Mn4Ca Cluster Research Articles

Mn4Ca cluster in photosynthesis: where and how water is oxidized to dioxygen.

Oxygen, which supports all aerobic life, is produced by photosynthetic water oxidation in plants, algae, and cyanobacteria. The water-oxidation reaction probably first appeared in nature ∼3 billion years ago in the precursors to present-day cyanobacteria, although the exact timing is not yet entirely clear.1−5 A key component in the appearance of oxygenic photosynthesis was a metal complex that could store oxidizing equivalents to facilitate the four-electron oxidation of two water molecules to dioxygen, meanwhile making the electrons available for the reductive carbon-fixing reactions required for sustaining life.6−8 This metal complex involved in oxygenic photosynthesis, the oxygen-evolving complex (OEC), consists of an oxo-bridged structure with four Mn atoms and one Ca atom. No variations have been observed so far among oxygenic photosynthetic organisms through higher plants and algae back to cyanobacteria, which represents the earliest oxygenic photosynthetic organisms. Oxygen itself is the byproduct of the photosynthetic water oxidation reaction shown in eq 1: 1 However, it was this oxygen that enabled oxygenic life to evolve and that led to the current diverse and complex life on earth by dramatically increasing the metabolic energy that became available from aerobic respiration. Oxygen produced by this process was also key for the development of the protective ozone layer, which allowed life to transition from marine forms to terrestrial life.

Mass spectroscopy locates the extrinsic proteins of photosystem II

The photosynthetic oxidation of water is catalyzed by photosystem II (PSII), a multisubunit pigment-protein embedded in the thylakoid membranes of cyanobacteria, algae, and plants. This remarkable photoenzyme is capable of generating the highly oxidizing chemical species required to extract four tightly bound electrons of two substrate water-molecules, yielding biosynthetically useful reductant and by-product O2. PSII is a large homodimeric complex in vivo, with a combined mass of ∼700 kDa. Each PSII monomer comprises more than 20 different proteins collectively coordinating ∼60 cofactors, including 35 chlorophylls, 2 pheophytins, 2 plastoquinone molecules, and the Mn4Ca cluster, responsible for H2O-oxidation (1, 2). Despite much progress, including the 1.9 A crystal structure of cyanobacterial PSII (1, 3), crucial structural information is missing. This lack includes extrinsic proteins affecting the Mn4Ca cluster, not observed in the PSII crystal structure because they are lost during the purification and crystallization process. In PNAS, Liu et al. use complementary technical approaches to construct a model for the binding of one of the lost extrinsic proteins, PsbQ, to the PSII, complex, and in doing so provide an example for the solution of the more general structural problem of defining the 3D structure of separately crystallized proteins that assemble into larger macromolecular complexes (4).

Mechanism of Water Splitting by Photosystem II

The light driven water splitting achieved in the oxygen evolving complex (OEC) of Photosystem II is a critical process that sustains our biosphere. Photosynthetic water splitting is fascinating in its complexity, inspiring due to its practical applications in designing artificial photosynthesis, and not yet understood. At the heart of the water splitting process is the Mn4Ca cluster embedded in a fine tuned protein environment.The electronic structure and geometry of this cluster were probed by X-ray spectroscopy at the functional room temperature state.1, 2 Detailed kinetic analysis of the X-ray induced damage was performed and allowed selection of undamaging conditions for experimentation. High-quality extended X-ray absorption fine structure (EXAFS) spectra of the OEC at room temperature will be presented and compared with XRD and DFT derived molecular models of the dark stable S1 state. The determined Debye-Waller factors, sensitive to dynamic processes, support the rigid structure of the Mn4Ca cluster at room temperature.Laser-pump X-ray probe time-resolved X-ray emission measurements (XES) allowed monitoring of changes in the electronic structure of the OEC in real time during the catalysis. Using time-resolved XES we monitored the evolution of the electronic structure of the OEC of Photosystem II during the most critical S3 to S0 transition which results in O2 evolution. Our data show no oxidation but only a gradual reduction of the Mn centers after excitation of the complex past the S3 state. These observations allow us to propose a unique O-O bond formation and water splitting mechanism. Combined with DFT modeling, our analysis reveals the mechanism of catalytic water splitting.1. Davis et al. J. Phys. Chem. B 117, 9161-9169 (2013).2. Davis et al. J. Phys. Chem. Lett. 3, 1858-1864 (2012).

1P254 光合成光化学系IIにおけるMnCaクラスターの歪んだ椅子型構造の起源(18B. 光生物:光合成,ポスター,第52回日本生物物理学会年会(2014年度))

1P254 光合成光化学系IIにおけるMnCaクラスターの歪んだ椅子型構造の起源(18B. 光生物:光合成,ポスター,第52回日本生物物理学会年会(2014年度))

Influence of the Ca2 + ion on the Mn4Ca conformation and the H-bond network arrangement in Photosystem II

In the crystal structure of Photosystem II (PSII) analyzed at a resolution of 1.9Å, most of the bond lengths between Mn and O atoms in the oxygen-evolving Mn4Ca cluster are 1.8–2.1Å. On the other hand, the Mn1O5 bond in the Mn3CaO4 cubane region of the Mn4Ca cluster is significantly elongated to 2.6Å. Using a quantum mechanical/molecular mechanical approach, we investigated factors that are responsible for distortion of the Mn3CaO4 cubane. Removal of Ca led to shortening the Mn1O5 bond by 0.2Å; however, Mn1O5 remained significantly elongated, at >2.5Å. Conversely, removal of Mn4 significantly shortens the Mn1O5 distance by 0.5Å to 2.2Å, resulting in a more symmetric cubane shape. These results suggest that Mn4, not Ca, is predominantly responsible for distortion of the Mn3CaO4 cubane. It was not the Ca component that was responsible for the existence of the two S2 conformers but two different Mn oxidation states (Mn1, Mn2, Mn3, M4)=(III, IV, IV, IV) and (IV, IV, IV, III); they were interconvertible by translocation of the O5 atom along the Mn1–O5–Mn4 axis. Depletion of Ca resulted in rearrangement of the H-bond network near TyrZ, which proceeds via a chloride ion (Cl-1 pathway). This may explain why Ca depletion inhibits the S2 to S3 transition, the same process that can also be inhibited by Cl− depletion.

Electronic structure of the Mn-cofactor of modified bacterial reaction centers measured by electron paramagnetic resonance and electron spin echo envelope modulation spectroscopies

The electronic structure of a Mn(II) ion bound to highly oxidizing reaction centers of Rhodobacter sphaeroides was studied in a mutant modified to possess a metal binding site at a location comparable to the Mn4Ca cluster of photosystem II. The Mn-binding site of the previously described mutant, M2, contains three carboxylates and one His at the binding site (Thielges et al., Biochemistry 44:389-7394, 2005). The redox-active Mn-cofactor was characterized using electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) spectroscopies. In the light without bound metal, the Mn-binding mutants showed an EPR spectrum characteristic of the oxidized bacteriochlorophyll dimer and reduced quinone whose intensity was significantly reduced due to the diminished quantum yield of charge separation in the mutant compared to wild type. In the presence of the metal and in the dark, the EPR spectrum measured at the X-band frequency of 9.4 GHz showed a distinctive spin 5/2 Mn(II) signal consisting of 16 lines associated with both allowed and forbidden transitions. Upon illumination, the amplitude of the spectrum is decreased by over 80 % due to oxidation of the metal upon electron transfer to the oxidized bacteriochlorophyll dimer. The EPR spectrum of the Mn-cofactor was also measured at the Q-band frequency of 34 GHz and was better resolved as the signal was composed of the six allowed electronic transitions with only minor contributions from other transitions. A fit of the Q-band EPR spectrum shows that the Mn-cofactor is a high spin Mn(II) species (S = 5/2) that is six-coordinated with an isotropic g-value of 2.0006, a weak zero-field splitting and E/D ratio of approximately 1/3. The ESEEM experiments showed the presence of one (14)N coordinating the Mn-cofactor. The nitrogen atom is assigned to a His by comparing our ESEEM results to those previously reported for Mn(II) ions bound to other proteins and on the basis of the X-ray structure of the M2 mutant that shows the presence of only one His, residue M193, that can coordinate the Mn-cofactor. Together, the data allow the electronic structure and coordination environment of the designed Mn-cofactor in the modified reaction centers to be characterized in detail and compared to those observed in other proteins with Mn-cofactors.

Inhibition of the water oxidizing complex of photosystem II and the reoxidation of the quinone acceptor QA- by Pb2+.

The action of the environmental toxic Pb2+ on photosynthetic electron transport was studied in thylakoid membranes isolated from spinach leaves. Fluorescence and thermoluminescence techniques were performed in order to determine the mode of Pb2+ action in photosystem II (PSII). The invariance of fluorescence characteristics of chlorophyll a (Chl a) and magnesium tetraphenylporphyrin (MgTPP), a molecule structurally analogous to Chl a, in the presence of Pb2+ confirms that Pb cation does not interact directly with chlorophyll molecules in PSII. The results show that Pb interacts with the water oxidation complex thus perturbing charge recombination between the quinone acceptors of PSII and the S2 state of the Mn4Ca cluster. Electron transfer between the quinone acceptors QA and QB is also greatly retarded in the presence of Pb2+. This is proposed to be owing to a transmembrane modification of the acceptor side of the photosystem.

Split Electron Paramagnetic Resonance Signal Induction in Photosystem II Suggests Two Binding Sites in the S2 State for the Substrate Analogue Methanol

Illuminating a photosystem II sample at low temperatures (here 5-10 K) yields so-called split signals detectable with continuous wave-electron paramagnetic resonance (CW-EPR). These signals reflect the oxidized, deprotonated radical of D1-Tyr161 (YZ(•)) in a magnetic interaction with the CaMn4 cluster in a particular S state. The intensity of the split EPR signals are affected by the addition of the water substrate analogue methanol. This was previously shown by the induction of split EPR signals from the S1, S3, and S0 states [Su, J.-H. et al. (2006) Biochemistry 45, 7617-7627.]. Here, we use two split EPR signals induced from photosystem II trapped in the S2 state to further probe the binding of methanol in an S state dependent manner. The signals are induced with either visible or near-infrared light illumination provided at 5-10 K where methanol cannot bind or unbind from its site. The results imply that the binding of methanol not only changes the magnetic properties of the CaMn4 cluster but also the hydrogen bond network in the oxygen evolving complex (OEC), thereby affecting the relative charge of the S2 state. The induction mechanisms for the two split EPR signals are different resulting in two different redox states, S2YZ(•) and S1YZ(•) respectively. The two states show different methanol dependence for their induction. This indicates the existence of two binding sites for methanol in the CaMn4 cluster. It is proposed that methanol binds to MnA with high affinity and to MnD with lower affinity. The molecular nature and S-state dependence of the methanol binding to each respective site are discussed.

An approach for catalyst design in artificial photosynthetic systems: focus on nanosized inorganic cores within proteins

Some enzymes can be considered as a catalyst having a nanosized inorganic core in a protein matrix. In some cases, the metal oxide or sulfide clusters, which can be considered as cofactors in enzymes, may be recruited for use in other related reactions in artificial photosynthetic systems. In other words, one approach to design efficient and environmentally friendly catalysts in artificial photosynthetic systems for the purpose of utilizing sunlight to generate high energy intermediates or useful material is to select and utilize inorganic cores of enzymes. For example, one of the most important goals in developing artificial photosynthesis is hydrogen production. However, first, it is necessary to find a "super catalyst" for water oxidation, which is the most challenging half reaction of water splitting. There is an efficient system for water oxidation in cyanobacteria, algae, and plants. Published data on the Mn-Ca cluster have provided details on the mechanism and structure of the water oxidizing complex as a Mn-Ca nanosized inorganic core in photosystem II. Progress has been made in introducing Mn-Ca oxides as efficient catalysts for water oxidation in artificial photosynthetic systems. Here, in the interest of designing efficient catalysts for other important reactions in artificial photosynthesis, a few examples of our knowledge of inorganic cores of proteins, and how Nature used them for important reactions, are discussed.

Photodamage of the manganese–calcium oxide: a model for UV-induced photodamage of the water oxidizing complex in photosystem II

The Mn-Ca cluster is proposed to play an important role in the ultraviolet (UV) photoinhibition of photosystem II, but the mechanism is still unknown. Here, we used Mn-Ca oxide as an important structural and functional model for the Mn-Ca cluster in photosystem II, and report the effect of UV radiation on the decomposition of the structure in the presence of organic groups. Our results show similarities between the reactions of the Mn-Ca oxide and the WOC of PSII in the presence of UV radiation.

Cyanamide route to calcium–manganese oxide foams for water oxidation

In nature, photosynthetic water oxidation is efficiently catalysed at a protein-bound μ-oxido Mn4Ca cluster. This cluster consists of earth abundant, non-toxic elements and serves as a paragon for development of synthetic catalysts. In this study we developed porous calcium-manganese oxides with a unique foam-like nanostructure prepared via a facile and robust synthetic route using cyanamide as a porogen. A series of such oxide foams annealed at different temperatures was characterized by TEM, SEM, XRD, N2 physisorption, and X-ray absorption spectroscopy (XAS) in order to correlate crystallinity, atomic structure, surface area and oxidation state of the materials with catalytic activity. Some of the resulting Ca-Mn oxides show high activity as catalysts for water oxidation in the presence of cerium(iv) ammonium nitrate as a non-oxo transfer oxidant. An amorphous calcium-manganese-oxide foam with 130 m(2) g(-1) surface area and Mn oxidation state of +3.6 was identified to be most active; its activity is superior to previously reported Ca-Mn oxides. At the atomic level, this material shares structural motifs with the biological paragon as revealed by dual-edge XAS at the Mn and Ca K-edge. Rather than nanostructure and surface area, the atomic structure of the Ca-Mn oxide and the extent of structural order appear to be crucial determinants of catalytic activity. Fully disordered low-valent Mn materials as well as high-valent but crystalline Mn-Ca oxides are unreactive. Highly disordered variants of layered manganese oxide with Ca and water molecules interfacing layer fragments are most reactive.

Influence of the PsbA1/PsbA3, Ca2+/Sr2+ and Cl−/Br− exchanges on the redox potential of the primary quinone QA in Photosystem II from Thermosynechococcus elongatus as revealed by spectroelectrochemistry

Ca2+ and Cl− ions are essential elements for the oxygen evolution activity of photosystem II (PSII). It has been demonstrated that these ions can be exchanged with Sr2+ and Br−, respectively, and that these ion exchanges modify the kinetics of some electron transfer reactions at the Mn4Ca cluster level (Ishida et al., J. Biol. Chem. 283 (2008) 13330–13340). It has been proposed from thermoluminescence experiments that the kinetic effects arise, at least in part, from a decrease in the free energy level of the Mn4Ca cluster in the S3 state though some changes on the acceptor side were also observed. Therefore, in the present work, by using thin-layer cell spectroelectrochemistry, the effects of the Ca2+/Sr2+ and Cl−/Br− exchanges on the redox potential of the primary quinone electron acceptor QA, Em(QA/QA−), were investigated. Since the previous studies on the Ca2+/Sr2+ and Cl−/Br− exchanges were performed in PsbA3-containing PSII purified from the thermophilic cyanobacterium Thermosynechococcus elongatus, we first investigated the influences of the PsbA1/PsbA3 exchange on Em(QA/QA−). Here we show that i) the Em(QA/QA−) was up-shifted by ca. +38mV in PsbA3-PSII when compared to PsbA1-PSII and ii) the Ca2+/Sr2+ exchange up-shifted the Em(QA/QA−) by ca. +27mV, whereas the Cl−/Br− exchange hardly influenced Em(QA/QA−). On the basis of the results of Em(QA/QA−) together with previous thermoluminescence measurements, the ion-exchange effects on the energetics in PSII are discussed.

Exploring the energetics of water permeation in photosystem II by multiple steered molecular dynamics simulations

The Mn4Ca cluster of the oxygen-evolving complex (OEC) of photosynthesis catalyzes the light-driven splitting of water into molecular oxygen, protons, and electrons. The OEC is buried within photosystem II (PSII), a multisubunit integral membrane protein complex, and water must find its way to the Mn4Ca cluster by moving through protein. Molecular dynamics simulations were used to determine the energetic barriers for water permeation though PSII extrinsic proteins. Potentials of mean force (PMFs) for water were derived by using the technique of multiple steered molecular dynamics (MSMD). Calculation of free energy profiles for water permeation allowed us to characterize previously identified water channels, and discover new pathways for water movement toward the Mn4Ca cluster. Our results identify the main constriction sites in these pathways which may serve as selectivity filters that restrict both the access of solutes detrimental to the water oxidation reaction and loss of Ca2+ and Cl− from the active site.

Theoretical EXAFS studies of a model of the oxygen‐evolving complex of photosystem II obtained with the quantum cluster approach

AbstractThe oxygen‐evolving complex (OEC) of photosystem II is the only natural system that can form O2 from water and sunlight and it consists of a Mn4Ca cluster. In a series of publications, Siegbahn has developed a model of the OEC with the quantum mechanical (QM) cluster approach that is compatible with available crystal structures, able to form O2 with a reasonable energetic barrier, and has a significantly lower energy than alternative models. In this investigation, we present a method to restrain a QM geometry optimization toward experimental polarized extended X‐ray absorption fine structure (EXAFS) data. With this method, we show that the cluster model is compatible with the EXAFS data and we obtain a refined cluster model that is an optimum compromise between QM and polarized EXAFS data. © 2012 Wiley Periodicals, Inc.

Light-driven oxygen production from superoxide by Mn-binding bacterial reaction centers

One of the outstanding questions concerning the early Earth is how ancient phototrophs made the evolutionary transition from anoxygenic to oxygenic photosynthesis, which resulted in a substantial increase in the amount of oxygen in the atmosphere. We have previously demonstrated that reaction centers from anoxygenic photosynthetic bacteria can be modified to bind a redox-active Mn cofactor, thus gaining a key functional feature of photosystem II, which contains the site for water oxidation in cyanobacteria, algae, and plants [Thielges M, et al. (2005) Biochemistry 44:7389-7394]. In this paper, the Mn-binding reaction centers are shown to have a light-driven enzymatic function; namely, the ability to convert superoxide into molecular oxygen. This activity has a relatively high efficiency with a k(cat) of approximately 1 s(-1) that is significantly larger than typically observed for designed enzymes, and a K(m) of 35-40 μM that is comparable to the value of 50 μM for Mn-superoxide dismutase, which catalyzes a similar reaction. Unlike wild-type reaction centers, the highly oxidizing reaction centers are not stable in the light unless they have a bound Mn. The stability and enzymatic ability of this type of Mn-binding reaction centers would have provided primitive phototrophs with an environmental advantage before the evolution of organisms with a more complex Mn(4)Ca cluster needed to perform the multielectron reactions required to oxidize water.

What spectroscopy reveals concerning the Mn oxidation levels in the oxygen evolving complex of photosystem II: X-ray to near infra-red

Photosystem II (PS II), found in oxygenic photosynthetic organisms, catalyses the most energetically demanding reaction in nature, the oxidation of water to molecular oxygen and protons. The water oxidase in PS II contains a Mn(4)Ca cluster (oxygen evolving complex, OEC), whose catalytic mechanism has been extensively investigated but is still unresolved. In particular the precise Mn oxidation levels through which the cluster cycles during functional turnover are still contentious. In this, the first of several planned parts, we examine a broad range of published data relating to this question, while considering the recent atomic resolution PS II crystal structure of Umena et al. (Nature, 2011, 473, 55). Results from X-ray, UV-Vis and NIR spectroscopies are considered, using an approach that is mainly empirical, by comparison with published data from known model systems, but with some reliance on computational or other theoretical considerations. The intention is to survey the extent to which these data yield a consistent picture of the Mn oxidation states in functional PS II - in particular, to test their consistency with two current proposals for the mean redox levels of the OEC during turnover; the so called 'high' and 'low' oxidation state paradigms. These systematically differ by two oxidation equivalents throughout the redox accumulating catalytic S state cycle (states S(0)···S(3)). In summary, we find that the data, in total, substantially favor the low oxidation proposal, particularly as a result of the new analyses we present. The low oxidation state scheme is able to resolve a number of previously 'anomalous' results in the observed UV-Visible S state turnover spectral differences and in the resonant inelastic X-ray spectroscopy (RIXS) of the Mn pre-edge region of the S(1) and S(2) states. Further, the low oxidation paradigm is able to provide a 'natural' explanation for the known sensitivity of the OEC Mn cluster to cryogenic near infra-red (NIR) induced turnover to alternative spin/redox states in S(2) and S(3).

The interaction of His337 with the Mn4Ca cluster of photosystem II

The most recent XRD studies of Photosystem II (PS II) reveal that the His337 residue is sufficiently close to the Mn(4)Ca core of the Water Oxidising Complex (WOC) to engage in H-bonding interactions with the μ(3)-oxo bridge connecting Mn(1), Mn(2) and Mn(3). Such interactions may account for the lengthening of the Mn-Mn distances observed in the most recent and highest resolution (1.9 Å) crystal structure of PS II compared to earlier, lower-resolution (2.9 Å or greater) XRD structures and EXAFS studies on functional PS II. Density functional theory is used to examine the influence on Mn-Mn distances of H-bonding interactions, mediated by the proximate His337 residue, which may lead to either partial or complete protonation of the μ(3)-oxo bridge on models of the WOC. Calculations were performed on a set of minimal-complexity models (in which WOC-ligating amino acid residues are represented as formate and imidazole ligands), and also on extended models in which a 13-peptide sequence (from His332 to Ala344) is treated explicitly. These calculations demonstrate that while the 2.9 Å structure is best described by models in which the μ(3)-oxo bridge is neither protonated nor involved in significant H-bonding, the 1.9 Å XRD structure is better reproduced by models in which the μ(3)-oxo bridge undergoes H-bonding interactions with the His337 residue leading to expansion of the 'close' Mn-Mn distances well known from EXAFS studies at ∼ 2.7 Å. Furthermore, full μ(3)-oxo-bridge protonation remains a distinct possibility during the process of water oxidation, as evidenced by the lengthening of the Mn-Mn vectors observed in EXAFS studies of the higher oxidation states of PS II. In this context, the Mn-Mn distances calculated in the protonated μ(3)-oxo bridge structures, particularly for the peptide extended models, are in close agreement with the EXAFS data.

Layered manganese oxides for water-oxidation: alkaline earth cations influence catalytic activity in a photosystem II-like fashion

In reaction sequences for light driven water-splitting into H2 and O2, water-oxidation is a crucial reaction step. In vivo, the process is catalysed within a photoenzyme called photosystem II (PSII) by a μ-oxido CaMn4 cluster, the oxygen-evolving complex (OEC). The OEC is known to be virtually inactive if Ca2+ is removed from its structure. Activity can be restored not only by the addition of Ca2+ but also Sr2+ ions. We have recently introduced layered calcium manganese oxides of the birnessite mineral family as functional synthetic model compounds for the OEC. Here, we present the syntheses of layered manganese oxides where we varied the interlayer cations, preparing a series of K-, Ca-, Sr- and Mg-containing birnessites. Structural motifs within these materials were determined using X-ray absorption spectroscopy (XAS) showing that all materials have similar atomic structures despite their different elemental compositions. Water-oxidation experiments were carried out to elucidate structure-reactivity relations. These experiments demonstrated that the oxides—like the OEC—require the presence of calcium in their structures to reach maximum catalytic activity. As another similarity to the OEC, Sr2+ is the “second best choice” for the secondary cation. The results thus support mechanistic proposals which involve an important catalytic role for Ca2+ in biological water-oxidation. Additionally, they offer valuable hints for the development of synthetic, manganese-based water-oxidation catalysts for artificial photosynthesis.

Hollandite as a Functional and Structural Model for the Biological Water Oxidizing Complex: Manganese-Calcium Oxide Minerals as a Possible Evolutionary Origin for the CaMn4 Cluster of the Biological Water Oxidizing Complex

Oxygen evolution was observed upon mixing either hollandite, which has been proposed as a structural model for the biological water oxidizing complex, or hausmannite with an aqueous solution of cerium (IV) ammonium nitrate. Oxygen evolution from water during irradiation with visible light (λ > 400 nm) was also observed upon adding either hollandite or hausmannite to an aqueous solution containing tris (2,2′-bipyridyl)ruthenium(II) chloride and chloro pentaammine cobalt(III) chloride in acetate buffer. These experiments showed that hollandite is a good catalyst for oxygen evolution in presence of cerium (IV) ammonium nitrate or tris (2,2′-bipyridyl)ruthenium (III). Thus, hollandite is not only a structural but also a functional model for the biological water oxidizing complex. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental file.

N-Formylkynurenine as a Marker of High Light Stress in Photosynthesis

Photosystem II (PSII) is the membrane protein complex that catalyzes the photo-induced oxidation of water at a manganese-calcium active site. Light-dependent damage and repair occur in PSII under conditions of high light stress. The core reaction center complex is composed of the D1, D2, CP43, and CP47 intrinsic polypeptides. In this study, a new chromophore formed from the oxidative post-translational modification of tryptophan is identified in the CP43 subunit. Tandem mass spectrometry peptide sequencing is consistent with the oxidation of the CP43 tryptophan side chain, Trp-365, to produce N-formylkynurenine (NFK). Characterization with ultraviolet visible absorption and ultraviolet resonance Raman spectroscopy supports this assignment. An optical assay suggests that the yield of NFK increases 2-fold (2.2 ± 0.5) under high light illumination. A concomitant 2.4 ± 0.5-fold decrease is observed in the steady-state rate of oxygen evolution under the high light conditions. NFK is the product formed from reaction of tryptophan with singlet oxygen, which can be produced under high light stress in PSII. Reactive oxygen species reactions lead to oxidative damage of the reaction center, D1 protein turnover, and inhibition of electron transfer. Our results are consistent with a role for the CP43 NFK modification in photoinhibition.