentration of NH4Cl and the temperature at which the S1 - S2 transition is carried out results in trapping different forms of the S2 state: the normal g = 2 multiline form, the g = 4 form, and the NH3-bound g = 2 altered multiline form.51,68 In the presence of 100 mM NH4Cl, the majority of centers are trapped in the g = 4 form upon illumination of the sample at 210 K, while the g = 2 form present is identical to the g = 2 form in the untreated sample.51,68 When this sample is annealed at 0 1C, an altered g = 2 signal is trapped with simultaneous loss of the original normal g = 2 and g = 4 signals, indicative of NH3 binding to the OEC in the S2 state.51 In the presence of only 10 mM NH4Cl, the majority of the centers are trapped in the g = 4 form, even when the S2 state is formed at 0 1C.51 However, with increasing concentrations of NH4Cl, upon advancement to the S2 state at 0 1C, there is an increase in the amount of centers trapped in the altered g = 2 form and a decrease in the centers trapped in the g = 4 form.51 These experiments suggest the following points: (1) There are two binding sites for NH3, of which one is to Mn ions in the OEC (site A) and the other is near the OEC (site B), possibly the Cl site. (2) NH3 can bind in site A only after formation of the S2 state, but can bind in site B in both the S1 and the S2 states. (3) The affinity of NH3 for site A is lower than for site B; thus, the g = 4 form is stabilized at a lower concentration of NH3 than the concentration of NH3 required for formation of the g = 2 altered multiline form. Binding to site A eliminates the effect of binding to site B to stabilize the g = 4 form. (4) Site A is specific for ammonia, but site B can accommodate other amines such as methylamine in addition to ammonia. ESEEM experiments suggest that NH3 binds to the OEC in the S2 state as a bridging ligand, proposed to be an imido or an amido species.67 However, a nitrido bridge is also a possibility. Furthermore, FTIR experiments looking at the low frequency Mn–O–Mn vibration in the S2 state indicate that NH3 replaces an oxo group, most likely the O5 oxo.64 Here, we propose a model for NH3 binding to the OEC that incorporates previous results for NH3 binding and the hydrogen-bonding requirements for oxo/NH3 exchange. The binding affinity of NH3 to site B is stronger than the binding


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affinity of NH3 to site A, and NH3 occupancy of site B stabilizes the S2 state g = 4 form instead of the g = 2 form. The stabilization of the S2 state g = 4 form when NH3 binds to site B can be explained by a change in the hydrogen-bonding environment around W1 and W2 when ammonia binds in place of chloride in site B, as discussed above. However, binding of ammonia to site A is triggered by formation of the S2 state. This indicates that the binding of ammonia to site A is activated by formation of Mn(IV), a stronger Lewis acid which may promote binding and deprotonation of a Lewis base such as ammonia. In our model, NH3 initially binds as a terminal ligand to Mn4 in the g = 4 form of the S2 state (Fig. 7); when O5 is dissociated from the ‘‘dangler’’ Mn4 in the g = 4 conformation, a putative additional exogenous ligand can bind to the ‘‘dangler’’ Mn4. Evidence for a Mn-bound ammonia in the g = 4 form comes from the observation that the g = 4 signal formed upon 200 K continuous illumination of an oriented PSII sample in the presence of 100 mM NH4Cl exhibits approximately 16 Mn hyperfine features, whereas, the g = 4 signal obtained in the absence of NH4Cl does not exhibit Mn hyperfine features.70,71 Warming up the NH4Cl-treated sample, which gives rise to the S2-state g = 4 form with hyperfine features to 273 K, induces formation of the altered g = 2 multiline signal at the expense of the g = 4 signal.70 Hence, NH3 bound in the g = 4 form of the OEC undergoes some structural rearrangement, possibly deprotonation steps to exchange with the O5 oxo, to form an imido or a nitrido bridge. Formation of the imido or the nitrido bridge between Mn3 and Mn4 stabilizes a Mn(IV) oxidation state of the ‘‘dangler’’ Mn4 instead of a Mn(IV) oxidation state of Mn1, thereby inducing the formation of the g = 2 altered multiline signal at the expense of the g = 4 signal (Fig. 7). The mechanism for an imido–nitrido bridge formation is proposed to be via the protonation of the O5 oxo in the g = 4 form of the OEC (Fig. 7). This scheme rationalizes the experimental observations regarding NH3 binding in the S1 and the S2 states. Our proposal for ammonia binding at the OEC differs from the proposal put forward by Navarro et al.72 These authors propose that ammonia binds as a terminal ligand to Mn4 by replacing W1. Their proposal is based on multiphase EPR results in the presence of ammonia.72 However, the authors’ primary argument for ammonia binding as a terminal ligand as opposed to a bridging ligand comes from the fact that addition of ammonia does not change the width of the signal envelope observed in the Q-band ENDOR spectrum used as a direct probe of protons in the vicinity of the OEC.72 It should be noted that if ammonia binds as a nitrido bridge by replacing the O5 m-oxo, there will be no extra protons on the N3 bridging ligand to alter the signal envelope observed in the Q-band ENDOR experiment. The authors’ also present 17O-EDNMR data to support their proposal.6,72 The authors’ show that addition of ammonia leads to an 17O m-oxo coupling that is unresolved.72 The authors attribute this to a weaker coupling of the 17O m-oxo instead of a loss of the 17O m-oxo coupling.72 However, these data could also be interpreted as a loss of the 17 O m-oxo coupling due to the exchange of the 17O m-oxo with a m-nitrido ligand. Therefore, we suggest that ammonia binding


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Fig. 7 Schematic representation of the proposal for NH3 binding to site A in the OEC. Changes in the oxidation state of Mn are shown in blue. The binding of ammonia to site A and other changes, such as protonation and movement of the O5 oxo, are shown in red. In our proposal, NH3 initially binds as a terminal ligand to Mn4 in the g = 4 form of the OEC, and several protonation/deprotonation steps of ammonia and the O5 oxo result in the formation of an imido–nitrido bridge between Mn3 and Mn4 as a replacement of the O5 oxo moiety.

as a m-nitrido ligand in place of the O5 m-oxo in the OEC is an alternative to the proposal presented by Navarro et al. The proposal for a m-nitrido ligand is also consistent with previous spectroscopic data such as FTIR data showing the loss of a Mn–O–Mn vibration upon addition of ammonia,64 ESEEM experiments that indicate binding of ammonia as a bridging ligand as opposed to a terminal ligand,67 and specificity for binding of ammonia over methylamine in site A.51 If ammonia binds as a terminal ligand to Mn4 in site A, it is not obvious why methylamine cannot bind to the same binding site in the OEC. However, if ammonia binds as a m-nitrido bridge, methylamine cannot bind to the same binding site because methylamine cannot form a nitrido bridge.

Perspective: for the centers that stabilize the g = 4 EPR signal in the S2 state, is the O5 oxo a ligand to His332-bound Mn1 in the S1 state? Analysis of the S1-state EPR spectrum in conjunction with the S2-state g = 2 and g = 4 signals can help answer this question. Unfortunately, there are not many EPR studies of the S1 state of the OEC in the literature to correlate the formation of the g = 2 and/or the g = 4 signal with the observed S1-state signal. Dexheimer and Klein first reported a 600 G wide g = 4.8 parallelmode S1-state EPR signal in spinach PSII; this signal has no resolved hyperfine features.73 Although this signal has been difficult to reproduce, Yamauchi et al. have also generated it.74


In both reports of the g = 4.8 S1-state signal, the authors showed that the g = 4.8 S1-state signal correlates only with the g = 2 multiline signal in the S2 state but does not correlate with the g = 4 S2-state signal.73,74 This observation is consistent with the idea that the S1 state has two different isomers, of which only one is observed by EPR, that give rise to the g = 2 or the g = 4 form of the S2 state. However, these interpretations are preliminary until the g = 4.8 parallel-mode S1-state EPR signal is more fully characterized. Campbell et al. reported a different parallel-mode S1-state EPR signal in PSII core complexes isolated from Synechocystis PCC 6803; this signal has 18 hyperfine lines and is centered at approximately g = 12.75 The g = 12 S1-state signal has also been reported in several mutants.38,53,54 The g = 12 S1-state signal has been shown to disappear upon formation of the S2-state multiline signal, but the correlation of the g = 12 S1-state signal with the S2-state g = 4 signal has not been probed. One major difficulty for this study is that the g = 4 signal is not normally observed in PSII core complexes isolated from Synechocystis PCC 6803. However, the g = 4 signal has been recently observed in the D2-K317R mutant of Synechocystis PCC 6803. Therefore, it is of interest to probe the correlation of the g = 12 S1-state signal with the g = 4 S2-state signal in this mutant. DFT calculations also support the existence of two isomeric substates in the S1 state that are close in energy.76 One of these substates has the O5 bound to the ‘‘dangler’’ Mn4 while the other substate has the O5 bound to the His332-bound Mn1; there are other differences in these two substates such as the protonation state of D1-D61 and the number of water/hydroxide ligands to the ‘‘dangler’’ Mn.76 Therefore, it is possible that the


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formation of the g = 2 or the g = 4 form of the S2 state is structurally predetermined in the S1 state, and that these two isomeric structures poise either the ‘‘dangler’’ Mn4 or the His-332 bound Mn1 for oxidation in the S1 - S2 transition. The alternative possibility is that there is only one stable form of the OEC in the S1 state. In this scenario, differences in the hydrogen bonding around the OEC would stabilize either the g = 2 form or the g = 4 form of the OEC after formation of the S2 state. Subtle differences in the ligand environment of the ‘‘dangler’’ Mn4 and the His332-bound Mn1 may favor one form over the other. If an identical S1-state structure transitions to either the g = 2 or the g = 4 forms of the S2 state, in the centers favoring the g = 4 form of the OEC, the O5 oxo must move to the His332-bound Mn during the S1 - S2 transition. The factors that trigger the oxidation of His332-bound Mn1 with a concomitant movement of the O5 oxo instead of oxidation of the ‘‘dangler’’ Mn4 have yet to be determined. We propose that the hydrogen bonding of W1 and W2 is an important factor differentiating stabilization of the ‘‘dangler’’ Mn4 versus the His332-bound Mn1 in the Mn(IV) oxidation state upon formation of the S2 state.

Conclusions In this review, we present a hypothesis for the origin of the g = 2 and the g = 4 forms of the OEC in the S2 state. We propose that the nature of the hydrogen-bonding interaction of the two water molecules bound to the ‘‘dangler’’ Mn4, W1 and W2, with the surrounding hydrogen-bonding network that includes D1-D61 is the critical factor for stabilizing either the g = 2 form or the g = 4 form of the S2 state. We also propose a mechanism for NH3 binding to the OEC in the S2 state. Finally, we discuss the possibility of two isomers of the OEC in the S1 state that lead to either the g = 2 form or the g = 4 form of the S2 state.

References 1 J. P. McEvoy and G. W. Brudvig, Chem. Rev., 2006, 106, 4455. 2 R. J. Debus, Coord. Chem. Rev., 2008, 252, 244. 3 A. F. Miller and G. W. Brudvig, Biochim. Biophys. Acta, 1991, 1056, 1. 4 T. Noguchi, Philos. Trans. R. Soc., B, 2008, 363, 1189. 5 M. Brynda and R. D. Britt, Biol. Magn. Reson., 2010, 29, 203. 6 L. Rapatskiy, N. Cox, A. Savitsky, W. M. Ames, J. Sander, M. M. Nowaczyk, M. Roegner, A. Boussac, F. Neese, J. Messinger and W. Lubitz, J. Am. Chem. Soc., 2012, 134, 16619. 7 N. Cox, L. Rapatskiy, J.-H. Su, D. A. Pantazis, M. Sugiura, L. Kulik, P. Dorlet, A. W. Rutherford, F. Neese, A. Boussac, W. Lubitz and J. Messinger, J. Am. Chem. Soc., 2011, 133, 3635. 8 L. V. Kulik, B. Epel, W. Lubitz and J. Messinger, J. Am. Chem. Soc., 2007, 129, 13421. 9 G. W. Brudvig, J. L. Casey and K. Sauer, Biochim. Biophys. Acta, Bioenerg., 1983, 723, 366.


Perspective

10 Y. Umena, K. Kawakami, J.-R. Shen and N. Kamiya, Nature, 2011, 473, 55. 11 F. H. M. Koua, Y. Umena, K. Kawakami and J.-R. Shen, Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 3889. 12 J. Yano, J. Kern, K. D. Irrgang, M. J. Latimer, U. Bergmann, P. Glatzel, Y. Pushkar, J. Biesiadka, B. Loll, K. Sauer, J. Messinger, A. Zouni and V. K. Yachandra, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 12047. 13 S. Luber, I. Rivalta, Y. Umena, K. Kawakami, J.-R. Shen, N. Kamiya, G. W. Brudvig and V. S. Batista, Biochemistry, 2011, 50, 6308. 14 D. A. Pantazis, W. Ames, N. Cox, W. Lubitz and F. Neese, Angew. Chem., Int. Ed., 2012, 51, 9935. 15 S. Schinzel, J. Schraut, A. V. Arbuznikov, P. E. M. Siegbahn and M. Kaupp, Chem. – Eur. J., 2010, 16, 10424. 16 P. E. M. Siegbahn, J. Am. Chem. Soc., 2013, 135, 9442. 17 P. E. M. Siegbahn, Phys. Chem. Chem. Phys., 2012, 14, 4849. 18 P. E. M. Siegbahn, J. Photochem. Photobiol., B, 2011, 104, 94. 19 D. Bovi, D. Narzi and L. Guidoni, Angew. Chem., Int. Ed., 2013, 52, 11744. 20 G. C. Dismukes and Y. Siderer, Proc. Natl. Acad. Sci. U. S. A., 1981, 78, 274. 21 R. D. Britt, G. A. Lorigan, K. Sauer, M. P. Klein and J.-L. Zimmermann, Biochim. Biophys. Acta, Bioenerg., 1992, 1140, 95. 22 A. Haddy, K. V. Lakshmi, G. W. Brudvig and H. A. Frank, Biophys. J., 2004, 87, 2885. 23 J. L. Casey and K. Sauer, Biochim. Biophys. Acta, Bioenerg., 1984, 767, 21. 24 P. van Vliet and A. W. Rutherford, Biochemistry, 1996, 35, 1829. 25 A. Boussac, J.-J. Girerd and A. W. Rutherford, Biochemistry, 1996, 35, 6984. 26 A. Seidler, Biochim. Biophys. Acta, Bioenerg., 1996, 1277, 35. 27 I. Enami, A. Okumura, R. Nagao, T. Suzuki, M. Iwai and J.-R. Shen, Photosynth. Res., 2008, 98, 349. 28 T. M. Bricker, J. L. Roose, R. D. Fagerlund, L. K. Frankel and J. J. Eaton-Rye, Biochim. Biophys. Acta, 2012, 1817, 121. 29 J. L. Roose, K. M. Wegener and H. B. Pakrasi, Photosynth. Res., 2007, 92, 369. 30 T. M. Bricker and L. K. Frankel, Photosynth. Res., 1998, 56, 157. 31 H. Popelkova and C. F. Yocum, J. Photochem. Photobiol., B, 2011, 104, 179. 32 H. Popelkova, A. Commet, T. Kuntzleman and C. F. Yocum, Biochemistry, 2008, 47, 12593. 33 M. A. Strickler, L. M. Walker, W. Hillier and R. J. Debus, Biochemistry, 2005, 44, 8571. 34 A. Boussac, N. Ishida, M. Sugiura and F. Rappaport, Biochim. Biophys. Acta, Bioenerg., 2012, 1817, 802. 35 N. Mizusawa, T. Yamanari, Y. Kimura, A. Ishii, S. Nakazawa and T.-A. Ono, Biochemistry, 2004, 43, 14644. 36 R. Pokhrel, R. J. Service, R. J. Debus and G. W. Brudvig, Biochemistry, 2013, 52, 4758. 37 R. J. Debus, K. A. Campbell, W. Gregor, Z.-L. Li, R. L. Burnap and R. D. Britt, Biochemistry, 2001, 40, 3690.


View Article Online


Perspective

38 R. J. Service, J. Yano, I. McConnell, H.-J. Hwang, D. Niks, R. Hille, T. Wydrzynski, R. L. Burnap, W. Hillier and R. J. Debus, Biochemistry, 2011, 50, 63. 39 A. Boussac and A. W. Rutherford, Biochemistry, 1988, 27, 3476. 40 T. Ono, J. L. Zimmermann, Y. Inoue and A. W. Rutherford, Biochim. Biophys. Acta, Bioenerg., 1986, 851, 193. ásson, Biochemistry, 2003, 41 K. Olesen and L. E. Andre 42, 2025. 42 T. Ono, H. Nakayama, H. Gleiter, Y. Inoue and A. Kawamori, Arch. Biochem. Biophys., 1987, 256, 618. 43 V. A. Szalai and G. W. Brudvig, Biochemistry, 1996, 35, 1946. 44 R. Pokhrel and G. W. Brudvig, Biochemistry, 2013, 52, 3781. 45 J. L. Zimmermann and A. W. Rutherford, Biochemistry, 1986, 25, 4609. 46 J. C. de Paula, W. F. Beck, A.-F. Miller, R. B. Wilson and G. W. Brudvig, J. Chem. Soc., Faraday Trans. 1, 1987, 83, 3635. 47 J. Yano, Y. Pushkar, P. Glatzel, A. Lewis, K. Sauer, J. Messinger, U. Bergmann and V. Yachandra, J. Am. Chem. Soc., 2005, 127, 14974. 48 R. Manchanda, G. W. Brudvig and R. H. Crabtree, Coord. Chem. Rev., 1995, 144, 1. 49 H. Dau, P. Liebisch and M. Haumann, Phys. Chem. Chem. Phys., 2004, 6, 4781. 50 W. Liang, M. J. Latimer, H. Dau, T. A. Roelofs, V. K. Yachandra, K. Sauer and M. P. Klein, Biochemistry, 1994, 33, 4923. 51 W. F. Beck and G. W. Brudvig, Biochemistry, 1986, 25, 6479. 52 M. Sugiura, F. Rappaport, W. Hillier, P. Dorlet, Y. Ohno, H. Hayashi and A. Boussac, Biochemistry, 2009, 48, 7856. 53 R. J. Debus, C. Aznar, K. A. Campbell, W. Gregor, B. A. Diner and R. D. Britt, Biochemistry, 2003, 42, 10600. 54 M. A. Strickler, L. M. Walker, W. Hillier, R. D. Britt and R. J. Debus, Biochemistry, 2007, 46, 3151. 55 H.-A. Chu, H. Sackett and G. T. Babcock, Biochemistry, 2000, 39, 14371. 56 Y. Kimura, A. Ishii, T. Yamanari and T.-A. Ono, Biochemistry, 2005, 44, 7613. 57 Y. Kimura, K. Hasegawa, T. Yamanari and T. Ono, Photosynth. Res., 2005, 84, 245.


PCCP

58 H.-A. Chu, R. J. Debus and G. T. Babcock, Biochemistry, 2001, 40, 2312. 59 I. L. McConnell, V. M. Grigoryants, C. P. Scholes, W. K. Myers, P.-Y. Chen, J. W. Whittaker and G. W. Brudvig, J. Am. Chem. Soc., 2012, 134, 1504. 60 J. Messinger, M. Badger and T. Wydrzynski, Proc. Natl. Acad. Sci. U. S. A., 1995, 92, 3209. 61 G. Hendry and T. Wydrzynski, Biochemistry, 2002, 41, 13328. 62 W. Hillier and T. Wydrzynski, Phys. Chem. Chem. Phys., 2004, 6, 4882. 63 T. Noguchi and M. Sugiura, Biochemistry, 2000, 39, 10943. 64 L.-H. Hou, C.-M. Wu, H.-H. Huang and H.-A. Chu, Biochemistry, 2011, 50, 9248. 65 R. J. Service, W. Hillier and R. J. Debus, Biochemistry, 2010, 49, 6655. 66 I. Rivalta, M. Amin, S. Luber, S. Vassiliev, R. Pokhrel, Y. Umena, K. Kawakami, J.-R. Shen, N. Kamiya, D. Bruce, G. W. Brudvig, M. R. Gunner and V. S. Batista, Biochemistry, 2011, 50, 6312. 67 R. D. Britt, J. L. Zimmermann, K. Sauer and M. P. Klein, J. Am. Chem. Soc., 1989, 111, 3522. 68 W. F. Beck, J. C. de Paula and G. W. Brudvig, J. Am. Chem. Soc., 1986, 108, 4018. 69 W. F. Beck and G. W. Brudvig, Chem. Scr., 1988, 28A, 93. 70 D. H. Kim, R. D. Britt, M. P. Klein and K. Sauer, J. Am. Chem. Soc., 1990, 112, 9389. 71 D. H. Kim, R. D. Britt, M. P. Klein and K. Sauer, Biochemistry, 1992, 31, 541. 72 M. P. Navarro, W. M. Ames, H. Nilsson, T. Lohmiller, D. A. Pantazis, L. Rapatskiy, M. M. Nowaczyk, F. Neese, A. Boussac, J. Messinger, W. Lubitz and N. Cox, Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 15561. 73 S. L. Dexheimer and M. P. Klein, J. Am. Chem. Soc., 1992, 114, 2821. 74 T. Yamauchi, H. Mino, T. Matsukawa, A. Kawamori and T.-A. Ono, Biochemistry, 1997, 36, 7520. 75 K. A. Campbell, J. M. Peloquin, D. P. Pham, R. J. Debus and R. D. Britt, J. Am. Chem. Soc., 1998, 120, 447. 76 M. Kusunoki, J. Photochem. Photobiol., B, 2011, 104, 100.


Thf1 and its association with the FtsH protease complex involved in photosystem II repair in cyanobacteria.

dynamics of photosystem II plastoquinone binding sites.

Removal of Ca(2+) from the Oxygen-Evolving Complex in Photosystem II Has Minimal Effect on the Mn4O5 Core Structure: A Polarized Mn X-ray Absorption Spectroscopy Study.

Photosystem II: its function, structure, and implications for artificial photosynthesis.

In Vivo Identification of Photosystem II Light Harvesting Complexes Interacting with PHOTOSYSTEM II SUBUNIT S.

The unusually strong stabilizing effects of glycine betaine on the structure and function of the oxygen-evolving Photosystem II complex.

Formation of the S2 state and structure of the Mn complex in photosystem II lacking the extrinsic 33 kilodalton polypeptide.

Carotenoid triplet states in photosystem II: coupling with low-energy states of the core complex.

Electronic structure of the oxygen evolving complex in photosystem II, as revealed by 55Mn Davies ENDOR studies at 2.5 K.

Regulation of Photosystem II.

Photosynthesis. Electronic structure of the oxygen-evolving complex in photosystem II prior to O-O bond formation.

Atomistic and Coarse Grain Topologies for the Cofactors Associated with the Photosystem II Core Complex.

Photosystem I complex.

Brevetoxin, the Dinoflagellate Neurotoxin, Localizes to Thylakoid Membranes and Interacts with the Light-Harvesting Complex II (LHCII) of Photosystem II.

Two photon absorption energy transfer in the light-harvesting complex of photosystem II (LHC-II) modified with organic boron dye.

Copper in photosystem II: association with LHC II.

Toward correlating structure and mechanics of platelets.

Current perceptions of Photosystem II.

Inactive photosystem II centers: A resolution of discrepencies in photosystem II quantitation.

Temperature-dependent changes in Photosystem II heterogeneity support a cycle of Photosystem II during photoinhibition.

Energy transfer between photosystem II and photosystem I in chloroplasts.

PHOTOSYSTEM II SUBUNIT R is required for efficient binding of LIGHT-HARVESTING COMPLEX STRESS-RELATED PROTEIN3 to photosystem II-light-harvesting supercomplexes in Chlamydomonas reinhardtii.

Editorial: Assembly of the Photosystem II Membrane-Protein Complex of Oxygenic Photosynthesis.

Photoactivation: The Light-Driven Assembly of the Water Oxidation Complex of Photosystem II.