Biological water-oxidizing complex: a nano-sized manganese-calcium oxide in a protein environment

Innovative Insights into Water-Oxidation Mechanism: Investigating Birnessite's Reaction with Cerium(IV) Ammonium Nitrate

Developing Mn-based water-oxidation reaction (WOR) catalysts is key for renewable energy storage, utilizing Mn's abundance, cost-effectiveness, and natural role. Cerium(IV) ammonium nitrate (CAN) has been widely utilized as a sacrificial oxidant in the exploration of WOR catalysts. In this study, advanced techniques, such as X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and in situ electron paramagnetic resonance (EPR), to delve into the WOR facilitated by CAN and birnessite were employed. XANES analysis has demonstrated that the average oxidation states (AOSs) of Mn in birnessite, a birnessite/CAN mixture, and in the birnessite/CAN mixture postwater addition are 3.7, 3.8, and 3.9, respectively. In situ Raman spectroscopy performed in the presence of birnessite and CAN revealed a distinct peak at 784 cm-1, which is attributed to Mn(IV)═O. A shift of this peak to 769 cm-1 in H218O confirms its association with Mn(IV)═O. No change in this peak was observed in D2O, further supporting the notion that it is linked to Mn(IV)═O rather than Mn-OH (D). Furthermore, EPR spectroscopy shows the presence of Mn(IV). It is suggested that the WOR mechanism initiates with the oxidation of birnessite by CAN, which enhances the concentration of Mn(IV) sites in the birnessite structure. Under acidic conditions, birnessite, enriched in Mn(IV), facilitates oxygen evolution and subsequently transitions into a form with reduced Mn(IV) levels. This process highlights the critical function of the Mn (hydr)oxide structure, similar to its role in the water-oxidizing complex of Photosystem II, where it serves as charge storage for oxidizing equivalents from CAN, paving the way for a four-electron reaction that drives the WOR.

Alloying with Mn Enhances the Activity and Durability of the CoPt Catalyst toward the Methanol Oxidation Reaction

To improve the catalytic performance and durability of Pt catalysts used for the methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs), alloying of Pt with other transition metals such as Ru, Co, Ni, and Fe is considered an effective approach. Despite the significant progress made in the preparation of bimetallic alloys and their utilization for MOR, improving the activity and durability of the catalysts to make them commercially viable remains a stiff challenge. In this work, trimetallic Pt_100-x(MnCo)_x (16 < x < 41) catalysts were successfully synthesized via borohydride reduction followed by hydrothermal treatment at 150 °C. The electrocatalytic performance of the synthesized trimetallic Pt_100-x(MnCo)_x (16 < x < 41) catalysts toward MOR was studied using cyclic voltammetry and chronoamperometry. The results affirm that all Pt_100-x(MnCo)_x (16 < x < 41) alloys have superior MOR activity and durability as compared to bimetallic PtCo alloys and commercially available Pt/C (comm. Pt/C) catalysts. Among all the compositions studied, the Pt₆₀Mn_1.7Co_38.3/C catalyst exhibited superior mass activity (1.3 and 1.9 times higher than those of Pt₈₁Co₁₉/C and comm. Pt/C, respectively) toward MOR. Furthermore, all the newly synthesized Pt_100-x(MnCo)_x/C (16 < x < 41) catalysts showed better CO tolerance when compared with comm. Pt/C. This improved performance of the Pt_100-x(MnCo)_x/C (16 < x < 41) catalyst can be attributed to the synergistic effect of Co and Mn on the Pt lattice.

Honoring two stalwarts of photosynthesis research: Eva-Mari Aro and Govindjee

On behalf of the entire photosynthesis community, it is an honor, for us, to write about two very eminent scientists who were recently recognised with a Lifetime Achievement Award from the International Society of Photosynthesis Research (ISPR) on August 5, 2022; this prestigious Award was given during the closing ceremony of the 18th International Congress on Photosynthesis Research in Dunedin, New Zealand. The awardees were: Professor Eva-Mari Aro (Finland) and Professor Emeritus Govindjee Govindjee (USA). One of the authors, Anjana Jajoo, is especially delighted to be a part of this tribute to professors Aro and Govindjee as she was lucky enough to have worked with both of them.

Role of Pt and PtO2 in the Oxygen-Evolution Reaction in the Presence of Iron under Alkaline Conditions

The oxygen-evolution reaction (OER) through water oxidation is an inevitable reaction for water splitting toward storing energy. However, OER is a four-electron and slow reaction, which is also a bottleneck for water splitting. To find the role of Pt and PtO₂ on the OER in the presence of Fe, the electrochemistry of Pt foil and PtO₂ is investigated in the absence/presence of K₂FeO₄ as a soluble Fe salt at pH ≈ 13. After the addition of K₂FeO₄, a remarkable increase in the OER is recorded in the presence of Pt or PtO₂. The obtained catalysts were characterized by operando visible spectroscopy, high-resolution transmission electron microscopy, scanning electron microscopy, electron-spin resonance spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and electrochemical methods. KOH solutions usually contain Fe and/or Ni impurities. It is found that neither Pt nor PtO₂ is an OER catalyst in a Ni/Fe-free KOH, and even at an overpotential of 570 mV in purified KOH (pH ≈ 13), no clear OER was observed.

An enzyme kinetics study of the pH dependence of chloride activation of oxygen evolution in photosystem II

Oxygen evolution by photosystem II (PSII) involves activation by Cl^- ion, which is regulated by extrinsic subunits PsbQ and PsbP. In this study, the kinetics of chloride activation of oxygen evolution was studied in preparations of PSII depleted of the PsbQ and PsbP subunits (NaCl-washed and Na₂SO₄/pH 7.5-treated) over a pH range from 5.3 to 8.0. At low pH, activation by chloride was followed by inhibition at chloride concentrations >100 mM, whereas at high pH activation continued as the chloride concentration increased above 100 mM. Both activation and inhibition were more pronounced at lower pH, indicating that Cl^- binding depended on protonation events in each case. The simplest kinetic model that could account for the complete data set included binding of Cl^- at two sites, one for activation and one for inhibition, and four protonation steps. The intrinsic (pH-independent) dissociation constant for Cl^- activation, K _S, was found to be 0.9 ± 0.2 mM for both preparations, and three of the four pK _as were determined, with the fourth falling below the pH range studied. The intrinsic inhibition constant, K _I, was found to be 64 ± 2 and 103 ± 7 mM for the NaCl-washed and Na₂SO₄/pH7.5-treated preparations, respectively, and is considered in terms of the conditions likely to be present in the thylakoid lumen. This enzyme kinetics analysis provides a more complete characterization of chloride and pH dependence of O₂ evolution activity than has been previously presented.

A nanosized Mn oxide/boron nitride composite as a catalyst for water oxidation

A nanosized Mn oxide/boron nitride composite is reported as a catalyst for water oxidation.

A very simple and high-yield method to synthesize nanolayered Mn oxide

Nanolayered Mn oxides have been prepared by a very simple, low-cost and high-yield method using soap, KOH, MnCl2 and H2O2. Scanning electron microscopy, transmission electron microscopy, dynamic light scattering, thermogravimetric analysis, Fourier transform infrared spectroscopy, and X-ray diffraction spectrometry have been used to characterize the phase and the morphology of the nanolayered Mn oxide. The nanolayered Mn oxide shows good catalytic activity toward water oxidation in the presence of cerium(iv) ammonium nitrate.

Nanolayered manganese oxide/C(60) composite: a good water-oxidizing catalyst for artificial photosynthetic systems

For the first time, we considered Mn oxide/C60 composites as water-oxidizing catalysts. The composites were synthesized by easy and simple procedures, and characterized by some methods. The water-oxidizing activities of these composites were also measured in the presence of cerium(iv) ammonium nitrate. We found that the nanolayered Mn oxide/C60 composites show promising activity toward water oxidation.

The role of nano-sized manganese oxides in the oxygen-evolution reactions by manganese complexes: towards a complete picture

Eighteen Mn complexes with N-donor and carboxylate ligands have been synthesized and characterized. Three Mn complexes among them are new and are reported for the first time. The reactions of oxygen evolution in the presence of oxone (2KHSO5·KHSO4·K2SO4) and cerium(iv) ammonium nitrate catalyzed by these complexes are studied and characterized by UV-visible spectroscopy, X-ray diffraction spectrometry, dynamic light scattering, Fourier transform infrared spectroscopy, electron paramagnetic resonance spectroscopy, transmission electron microscopy, scanning electron microscopy, membrane-inlet mass spectrometry and electrochemistry. Some of these complexes evolve oxygen in the presence of oxone as a primary oxidant. CO2 and MnO4(-) are other products of these reactions. Based on spectroscopic studies, the true catalysts for oxygen evolution in these reactions are different. We proposed that for the oxygen evolution reactions in the presence of oxone, the true catalysts are both high valent Mn complexes and Mn oxides, but for the reactions in the presence of cerium(iv) ammonium nitrate, the active catalyst is most probably a Mn oxide.

Nanostructured manganese oxide/carbon nanotubes, graphene and graphene oxide as water-oxidizing composites in artificial photosynthesis

Herein, we report on nano-sized Mn oxide/carbon nanotubes, graphene and graphene oxide as water-oxidizing compounds in artificial photosynthesis. The composites are synthesized by different and simple procedures and characterized by a number of methods. The water-oxidizing activities of these composites are also considered in the presence of cerium(IV) ammonium nitrate. Some composites are efficient Mn-based catalysts with TOF (mmol O2 per mol Mn per second) ~ 2.6.

A nano-sized manganese oxide in a protein matrix as a natural water-oxidizing site

The purpose of this review is to present recent advances in the structural and functional studies of water-oxidizing center of Photosystem II and its surrounding protein matrix in order to synthesize artificial catalysts for production of clean and efficient hydrogen fuel.

Action and target sites of nitric oxide in chloroplasts

Nitric oxide (NO) is an important signalling molecule in plants under physiological and stress conditions. Here we review the influence of NO on chloroplasts which can be directly induced by interaction with the photosynthetic apparatus by influencing photophosphorylation, electron transport activity and oxido-reduction state of the Mn clusters of the oxygen-evolving complex or by changes in gene expression. The influence of NO-induced changes in the photosynthetic apparatus on its functions and sensitivity to stress factors are discussed.

Water exchange in manganese-based water-oxidizing catalysts in photosynthetic systems: from the water-oxidizing complex in photosystem II to nano-sized manganese oxides

The water-oxidizing complex (WOC), also known as the oxygen-evolving complex (OEC), of photosystem II in oxygenic photosynthetic organisms efficiently catalyzes water oxidation. It is, therefore, responsible for the presence of oxygen in the Earth's atmosphere. The WOC is a manganese-calcium (Mn₄CaO₅(H₂O)₄) cluster housed in a protein complex. In this review, we focus on water exchange chemistry of metal hydrates and discuss the mechanisms and factors affecting this chemical process. Further, water exchange rates for both the biological cofactor and synthetic manganese water splitting are discussed. The importance of fully unveiling the water exchange mechanism to understand the chemistry of water oxidation is also emphasized here. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Nano-sized layered manganese oxide in a poly-L-glutamic acid matrix: a biomimetic, homogenized, heterogeneous structural model for the water-oxidizing complex in photosystem II

Here, we report a nano-sized layered Mn–Ca oxide in poly-L-glutamic acid as a structural model for a biological water-oxidizing site in plants, algae and cyanobacteria.

Mn oxide/nanodiamond composite: a new water-oxidizing catalyst for water oxidation

Herein, we reported nanosized Mn oxide/nanodiamond composites as water-oxidizing compounds.

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.

Imidazolium or guanidinium/layered manganese (III, IV) oxide hybrid as a promising structural model for the water-oxidizing complex of Photosystem II for artificial photosynthetic systems

Photosystem II is responsible for the light-driven biological water-splitting system in oxygenic photosynthesis and contains a cluster of one calcium and four manganese ions at its water-oxidizing complex. This cluster may serve as a model for the design of artificial or biomimetic systems capable of splitting water into oxygen and hydrogen. In this study, we consider the ability of manganese oxide monosheets to self-assemble with organic compounds. Layered structures of manganese oxide, including guanidinium and imidazolium groups, were synthesized and characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction spectrometry, and atomic absorption spectroscopy. The compounds can be considered as new structural models for the water-oxidizing complex of Photosystem II. The overvoltage of water oxidation for the compounds in these conditions at pH = 6.3 is ~0.6 V. These compounds may represent the first step to synthesize a hybrid of guanidinium or imidazole together with manganese as a biomimetic system for the water-oxidizing complex of Photosystem II.

Gold or silver deposited on layered manganese oxide: a functional model for the water-oxidizing complex in photosystem II

In this report, gold or silver deposited on layered manganese oxide has been synthesized by a simple method and characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction spectrometry, atomic absorption spectroscopy, and energy-dispersive X-ray mapping. The gold deposited on layered manganese oxide showed efficient catalytic activity toward water oxidation in the presence of cerium(IV) ammonium nitrate. The properties associated with this compound suggest it is a functional model for the water-oxidizing complex in photosystem II.

The GreenCut: re-evaluation of physiological role of previously studied proteins and potential novel protein functions

Based on comparative genomics, a list of proteins present in the green algal, flowering and nonflowering plant lineages, but not detected in nonphotosynthetic organisms, was assembled (Merchant et al., Science 318:245-250, 2007; Karpowicz et al., J Biol Chem 286:21427-21439, 2011). This protein grouping, previously designated the GreenCut, was established using stringent comparative genomic criteria; they are those Chlamydomonas reinhardtii proteins with orthologs in Arabidopsis thaliana, Physcomitrella patens, Oryza sativa, Populus tricocarpa and at least one of the three Ostreococcus species with fully sequenced genomes, but not in bacteria, yeast, fungi or mammals. Many GreenCut proteins are also present in red algae and diatoms and a subset of 189 have been identified as encoded on nearly all cyanobacterial genomes. Of the current GreenCut proteins (597 in total), approximately half have been studied previously. The functions or activities of a number of these proteins have been deduced from phenotypic analyses of mutants (defective for genes encoding specific GreenCut proteins) of A. thaliana, and in many cases the assigned functions do not exist in C. reinhardtii. Therefore, precise physiological functions of several previously studied GreenCut proteins are still not clear. The GreenCut also contains a number of proteins with certain conserved domains. Three of the most highly conserved domains are the FK506 binding, cyclophilin and PAP fibrillin domains; most members of these gene families are not well characterized. In general, our analysis of the GreenCut indicates that many processes critical to green lineage organisms remain unstudied or poorly characterized. We have begun to examine the functions of some GreenCut proteins in detail. For example, our work on the CPLD38 protein has demonstrated that it has an essential role in photosynthetic function and the stability of the cytochrome b 6 f complex.

A 2-(2-hydroxyphenyl)-1H-benzimidazole-manganese oxide hybrid as a promising structural model for the tyrosine 161/histidine 190-manganese cluster in photosystem II

In this communication, we report the synthesis, characterization, and electrochemistry of a 2-(2-hydroxyphenyl)-1H-benzimidazole-manganese oxide hybrid. Our results suggest that this compound is a promising model for the manganese cluster together with tyrosine-161 and histidine-190 in photosystem II of plants, algae and cyanobacteria.