Water Ligands Regulate the Redox Leveling Mechanism of the Oxygen-Evolving Complex of the Photosystem II.

Understanding how water ligands regulate the conformational changes and functionality of the oxygen-evolving complex (OEC) in photosystem II (PSII) throughout the catalytic cycle of oxygen evolution remains a highly intriguing and unresolved challenge. In this study, we investigate the effect of water insertion (WI) on the redox state of the OEC by using the molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) hybrid methods. We find that water binding significantly reduces the free energy change for proton-coupled electron transfer (PCET) from Mn to Y, underscoring the important regulatory role of water binding, which is essential for enabling the OEC redox-leveling mechanism along the catalytic cycle.

Synthesis, crystal structure, EPR, and DFT studies of an unusually distorted vanadium(II) complex.

We report the synthesis and structure of the most highly distorted four-coordinate d ion known to date that also serves as the second known example of a bis(biphenolato) transition metal complex. We demonstrate the application of density functional theory to calculate the magnetic parameters derived from the experimental and simulated EPR spectra.

High-resolution cryo-electron microscopy structure of photosystem II from the mesophilic cyanobacterium, sp. PCC 6803.

Photosystem II (PSII) enables global-scale, light-driven water oxidation. Genetic manipulation of PSII from the mesophilic cyanobacterium sp. PCC 6803 has provided insights into the mechanism of water oxidation; however, the lack of a high-resolution structure of oxygen-evolving PSII from this organism has limited the interpretation of biophysical data to models based on structures of thermophilic cyanobacterial PSII.

Structure of a monomeric photosystem II core complex from a cyanobacterium acclimated to far-red light reveals the functions of chlorophylls d and f.

Far-red light (FRL) photoacclimation in cyanobacteria provides a selective growth advantage for some terrestrial cyanobacteria by expanding the range of photosynthetically active radiation to include far-red/near-infrared light (700-800 nm). During this photoacclimation process, photosystem II (PSII), the water:plastoquinone photooxidoreductase involved in oxygenic photosynthesis, is modified. The resulting FRL-PSII is comprised of FRL-specific core subunits and binds chlorophyll (Chl) d and Chl f molecules in place of several of the Chl a molecules found when cells are grown in visible light.

Heterogeneous Composition of Oxygen-Evolving Complexes in Crystal Structures of Dark-Adapted Photosystem II.

Photosystem II (PSII) is a homodimeric protein complex that catalyzes water oxidation at the oxygen-evolving complex (OEC), a heterocubanoid calcium-tetramanganese cluster. Here, we analyze the omit electron density peaks of the OEC's metal ions in five X-ray free-electron laser PSII structures at resolutions between 2.15 and 1.

Three overlooked photosynthesis papers of Otto Warburg (1883-1970), published in the 1940s in German and in Russian, on light-driven water oxidation coupled to benzoquinone reduction.

After a brief background on Otto Heinrich Warburg (1883-1970), and some of his selected research, we provide highlights, in English, of three of his papers in the 1940s-unknown to many as they were not originally published in English. They are: two brief reports on Photosynthesis, with Wilhelm Lüttgens, originally published in German, in 1944: 'Experiment on assimilation of carbonic acid'; and 'Further experiments on carbon dioxide assimilation'. This is followed by a regular paper, originally published in Russian, in 1946: 'The photochemical reduction of quinone in green granules'.

Studies of iron(II) and iron(III) complexes with fac-N2O, cis-N2O2 and N2O3 donor ligands: models for the 2-His 1-carboxylate motif of non-heme iron monooxygenases.

Enzymes in the oxygen-activating class of mononuclear non-heme iron oxygenases (MNOs) contain a highly conserved iron center facially ligated by two histidine nitrogen atoms and one carboxylate oxygen atom that leave one face of the metal center (three binding sites) open for coordination to cofactor, substrate, and/or dioxygen. A comparative family of [Fe(II/III)(N(2)O(n))(L)(4-n))](±x), n = 1-3, L = solvent or Cl(-), model complexes, based on a ligand series that supports a facially ligated N,N,O core that is then modified to contain either one or two additional carboxylate chelate arms, has been structurally and spectroscopically characterized. EPR studies demonstrate that the high-spin d(5) Fe(III)g = 4.

13C ENDOR reveals that the D1 polypeptide C-terminus is directly bound to Mn in the photosystem II oxygen evolving complex.

Antiferromagnetically coupled Mn(III)Mn(IV) dimers have been commonly used to study biological systems that exhibit complex exchange interactions. Such is the case for the oxygen evolving complex (OEC) in photosystem II (PSII), where we have studied whether the C-terminal carboxylate of D1-Ala344 is directly bound to the Mn cluster. To probe these protein-derived carboxylate hyperfine interactions, which give direct bonding information, Q-band (34 GHz) Mims ENDOR was performed on a Mn(III)Mn(IV) dimer ([Mn(III)Mn(IV)(mu-O)(2)mu-OAc(TACN)(2)](BPh(4))(2)) (1) that was labeled with (13)C (I = (1)/(2)) at the carboxylate position of the acetate bridge.

Illumination with ultraviolet or visible light induces chemical changes in the water-soluble manganese complex, [Mn(4)O(6)(bpea)(4)]Br(4).

We measured the photosensitivity of an artificial tetranuclear oxo-Mn(IV) complex, [Mn(4)O(6)(bpea)(4)]Br(4), which has an adamantane-shaped {Mn(4)O(6)}(4+) core. Illumination caused changes in the absorption spectrum of the compound consistent with a one-electron reduction in the compound. Bromide appears to be the most probable electron donor in the reaction system.

Tuning tetranuclear manganese-oxo core electronic properties: adamantane-shaped complexes synthesized by ligand exchange.

A series of adamantane-shaped [Mn4O6]4+ aggregates has been prepared. Ligand substitution reactions of [Mn4O6(bpea)4](ClO4)4 (1) with tridentate amine and iminodicarboxylate ligands in acetonitrile affords derivative clusters [Mn4O6(tacn)4](ClO4)4 (4), [Mn4O6(bpea)2(dien)2](ClO4)4)(5), [Mn4O6(Medien)4](ClO4)4 (6), [Mn4O6(tach)4](ClO4)4 (7), [Mn4O6(bpea)2(me-ida)2] (8), [Mn4O6(bpea)2(bz-ida)2] (9), [Mn4O6(bpea)2((t)bu-ida)2] (10), and [Mn4O6(bpea)2((c)pent-ida)2] (11) generally on the order of 10 min with retention of core nuclearity and oxidation state. Of these complexes, only 4 had been synthesized previously.

Shape-shifting tetranuclear oxo-bridged manganese cluster: relevance to photosystem II water oxidase active site.

The redox properties of the "dimer-of-dimers" complex, [{Mn2(mu-O)2(tphpn)}2]4+ (1) (where Htphpn = N,N,N',N'-tetra(2-methylpyridyl)-2-hydroxypropane-diamine) were investigated. The structure changes dramatically to an adamantane-shaped core upon one-electron oxidation. On the other hand, the one-electron reduced product of 1, [Mn4O4(tphpn)2]3+, exhibits a hyperfine-structured multiline EPR signal very similar to the so-called S0 state of the tetramanganese cluster, which resides at the Photosystem II water oxidase active site.

Chloride ligation in inorganic manganese model compounds relevant to photosystem II studied using X-ray absorption spectroscopy.

Chloride ions are essential for proper function of the photosynthetic oxygen-evolving complex (OEC) of Photosystem II (PS II). Although proposed to be directly ligated to the Mn cluster of the OEC, the specific structural and mechanistic roles of chloride remain unresolved. This study utilizes X-ray absorption spectroscopy (XAS) to characterize the Mn-Cl interaction in inorganic compounds that contain structural motifs similar to those proposed for the OEC.

A new class of oxo-bridged high-valent hexamanganese clusters supported by sterically hindered carboxylate ligands.

A new class of oxo-bridged high-valent hexamanganese (Mn6) clusters containing a novel (Mn6O8)6+ core, [MnIV(4)MnIII2(mu-O)4(mu3-O)4(dmb)6(O2CR)2]4+ (where dmb=4,4'-dimethyl-2,2'-bipyridine, and RCO2=2,6-di(p-tolyl)benzoate (Ar(Tol)CO2-) (3) or 2,6-di(4-tert-butylphenyl)benzoate (Ar(4-tBuPh)CO2-) (4)), was synthesized using sterically hindered m-terphenyl-derived carboxylate ligands. These complexes can be synthesized by oxidizing the MnII mononuclear complexes, [Mn(dmb)2(OH2)(O2CR)]+ (where RCO2=Ar(Tol)CO2- (1) or Ar(4-tBuPh)CO2- (2)) with (n-Bu4N)MnO4, by direct Mn(II) + Mn(VII) in situ comproportionation reactions, or by ligand substitution on the dinuclear manganese (III,IV) or (IV,IV) complexes, [(Mn2(mu-O)2(dmb)4)](3+/4+). The compound [MnIV4MnIII2(mu-O)4(mu3-O)4(dmb)6(Ar(Tol)CO2)2](OTf)4 [3(OTf)4] crystallizes in the monoclinic space group P2(1)/n, with the cell parameters a=15.

Multinuclear oxo-bridged manganese complexes with a bulky substituted benzoate ligand: novel species obtained by using steric control.

A sterically hindered carboxylate ligand is used to synthesize the first transition metal complex containing both bis-mu-oxo and bis-mu-carboxylato groups, [Mn2(mu-O)2(mu-ArtolCO2)2(bpy)2]+. However, methyl substitution on the chelating bipyridine ligand results in the formation of a strikingly different and novel hexanuclear species, [Mn6(mu-O)4(mu3-O)4(mu-ArtolCO2)2(dmb)6]4+. Steric interactions between the bridging carboxylates and chelating pyridine-based ligands determine the nuclearity of the complexes formed.

FTIR spectra and normal-mode analysis of a tetranuclear manganese adamantane-like complex in two electrochemically prepared oxidation states: relevance to the oxygen-evolving complex of photosystem II.

The IR spectra and normal-mode analysis of the adamantane-like compound [Mn(4)O(6)(bpea)(4)](n+) (bpea = N,N-bis(2-pyridylmethyl)ethylamine) in two oxidation states, Mn(IV)(4) and Mn(III)Mn(IV)(3), that are relevant to the oxygen-evolving complex of photosystem II are presented. Mn-O vibrational modes are identified with isotopic exchange, (16)O-->(18)O, of the mono-micro-oxo bridging atoms in the complex. IR spectra of the Mn(III)Mn(IV)(3) species are obtained by electrochemical reduction of the Mn(IV)(4) species using a spectroelectrochemical cell, based on attenuated total reflection [Visser, H.

Toward synthetic models for high oxidation state forms of the photosystem II active site metal cluster: the first tetranuclear manganese cluster containing a [Mn4(mu-O)5]6+ core.

The first tetrameric high valent manganese complex consisting of a MnIV4(mu-O)5 bridged core, [Mn4(mu-O)5(dmb)4(dmbO)2](ClO4)4, [symbol: see text] was isolated via dimanganese (III,IV) and (IV,IV) intermediates in presence of the oxidant tert-butyl hydroperoxide and was characterized by X-ray crystallography, electrochemistry, infrared, UV-visible, 1H NMR, and mass spectroscopy; the structure found differs greatly from a proposal for the putative Mn4O5 aggregate found in Photosystem II.

Proton NMR Study of Oxo-Bridged Dimanganese(III) Complexes: Solution State Structures and an Isotropic Shift/Magnetic Exchange Correlation.

The (1)H NMR spectra of a series of manganese-oxo aggregates have been examined, and a characteristic signature was found for each complex. For the dimanganese(III,III) complexes [Mn(2)O(OAc)(2)(HB(pz)(3))(2)], [Mn(2)O(OAc)(2)(tacn)(2)](2+), [Mn(2)O(OAc)(2)(H(2)O)(2)(bpy)(2)](2+), and [Mn(2)O(OAc)(2)(bpta)(2)](2+) (HB(pz)(3) = hydrotris(pyrazol-1-yl)borate; tacn = 1,4,7-triazacyclononane; bpy = 2,2'-bipyridine, and bpta = N,N-bis(2-pyridylmethyl)-tert-butylamine), the (1)H NMR spectra reveal a resonance associated with acetate, found downfield between 58 and 80 ppm, and a generally well resolved set of terminal ligand resonances which can be divided into two classes: those resonances associated with pyridyl or pyrazolyl ring protons and those of methylene groups. A number of the pyridine ring resonances have been unambiguously assigned by the examination of methyl-substituted derivatives.