Investigation of the Inhibitory Effect of Nitrite on Photosystem II

The role of chloride in photosystem II (PSII) is unclear. Several monovalent anions compete for the Cl^– site(s) in PSII, and some even support activity. NO₂^– has been reported to be an activator in Cl^–-depleted PSII membranes. In this paper, we report a detailed investigation of the chemistry of NO₂^– with PSII. NO₂^– is shown to inhibit PSII activity, and the effects on the donor side as well as the acceptor side are characterized using steady-state O₂-evolution assays, electron paramagnetic resonance (EPR) spectroscopy, electron-transfer assays, and flash-induced polarographic O₂ yield measurements. Enzyme kinetics analysis shows multiple sites of NO₂^– inhibition in PSII with significant inhibition of oxygen evolution at <5 mM NO₂^–. By EPR spectroscopy, the yield of the S₂ state remains unchanged up to 15 mM NO₂^–. However, the S₂-state <i>g</i> = 4.1 signal is favored over the <i>g</i> = 2 multiline signal with increasing NO₂^– concentrations. This could indicate competition of NO₂^– for the Cl^– site at higher NO₂^– concentrations. In addition to the donor-side chemistry, there is clear evidence of an acceptor-side effect of NO₂^–. The <i>g</i> = 1.9 Fe­(II)-Q_A^–• signal is replaced by a broad <i>g</i> = 1.6 signal in the presence of NO₂^–. Additionally, a <i>g</i> = 1.8 Fe­(II)-Q^–• signal is present in the dark, indicating the formation of a NO₂^–-bound Fe­(II)-Q_B^–• species in the dark. Electron-transfer assays suggest that the inhibitory effect of NO₂^– on the activity of PSII is largely due to the donor-side chemistry of NO₂^–. UV–visible spectroscopy and flash-induced polarographic O₂ yield measurements indicate that NO₂^– is oxidized by the oxygen-evolving complex in the higher S states, contributing to the donor-side inhibition by NO₂^–

Insights into Substrate Binding to the Oxygen-Evolving Complex of Photosystem II from Ammonia Inhibition Studies

Water oxidation in Photosystem II occurs at the oxygen-evolving complex (OEC), which cycles through distinct intermediates, S₀–S₄. The inhibitor ammonia selectively binds to the S₂ state at an unresolved site that is not competitive with substrate water. By monitoring the yields of flash-induced oxygen production, we show that ammonia decreases the net efficiency of OEC turnover and slows the decay kinetics of S₂ to S₁. The temperature dependence of biphasic S₂ decay kinetics provides activation energies that do not vary in control and ammonia conditions. We interpret our data in the broader context of previous studies by introducing a kinetic model for both the formation and decay of ammonia-bound S₂. The model predicts ammonia binds to S₂ rapidly (<i>t</i>_1/2 = 1 ms) with a large equilibrium constant. This finding implies that ammonia decreases the reduction potential of S₂ by at least 2.7 kcal mol^–1 (>120 mV), which is not consistent with ammonia substitution of a terminal water ligand of Mn­(IV). Instead, these data support the proposal that ammonia binds as a bridging ligand between two Mn atoms. Implications for the mechanism of O–O bond formation are discussed

One-Step Trimethylstannylation of Benzyl and Alkyl Halides

Trialkylstannanes are good leaving groups that have been used for the formation of carbon–metal bonds to electrode surfaces for analyses of single-molecule conductivity. Here, we report the multistep synthesis of two amide-containing compounds that are of interest in studies of molecular rectifiers. Each molecule has two trimethylstannyl units, one linked by a methylene and the other by an ethylene group. To account for the very different reactivities of the parent halides, a new methodology for one-step trimethylstannylation was developed and optimized

Cation Effects on the Electron-Acceptor Side of Photosystem II

The normal pathway of electron transfer on the electron-acceptor side of photosystem II (PSII) involves electron transfer from quinone A, Q_A, to quinone B, Q_B. It is possible to redirect electrons from Q_A^– to water-soluble Co^III complexes, which opens a new avenue for harvesting electrons from water oxidation by immobilization of PSII on electrode surfaces. Herein, the kinetics of electron transfer from Q_A^– to [Co­(III)­(terpy)₂]³⁺ (terpy = 2,2′;6′,2″-terpyridine) are investigated with a spectrophotometric assay revealing that the reaction follows Michaelis–Menten saturation kinetics, is inhibited by cations, and is not affected by variation of the Q_A reduction potential. A negatively charged site on the stromal surface of the PSII protein complex, composed of glutamic acid residues near Q_A, is hypothesized to bind cations, especially divalent cations. The cations are proposed to tune the redox properties of Q_A through electrostatic interactions. These observations may thus explain the molecular basis of the effect of divalent cations like Ca²⁺, Sr²⁺, Mg²⁺, and Zn²⁺ on the redox properties of the quinones in PSII, which has previously been attributed to long-range conformational changes propagated from divalent cations binding to the Ca­(II)-binding site in the oxygen-evolving complex on the lumenal side of the PSII complex

Probing the Effect of Mutations of Asparagine 181 in the D1 Subunit of Photosystem II

Efficient proton removal from the oxygen-evolving complex (OEC) of photosystem II (PSII) and activation of substrate water molecules are some of the key aspects optimized in the OEC for high turnover rates. The hydrogen-bonding network around the OEC is critical for efficient proton transfer and for tuning the position and p<i>K</i>_a values of the substrate water/hydroxo/oxo molecules. The D1-N181 residue is part of the hydrogen-bonding network on the active face of the OEC. D1-N181 is also associated with the chloride ion in the D2-K317 site and is one of the residues closest to a putative substrate water molecule bound as a terminal ligand to Mn4. We studied the effect of the D1-N181A and D1-N181S mutations on the water oxidation chemistry at the OEC. PSII core complexes isolated from the D1-N181A and D1-N181S mutants have steady-state O₂ evolution rates lower than those of wild-type PSII core complexes. Fourier transform infrared spectroscopy indicates slight perturbations of the hydrogen-bonding network in D1-N181A and D1-N181S PSII core complexes, similar to the effects of some other mutations in the same region, but to a lesser extent. Unlike in wild-type PSII core complexes, a <i>g</i> = 4 signal was observed in the S₂-state EPR spectra of D1-N181A and D1-N181S PSII core complexes in addition to the normal <i>g</i> = 2 multiline signal. The S-state cycling of D1-N181A and D1-N181S PSII core complexes was similar to that of wild-type PSII core complexes, whereas the O₂-release kinetics of D1-N181A and D1-N181S PSII core complexes were much slower than the O₂-release kinetics of wild-type PSII core complexes. On the basis of these results, we conclude that proton transfer is not compromised in the D1-N181A and D1-N181S mutants but that the O–O bond formation step is retarded in these mutants

Water Oxidation Catalyzed by the Tetranuclear Mn Complex [Mn^IV₄O₅(terpy)₄(H₂O)₂](ClO₄)₆

The tetranuclear manganese complex [Mn^IV₄O₅(terpy)₄(H₂O)₂]­(ClO₄)₆ (<b>1</b>; terpy = 2,2′:6′,2″-terpyridine) gives catalytic water oxidation in aqueous solution, as determined by electrochemistry and GC-MS. Complex <b>1</b> also exhibits catalytic water oxidation when adsorbed on kaolin clay, with Ce^IV as the primary oxidant. The redox intermediates of complex <b>1</b> adsorbed on kaolin clay upon addition of Ce^IV have been characterized by using diffuse reflectance UV/visible and EPR spectroscopy. One of the products in the reaction on kaolin clay is Mn^III, as determined by parallel-mode EPR spectroscopic studies. When <b>1</b> is oxidized in aqueous solution with Ce^IV, the reaction intermediates are unstable and decompose to form Mn^II, detected by EPR spectroscopy, and MnO₂. DFT calculations show that the oxygen in the mono-μ-oxo bridge, rather than Mn^IV, is oxidized after an electron is removed from the Mn­(IV,IV,IV,IV) tetramer. On the basis of the calculations, the formation of O₂ is proposed to occur by reaction of water with an electrophilic manganese-bound oxyl radical species, ^•O–Mn₂^IV/IV, produced during the oxidation of the tetramer. This study demonstrates that [Mn^IV₄O₅(terpy)₄(H₂O)₂]­(ClO₄)₆ may be relevant for understanding the role of the Mn tetramer in photosystem II

An Anionic N‑Donor Ligand Promotes Manganese-Catalyzed Water Oxidation

Four manganese complexes of pentadentate ligands have been studied for their ability to act as oxygen evolution catalysts in the presence of Oxone or hydrogen peroxide. The complexes [Mn­(PaPy₃)­(NO₃)]­(ClO₄) (<b>1</b>) (PaPy₃H = <i>N</i>,<i>N</i>-bis­(2-pyridylmethyl)-amine-<i>N</i>-ethyl-2-pyridine-2-carboxamide) and [Mn­(PaPy₃)­(μ-O)­(PaPy₃)­Mn]­(ClO₄)₂ (<b>2</b>) feature an anionic carboxamido ligand <i>trans</i> to the labile sixth coordination site, while [Mn­(N4Py)­OTf]­(OTf) (<b>3</b>) (N4Py = <i>N</i>,<i>N</i>-bis­(2-pyridylmethyl)-<i>N</i>-bis­(2-pyridyl)­methylamine) and [Mn­(PY5)­(OH₂)]­(ClO₄)₂ (<b>4</b>) (PY5 = 2,6-bis­(bis­(2-pyridyl)­methoxymethane)-pyridine) have neutral ligands of varying flexibility. <b>1</b> and <b>2</b> are shown to evolve oxygen in the presence of either Oxone or hydrogen peroxide, but <b>3</b> evolves oxygen only in the presence of hydrogen peroxide. <b>4</b> is inactive. The activity of <b>1</b> and <b>2</b> with Oxone suggests that the presence of an anionic N-donor ligand plays a role in stabilizing putative high-valent intermediates. Anionic N-donor ligands may be viewed as alternatives to μ-oxo ligands that are prone to protonation in low-valent Mn species formed during a catalytic cycle, resulting in loss of catalyst structure

An Anionic N‑Donor Ligand Promotes Manganese-Catalyzed Water Oxidation

Four manganese complexes of pentadentate ligands have been studied for their ability to act as oxygen evolution catalysts in the presence of Oxone or hydrogen peroxide. The complexes [Mn­(PaPy₃)­(NO₃)]­(ClO₄) (<b>1</b>) (PaPy₃H = <i>N</i>,<i>N</i>-bis­(2-pyridylmethyl)-amine-<i>N</i>-ethyl-2-pyridine-2-carboxamide) and [Mn­(PaPy₃)­(μ-O)­(PaPy₃)­Mn]­(ClO₄)₂ (<b>2</b>) feature an anionic carboxamido ligand <i>trans</i> to the labile sixth coordination site, while [Mn­(N4Py)­OTf]­(OTf) (<b>3</b>) (N4Py = <i>N</i>,<i>N</i>-bis­(2-pyridylmethyl)-<i>N</i>-bis­(2-pyridyl)­methylamine) and [Mn­(PY5)­(OH₂)]­(ClO₄)₂ (<b>4</b>) (PY5 = 2,6-bis­(bis­(2-pyridyl)­methoxymethane)-pyridine) have neutral ligands of varying flexibility. <b>1</b> and <b>2</b> are shown to evolve oxygen in the presence of either Oxone or hydrogen peroxide, but <b>3</b> evolves oxygen only in the presence of hydrogen peroxide. <b>4</b> is inactive. The activity of <b>1</b> and <b>2</b> with Oxone suggests that the presence of an anionic N-donor ligand plays a role in stabilizing putative high-valent intermediates. Anionic N-donor ligands may be viewed as alternatives to μ-oxo ligands that are prone to protonation in low-valent Mn species formed during a catalytic cycle, resulting in loss of catalyst structure

Mapping the Oxygens in the Oxygen-Evolving Complex of Photosystem II by Their Nucleophilicity Using Quantum Descriptors

The oxygen-evolving complex (OEC) of Photosystem II catalyzes the water-splitting reaction using solar energy. Thus, understanding the reaction mechanism will inspire the design of biomimetic artificial catalysts that convert solar energy to chemical energy. Conceptual Density Functional Theory (CDFT) focuses on understanding the reactivity of molecules and the atomic contribution to the overall nucleophilicity and electrophilicity of the molecule using quantum descriptors. However, this method has not been applied to the OEC before. Here, we use Fukui functions and the dual descriptor to provide quantitative measures of the nucleophilicity and electrophilicity of oxygens in the OEC for different models in different S states. Our results show that the μ-oxo bridges connected to terminal Mn4 are nucleophilic, and those in the cube formed by Mn1, Mn2, and Mn3 are mostly electrophilic. The dual descriptors of the bridging oxygens in the OEC showed a similar reactivity to that of bridging oxygens in Mn model compounds. However, the terminal water W1, which is bound to Mn4, showed very strong reactivity in some of the S3 models. Thus, our calculations support the model that proposes the formation of the O2 molecule through nucleophilic attack by a terminal water

Mapping the Oxygens in the Oxygen-Evolving Complex of Photosystem II by Their Nucleophilicity Using Quantum Descriptors

The oxygen-evolving complex (OEC) of Photosystem II catalyzes the water-splitting reaction using solar energy. Thus, understanding the reaction mechanism will inspire the design of biomimetic artificial catalysts that convert solar energy to chemical energy. Conceptual Density Functional Theory (CDFT) focuses on understanding the reactivity of molecules and the atomic contribution to the overall nucleophilicity and electrophilicity of the molecule using quantum descriptors. However, this method has not been applied to the OEC before. Here, we use Fukui functions and the dual descriptor to provide quantitative measures of the nucleophilicity and electrophilicity of oxygens in the OEC for different models in different S states. Our results show that the μ-oxo bridges connected to terminal Mn4 are nucleophilic, and those in the cube formed by Mn1, Mn2, and Mn3 are mostly electrophilic. The dual descriptors of the bridging oxygens in the OEC showed a similar reactivity to that of bridging oxygens in Mn model compounds. However, the terminal water W1, which is bound to Mn4, showed very strong reactivity in some of the S3 models. Thus, our calculations support the model that proposes the formation of the O2 molecule through nucleophilic attack by a terminal water