Covalent Immobilization of Oriented Photosystem II on a Nanostructured Electrode for Solar Water Oxidation

Photosystem II (PSII) offers a biological and sustainable route of photochemical water oxidation to O₂ and can provide protons and electrons for the generation of solar fuels, such as H₂. We present a rational strategy to electrostatically improve the orientation of PSII from a thermophilic cyanobacterium, Thermosynechococcus elongatus, on a nanostructured indium tin oxide (ITO) electrode and to covalently immobilize PSII on the electrode. The ITO electrode was modified with a self-assembled monolayer (SAM) of phosphonic acid ITO linkers with a dangling carboxylate moiety. The negatively charged carboxylate attracts the positive dipole on the electron acceptor side of PSII via Coulomb interactions. Covalent attachment of PSII in its electrostatically improved orientation to the SAM-modified ITO electrode was accomplished via an amide bond to further enhance red-light-driven, direct electron transfer and stability of the PSII hybrid photoelectrode

Photoelectrochemical Water Oxidation with Photosystem II Integrated in a Mesoporous Indium–Tin Oxide Electrode

We report on a hybrid photoanode for water oxidation consisting of a cyanobacterial photosystem II (PSII) from <i>Thermosynechococcus elongatus</i> on a mesoporous indium–tin oxide (<i>meso</i>ITO) electrode. The three-dimensional metal oxide environment allows for high protein coverage (26 times an ideal monolayer coverage) and direct (mediator-free) electron transfer from PSII to <i>meso</i>ITO. The oxidation of water occurs with 1.6 ± 0.3 μA cm^–2 and a corresponding turnover frequency of approximately 0.18 ± 0.04 (mol O₂) (mol PSII)⁻¹ s^–1 during red light irradiation. Mechanistic studies are consistent with interfacial electron transfer occurring not only from the terminal quinone Q_B, but also from the quinone Q_A through an unnatural electron transfer pathway to the ITO surface

Reversible Interconversion of CO₂ and Formate by a Molybdenum-Containing Formate Dehydrogenase

CO₂ and formate are rapidly, selectively, and efficiently interconverted by tungsten-containing formate dehydrogenases that surpass current synthetic catalysts. However, their mechanism of catalysis is unknown, and no tractable system is available for study. Here, we describe the catalytic properties of the molybdenum-containing formate dehydrogenase H from the model organism Escherichia coli (<i>Ec</i>FDH-H). We use protein film voltammetry to demonstrate that <i>Ec</i>FDH-H is a highly active, reversible electrocatalyst. In each voltammogram a single point of zero net current denotes the CO₂ reduction potential that varies with pH according to the Nernst equation. By quantifying formate production we show that electrocatalytic CO₂ reduction is specific. Our results reveal the capabilities of a Mo-containing catalyst for reversible CO₂ reduction and establish <i>Ec</i>FDH-H as an attractive model system for mechanistic investigations and a template for the development of synthetic catalysts

Selective Photocatalytic CO₂ Reduction in Water through Anchoring of a Molecular Ni Catalyst on CdS Nanocrystals

Photocatalytic conversion of CO₂ into carbonaceous feedstock chemicals is a promising strategy to mitigate greenhouse gas emissions and simultaneously store solar energy in chemical form. Photocatalysts for this transformation are typically based on precious metals and operate in nonaqueous solvents to suppress competing H₂ generation. In this work, we demonstrate selective visible-light-driven CO₂ reduction in water using a synthetic photocatalyst system that is entirely free of precious metals. We present a series of self-assembled nickel terpyridine complexes as electrocatalysts for the reduction of CO₂ to CO in organic media. Immobilization on CdS quantum dots allows these catalysts to be active in purely aqueous solution and photocatalytically reduce CO₂ with >90% selectivity under UV-filtered simulated solar light irradiation (AM 1.5G, 100 mW cm^–2, λ > 400 nm, pH 6.7, 25 °C). Correlation between catalyst immobilization efficiency and product selectivity shows that anchoring the molecular catalyst on the semiconductor surface is key in controlling the selectivity for CO₂ reduction over H₂ evolution in aqueous solution

Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO₂ as a Reversible Electron Acceptor in a TiO₂–Re Catalyst System for CO₂ Photoreduction

Attaching the phosphonated molecular catalyst [Re^IBr­(bpy)­(CO)₃]⁰ to the wide-bandgap semiconductor TiO₂ strongly enhances the rate of visible-light-driven reduction of CO₂ to CO in dimethylformamide with triethanolamine (TEOA) as sacrificial electron donor. Herein, we show by transient mid-IR spectroscopy that the mechanism of catalyst photoreduction is initiated by ultrafast electron injection into TiO₂, followed by rapid (ps-ns) and sequential two-electron oxidation of TEOA that is coordinated to the Re center. The injected electrons can be stored in the conduction band of TiO₂ on an ms-s time scale, and we propose that they lead to further reduction of the Re catalyst and completion of the catalytic cycle. Thus, the excited Re catalyst gives away one electron and would eventually get three electrons back. The function of an electron reservoir would represent a role for TiO₂ in photocatalytic CO₂ reduction that has previously not been considered. We propose that the increase in photocatalytic activity upon heterogenization of the catalyst to TiO₂ is due to the slow charge recombination and the high oxidative power of the Re^II species after electron injection as compared to the excited MLCT state of the unbound Re catalyst or when immobilized on ZrO₂, which results in a more efficient reaction with TEOA

Photoelectrochemistry of Photosystem II <i>in Vitro</i> vs <i>in Vivo</i>

Factors governing the photoelectrochemical output of photosynthetic microorganisms are poorly understood, and energy loss may occur due to inefficient electron transfer (ET) processes. Here, we systematically compare the photoelectrochemistry of photosystem II (PSII) protein-films to cyanobacteria biofilms to derive: (i) the losses in light-to-charge conversion efficiencies, (ii) gains in photocatalytic longevity, and (iii) insights into the ET mechanism at the biofilm interface. This study was enabled by the use of hierarchically structured electrodes, which could be tailored for high/stable loadings of PSII core complexes and Synechocystis sp. PCC 6803 cells. The mediated photocurrent densities generated by the biofilm were 2 orders of magnitude lower than those of the protein-film. This was partly attributed to a lower photocatalyst loading as the rate of mediated electron extraction from PSII <i>in vitro</i> is only double that of PSII <i>in vivo</i>. On the other hand, the biofilm exhibited much greater longevity (>5 days) than the protein-film (<6 h), with turnover numbers surpassing those of the protein-film after 2 days. The mechanism of biofilm electrogenesis is suggested to involve an intracellular redox mediator, which is released during light irradiation

Reaction of Thiosulfate Dehydrogenase with a Substrate Mimic Induces Dissociation of the Cysteine Heme Ligand Giving Insights into the Mechanism of Oxidative Catalysis

Thiosulfate dehydrogenases are bacterial cytochromes that contribute to the oxidation of inorganic sulfur. The active sites of these enzymes contain low-spin c-type heme with Cys–/His axial ligation. However, the reduction potentials of these hemes are several hundred mV more negative than that of the thiosulfate/tetrathionate couple (Em, +198 mV), making it difficult to rationalize the thiosulfate oxidizing capability. Here, we describe the reaction of Campylobacter jejuni thiosulfate dehydrogenase (TsdA) with sulfite, an analogue of thiosulfate. The reaction leads to stoichiometric conversion of the active site Cys to cysteinyl sulfonate (Cα-CH2-S-SO3–) such that the protein exists in a form closely resembling a proposed intermediate in the pathway for thiosulfate oxidation that carries a cysteinyl thiosulfate (Cα-CH2-S-SSO3–). The active site heme in the stable sulfonated protein displays an Em approximately 200 mV more positive than the Cys–/His-ligated state. This can explain the thiosulfate oxidizing activity of the enzyme and allows us to propose a catalytic mechanism for thiosulfate oxidation. Substrate-driven release of the Cys heme ligand allows that side chain to provide the site of substrate binding and redox transformation; the neighboring heme then simply provides a site for electron relay to an appropriate partner. This chemistry is distinct from that displayed by the Cys-ligated hemes found in gas-sensing hemoproteins and in enzymes such as the cytochromes P450. Thus, a further class of thiolate-ligated hemes is proposed, as exemplified by the TsdA centers that have evolved to catalyze the controlled redox transformations of inorganic oxo anions of sulfur