Unraveling the Mechanism of the Oxidative C-C Bond Coupling Reaction Catalyzed by Deoxypodophyllotoxin Synthase.

Deoxypodophyllotoxin synthase (DPS), a nonheme Fe(II)/2-oxoglutarate (2OG)-dependent oxygenase, is a key enzyme that is involved in the construction of the fused-ring system in (-)-podophyllotoxin biosynthesis by catalyzing the C-C coupling reaction. However, the mechanistic details of DPS-catalyzed ring formation remain unclear. Herein, our quantum mechanics/molecular mechanics (QM/MM) calculations reveal a novel mechanism that involves the recycling of CO2 (a product of decarboxylation of 2OG) to prevent the formation of hydroxylated byproducts. Our results show that CO2 can react with the FeIII-OH species to generate an unusual FeIII-bicarbonate species. In this way, hydroxylation is avoided by consuming the OH group. Then, the C-C coupling followed by desaturation yields the final product, deoxypodophyllotoxin. This work highlights the crucial role of the CO2 molecule, generated in the crevice between the iron active site and the substrate, in controlling the reaction selectivity.

Hyperspectral Compressive Snapshot Reconstruction via Coupled Low-Rank Subspace Representation and Self-Supervised Deep Network.

Coded aperture snapshot spectral imaging (CASSI) is an important technique for capturing three-dimensional (3D) hyperspectral images (HSIs), and involves an inverse problem of reconstructing the 3D HSI from its corresponding coded 2D measurements. Existing model-based and learning-based methods either could not explore the implicit feature of different HSIs or require a large amount of paired data for training, resulting in low reconstruction accuracy or poor generalization performance as well as interpretability. To remedy these deficiencies, this paper proposes a novel HSI reconstruction method, which exploits the global spectral correlation from the HSI itself through a formulation of model-driven low-rank subspace representation and learns the deep prior by a data-driven self-supervised deep learning scheme. Specifically, we firstly develop a model-driven low-rank subspace representation to decompose the HSI as the product of an orthogonal basis and a spatial representation coefficient, then propose a data-driven deep guided spatial-attention network (called DGSAN) to adaptively reconstruct the implicit spatial feature of HSI by learning the deep coefficient prior (DCP), and finally embed these implicit priors into an iterative optimization framework through a self-supervised training way without requiring any training data. Thus, the proposed method shall enhance the reconstruction accuracy, generalization ability, and interpretability. Extensive experiments on several datasets and imaging systems validate the superiority of our method. The source code and data of this article will be made publicly available at https://github.com/ChenYong1993/LRSDN.

Host- and virus-induced gene silencing of HOG1-MAPK cascade genes in Rhizophagus irregularis inhibit arbuscule development and reduce resistance of plants to drought stress.

Arbuscular mycorrhizal (AM) fungi can form beneficial associations with the most terrestrial vascular plant species. AM fungi not only facilitate plant nutrient acquisition but also enhance plant tolerance to various environmental stresses such as drought stress. However, the molecular mechanisms by which AM fungal mitogen-activated protein kinase (MAPK) cascades mediate the host adaptation to drought stimulus remains to be investigated. Recently, many studies have shown that virus-induced gene silencing (VIGS) and host-induced gene silencing (HIGS) strategies are used for functional studies of AM fungi. Here, we identify the three HOG1 (High Osmolarity Glycerol 1)-MAPK cascade genes RiSte11, RiPbs2 and RiHog1 from Rhizophagus irregularis. The expression levels of the three HOG1-MAPK genes are significantly increased in mycorrhizal roots of the plant Astragalus sinicus under severe drought stress. RiHog1 protein was predominantly localized in the nucleus of yeast in response to 1 M sorbitol treatment, and RiPbs2 interacts with RiSte11 or RiHog1 directly by pull-down assay. Importantly, VIGS or HIGS of RiSte11, RiPbs2 or RiHog1 hampers arbuscule development and decreases relative water content in plants during AM symbiosis. Moreover, silencing of HOG1-MAPK cascade genes led to the decreased expression of drought-resistant genes (RiAQPs, RiTPSs, RiNTH1 and Ri14-3-3) in the AM fungal symbiont in response to drought stress. Taken together, this study demonstrates that VIGS or HIGS of AM fungal HOG1-MAPK cascade inhibits arbuscule development and expression of AM fungal drought-resistant genes under drought stress.

Mechanistic Insight into Peptidyl-Cysteine Oxidation by the Copper-Dependent Formylglycine-Generating Enzyme.

The copper-dependent formylglycine-generating enzyme (FGE) catalyzes the oxygen-dependent oxidation of specific peptidyl-cysteine residues to formylglycine. Our QM/MM calculations provide a very likely mechanism for this transformation. The reaction starts with dioxygen binding to the tris-thiolate CuI center to form a triplet CuII -superoxide complex. The rate-determining hydrogen atom abstraction involves a triplet-singlet crossing to form a CuII -OOH species that couples with the substrate radical, leading to a CuI -alkylperoxo intermediate. This is accompanied by proton transfer from the hydroperoxide to the S atom of the substrate via a nearby water molecule. The subsequent O-O bond cleavage is coupled with the C-S bond breaking that generates the formylglycine and a CuII -oxyl complex. Moreover, our results suggest that the aldehyde oxygen of the final product originates from O2 , which will be useful for future experimental work.

C(sp3 )-H Hydroxylation in Diiron ß-Hydroxylase CmlA Transpires by Amine-Assisted O2 Activation Avoiding FeIV 2 O2 Species.

Through QM/MM modeling, we discovered that C(sp3 )-H ß-hydroxylation in the diiron hydroxylase CmlA transpires by traceless amine-assisted O2 activation. Different from the canonical diiron hydroxylase sMMO, this aliphatic-amine-assisted O2 activation avoids generating the high-valent diferryl FeIV 2 O2 species, but alternatively renders a diferric FeIII 2 O species as the reactive oxidant. From this unprecedented O2 activation mode, the derived C(sp3 )-H hydroxylation mechanism in CmlA also differs drastically from the toluene aromatic C(sp2 )-H hydroxylation in the diiron hydroxylase T4MO. This substrate-modulated O2 activation in CmlA has rich mechanistic implications for other diiron hydroxylases with an amine group adjacent to the C-H bond under hydroxylation in substrates, such as hDOHH. Furthermore, the adapted coordination environment of the diiron cofactor upon O2 binding in CmlA opens up more structural and mechanistic possibilities for O2 activation in non-heme diiron enzymes.

Multiple PHT1 family phosphate transporters are recruited for mycorrhizal symbiosis in Eucalyptus grandis and conserved PHT1;4 is a requirement for the arbuscular mycorrhizal symbiosis.

Eucalypts engage in a mutualistic endosymbiosis with arbuscular mycorrhizal (AM) fungi to acquire mineral nutrients from soils, particularly inorganic phosphate (Pi). In return, the host plant provides organic carbons to its fungal partners. However, the mechanism by which the Eucalyptus plants acquire Pi released from the AM fungi has remained elusive. In this study, we investigated the characterization of potential PHOSPHATE TRANSPORTER1 (PHT1) family Pi transporters in AM symbiosis in Eucalyptus grandis W. Hill ex Maiden. We show that multiple PHT1 family Pi transporters were recruited for AM symbiosis in E. grandis. We further report that EgPT4, an E. grandis member of the PHT1 family, is conserved across angiosperms and is exclusively expressed in AM roots with arbuscule-containing cells and localizes to the periarbuscular membrane (PAM). EgPT4 was able to complement a yeast mutant strain defective in all inorganic Pi transporters and mediate Pi uptake. Importantly, EgPT4 is essential for improved E. grandis growth, total phosphorus concentration and arbuscule development during symbiosis. Moreover, silencing of EgPT4 led to the induction of polyphosphate accumulation relevant genes of Rhizophagus irregularis DAOM 197198. Collectively, our results unravel a pivotal role for EgPT4 in symbiotic Pi transport across the PAM required for arbuscule development in E. grandis.

Ligand-Promoted RhI -Catalyzed C2-Selective C-H Alkenylation and Polyenylation of Imidazoles with Alkenyl Carboxylic Acids.

The first RhI -catalyzed, directed decarbonylative C2-H alkenylation of imidazoles with readily available alkenyl carboxylic acids is reported. The reaction proceeds in a highly regio- and stereoselective manner, providing efficient access to C2-alkenylated imidazoles that are generally inaccessible by known C-H alkenylation methods. This transformation accommodates a wide range of alkenyl carboxylic acids, including challenging conjugated polyene carboxylic acids, and diversely decorated imidazoles with high functional group compatibility. The presence of a removable pyrimidine directing group and the use of a bidentate phosphine ligand are pivotal to the success of the catalytic reaction. This process is also suitable for benzimidazoles. Importantly, the scalability and diversification of the products highlight the potential of this protocol in practical applications. Detailed experimental and computational studies provide important insights into the underlying reaction mechanism.

A SPX domain-containing phosphate transporter from Rhizophagus irregularis handles phosphate homeostasis at symbiotic interface of arbuscular mycorrhizas.

Reciprocal symbiosis of > 70% of terrestrial vascular plants with arbuscular mycorrhizal (AM) fungi provides the fungi with fatty acids and sugars. In return, AM fungi facilitate plant phosphate (Pi) uptake from soil. However, how AM fungi handle Pi transport and homeostasis at the symbiotic interface of AM symbiosis is poorly understood. Here, we identify an SPX (SYG1/Pho81/XPR1) domain-containing phosphate transporter, RiPT7 from Rhizophagus irregularis. To characterize the RiPT7 transporter, we combined subcellular localization and heterologous expression studies in yeasts with reverse genetics approaches during the in planta phase. The results show that RiPT7 is conserved across fungal species and expressed in the intraradical mycelia. It is expressed in the arbuscules, intraradical hyphae and vesicles, independently of Pi availability. The plasma membrane-localized RiPT7 facilitates bidirectional Pi transport, depending on Pi gradient across the plasma membrane, whereas the SPX domain of RiPT7 inhibits Pi transport activity and mediates the vacuolar targeting of RiPT7 in yeast in response to Pi starvation. Importantly, RiPT7 silencing hampers arbuscule development of R. irregularis and symbiotic Pi delivery under medium- to low-Pi conditions. Collectively, our findings reveal a role for RiPT7 in fine-tuning of Pi homeostasis across the fungal membrane to maintain the AM development.

Mechanistic insights into dioxygen activation by a manganese corrole complex: a broken-symmetry DFT study.

The Mn-oxygen species have been implicated as key intermediates in various Mn-mediated oxidation reactions. However, artificial oxidants were often used for the synthesis of the Mn-oxygen intermediates. Remarkably, the Mn(v)-oxo and Mn(iv)-peroxo species have been observed in the activation of O2 by Mn(iii) corroles in the presence of base (OH-) and hydrogen donors. In this work, density functional theory methods were used to get insight into the mechanism of dioxygen activation and formation of Mn(v)-oxo. The results demonstrated that the dioxygen cannot bind to Mn without the axial OH- ligand. Upon the addition of the axial OH- ligand, the dioxygen can bind to Mn in an end-on fashion to give the Mn(iv)-superoxo species. The hydrogen atom transfer from the hydrogen donor (substrate) to the Mn(iv)-superoxo species is the rate-limiting step, having a high reaction barrier and a large endothermicity. Subsequently, the O-C bond formation is concerted with an electron transfer from the substrate radical to the Mn and a proton transfer from the hydroperoxo moiety to the nearby N atom of the corrole ring, generating an alkylperoxo Mn(iii) complex. The alkylperoxo O-O bond cleavage affords a Mn(v)-oxo complex and a hydroxylated substrate. This novel mechanism for the Mn(v)-oxo formation via an alkylperoxo Mn(iii) intermediate gives insight into the O-O bond activation by manganese complexes.

Monophenolase and catecholase activity of Aspergillus oryzae catechol oxidase: insights from hybrid QM/MM calculations.

Catechol oxidase from Aspergillus oryzae (AoCO4) can not only catalyze oxidation of o-diphenols to o-quinones, but can also catalyze monooxygenation of small phenolics. To gain insight into the catecholase and monophenolase activities of AoCO4, the reaction mechanism of catechol oxidation was investigated by means of hybrid quantum mechanical/molecular mechanical (QM/MM) calculations. The oxy-form of AoCO4 was found to be a µ-η2:η2 side-on peroxo dicopper(ii) complex, which can undergo a proton coupled electron transfer from the substrate rather than a proton transfer from the nearby Ser302 residue to generate a hydroperoxide. The µ-1,1-OOH Cu2(i,ii) complex is thermodynamically more stable than the µ-η1:η2 hydroperoxide. Moreover, the cleavage of the O-O bond in the µ-1,1-OOH Cu2(i,ii) intermediate has a much lower barrier than that in the µ-η1:η2 hydroperoxide species. In both cases, the O-O bond cleavage is the rate-limiting step, generating the reactive (µ-O˙)(µ-OH) dicopper(ii) complex. In addition, our results demonstrated that the oxidation of catechol to quinone is much more preferred than the hydroxylation reaction. These findings may provide useful information for understanding the reactivity of the Cu2O2 active site of coupled binuclear copper enzymes.

The auxin-inducible phosphate transporter AsPT5 mediates phosphate transport and is indispensable for arbuscule formation in Chinese milk vetch at moderately high phosphate supply.

Phosphorus is a macronutrient that is essential for plant survival. Most land plants have evolved the ability to form a mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi, which enhances phosphate (Pi) acquisition. Modulation of Pi transporter systems is the master strategy used by mycorrhizal plants to adapt to ambient Pi concentrations. However, the specific functions of PHOSPHATE TRANSPORTER 1 (PHT1) genes, which are Pi transporters that are responsive to high Pi availability, are largely unknown. Here, we report that AsPT5, an Astragalus sinicus (Chinese milk vetch) member of the PHT1 gene family, is conserved across dicotyledons and is constitutively expressed in a broad range of tissues independently of Pi supply, but is remarkably induced by indole-3-acetic acid (auxin) treatment under moderately high Pi conditions. Subcellular localization experiments indicated that AsPT5 localizes to the plasma membrane of plant cells. Using reverse genetics, we showed that AsPT5 not only mediates Pi transport and remodels root system architecture but is also essential for arbuscule formation in A. sinicus under moderately high Pi concentrations. Overall, our study provides insight into the function of AsPT5 in Pi transport, AM development and the cross-talk between Pi nutrition and auxin signalling in mycorrhizal plants.

Mechanistic insights into ring cleavage of hydroquinone by PnpCD from quantum mechanical/molecular mechanical calculations.

PnpCD is a mononuclear non-heme iron(ii) dioxygenase containing an unusual 2His-1Glu-1Asn metal-binding motif. To gain insights into the catalytic mechanism of the ring opening of hydroquinone by PnpCD, hybrid quantum mechanics/molecular mechanics calculations have been performed by using two models with different protonation states of the substrate (nonionized and ionized forms of the Fe-bound hydroxyl group of hydroquinone). In both cases, the structure of the reactive Fe-O2 species reveals a trigonal bipyramidal complex, in which Asn258 is no longer coordinated to the iron center. The catalytic process mainly involves the attack of the superoxo radical, O-O bond cleavage, three-membered ring closure and opening, attack of the Fe-bound oxyl radical, and ring-opening of the seven-membered ring. The transition state for the peroxo O-O bond cleavage was found to be the rate-determining transition state. The second-sphere Glu248 serves as a proton acceptor to deprotonate the unbound substrate hydroxyl group, thereby facilitating the electron transfer between the substrate and dioxygen. The first-sphere Glu262 can act as an acid-base catalyst to lower the rate-limiting barrier, thus providing a useful clue for improving catalytic efficiency.

Mechanistic insights into a non-heme 2-oxoglutarate-dependent ethylene-forming enzyme: selectivity of ethylene-formation versusl-Arg hydroxylation.

The ethylene-forming enzyme (EFE) is a unique member of the Fe(ii)- and 2-oxoglutarate-dependent (Fe/2OG) oxygenases. It converts 2OG into ethylene plus three CO2 molecules (ethylene-forming reaction) and also catalyzes the C5 hydroxylation of l-arginine coupled to the oxidative decarboxylation of 2OG (l-Arg hydroxylation reaction). To uncover the mechanisms of the dual transformations by EFE, quantum mechanical/molecular mechanical (QM/MM) calculations were carried out. Based on the results, a branched mechanism was proposed. An FeII-peroxysuccinate complex with a dissociated CO2 generated through the nucleophilic attack of the superoxo moiety of the Fe-O2 species on the keto carbon of 2OG is the key common intermediate in both reactions. A competition between the subsequent CO2 insertion (a key step in the ethylene-forming pathway) and the O-O bond cleavage (leading to the formation of succinate) governs the product selectivity. The calculated reaction barriers suggested that the CO2 insertion is favored over the O-O bond cleavage. This is consistent with the product preference observed in experiments. By comparison with the results of AsqJ (an Fe/2OG oxygenase that leads to substrate oxidation exclusively), the protein environment was found to be crucial for the selectivity. Further calculations demonstrated that the local electric field of the protein environment in EFE promotes ethylene formation by acting as a charge template, exemplifying the importance of the electrostatic interaction in enzyme catalysis. These findings offer mechanistic insights into the EFE catalysis and provide important clues for better understanding the unique ethylene-forming capability of EFE compared with other Fe/2OG oxygenases.

Low overpotential water oxidation at neutral pH catalyzed by a copper(ii) porphyrin.

Low overpotential water oxidation under mild conditions is required for new energy conversion technologies with potential application prospects. Extensive studies on molecular catalysis have been performed to gain fundamental knowledge for the rational designing of cheap, efficient and robust catalysts. We herein report a water-soluble CuII complex of tetrakis(4-N-methylpyridyl)porphyrin (1), which catalyzes the oxygen evolution reaction (OER) in neutral aqueous solutions with small overpotentials: the onset potential of the catalytic water oxidation wave measured at current density j = 0.10 mA cm-2 is 1.13 V versus a normal hydrogen electrode (NHE), which corresponds to an onset overpotential of 310 mV. Constant potential electrolysis of 1 at neutral pH and at 1.30 V versus NHE displayed a substantial and stable current for O2 evolution with a faradaic efficiency of >93%. More importantly, in addition to the 4e water oxidation to O2 at neutral pH, 1 can catalyze the 2e water oxidation to H2O2 in acidic solutions. The produced H2O2 is detected by rotating ring-disk electrode measurements and by the sodium iodide method after bulk electrolysis at pH 3.0. This work presents an efficient and robust Cu-based catalyst for water oxidation in both neutral and acidic solutions. The observation of H2O2 during water oxidation catalysis is rare and will provide new insights into the water oxidation mechanism.

Mechanistic Insights into a Stibene Cleavage Oxygenase NOV1 from Quantum Mechanical/Molecular Mechanical Calculations.

NOV1, a stilbene cleavage oxygenase, catalyzes the cleavage of the central double bond of stilbenes to two phenolic aldehydes, using a 4-His Fe(II) center and dioxygen. Herein, we use in-protein quantum mechanical/molecular mechanical (QM/MM) calculations to elucidate the reaction mechanism of the central double bond cleavage of phytoalexin resveratrol by NOV1. Our results showed that the oxygen molecule prefers to bind to the iron center in a side-on fashion, as suggested from the experiment. The quintet Fe-O2 complex with the side-on superoxo antiferromagnetic coupled to the resveratrol radical is identified as the reactive oxygen species. The QM/MM results support the dioxygenase mechanism involving a dioxetane intermediate with a rate-limiting barrier of 10.0 kcal mol-1. The alternative pathway through an epoxide intermediate is ruled out due to a larger rate-limiting barrier (26.8 kcal mol-1). These findings provide important insight into the catalytic mechanism of carotenoid cleavage oxygenases and also the dioxygen activation of non-heme enzymes.

Origin of Nitric Oxide Reduction Activity in Flavo-Diiron NO Reductase: Key Roles of the Second Coordination Sphere.

The second coordination sphere constitutes a distinguishing factor in the active site to modulate enzymatic reactivity. To unravel the origin of NO-to-N2 O reduction activity of non-heme diiron enzymes, herein we report a strong second-coordination-sphere interaction between a conserved Tyr197 and the key iron-nitrosyl intermediate of Tm FDP (flavo-diiron protein), which leads to decreased reaction barriers towards N-N formation and N-O cleavage in NO reduction. This finding supports the direct coupling of diiron dinitrosyl as the N-N formation mode in our QM/MM modeling, and reconciles the mechanistic controversy of external reduction between FDPs and synthetic biomimetics of the iron-nitrosyls. This work highlights the application of QM/MM 57 Fe Mössbauer modeling in elucidating the structural features of not only first, but also second coordination spheres of the key transient species involved in NO/O2 activation by non-heme diiron enzymes.

Electrocatalytic Water Oxidation by a Water-Soluble Copper(II) Complex with a Copper-Bound Carbonate Group Acting as a Potential Proton Shuttle.

Water-soluble copper(II) complexes of the dianionic tridentate pincer ligand N,N'-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate (L) are catalysts for water oxidation. In [L-CuII-DMF] (1, DMF = dimethylformamide) and [L-CuII-OAc]- (2, OAc = acetate), ligand L binds CuII through three N atoms, which define an equatorial plane. The fourth coordination site of the equatorial plane is occupied by DMF in 1 and by OAc- in 2. These two complexes can electrocatalyze water oxidation to evolve O2 in 0.1 M pH 10 carbonate buffer. Spectroscopic, titration, and crystallographic studies show that both 1 and 2 undergo ligand exchange when they are dissolved in carbonate buffer to give [L-CuII-CO3H]- (3). Complex 3 has a similar structure as those of 1 and 2 except for having a carbonate group at the fourth equatorial position. A catalytic cycle for water oxidation by 3 is proposed based on experimental and theoretical results. The two-electron oxidized form of 3 is the catalytically active species for water oxidation. Importantly, for these two oxidation events, the calculated potential values of Ep,a = 1.01 and 1.59 V vs normal hydrogen electrode (NHE) agree well with the experimental values of Ep,a = 0.93 and 1.51 V vs NHE in pH 10 carbonate buffer. The potential difference between the two oxidation events is 0.58 V for both experimental and calculated results. With computational evidence, this Cu-bound carbonate group may act as a proton shuttle to remove protons for water activation, a key role resembling intramolecular bases as reported previously.

Mechanistic insights into dioxygen activation, oxygen atom exchange and substrate epoxidation by AsqJ dioxygenase from quantum mechanical/molecular mechanical calculations.

Herein, we use in-protein quantum mechanical/molecular mechanical (QM/MM) calculations to elucidate the mechanism of dioxygen activation, oxygen atom exchange and substrate epoxidation processes by AsqJ, an FeII/α-ketoglutarate-dependent dioxygenase (α-KGD) using a 2-His-1-Asp facial triad. Our results demonstrated that the whole reaction proceeds through a quintet surface. The dioxygen activation by AsqJ leads to a quintet penta-coordinated FeIV-oxo species, which has a square pyramidal geometry with the oxo group trans to His134. This penta-coordinated FeIV-oxo species is not the reactive one in the substrate epoxidation reaction since its oxo group is pointing away from the target C[double bond, length as m-dash]C bond. Instead, it can undergo the oxo group isomerization followed by water binding or the water binding followed by oxygen atom exchange to form the reactive hexa-coordinated FeIV-oxo species with the oxo group trans to His211. The calculated parameters of Mössbauer spectra for this hexa-coordinated FeIV-oxo intermediate are in excellent agreement with the experimental values, suggesting that it is most likely the experimentally trapped species. The calculated energetics indicated that the rate-limiting step is the substrate C[double bond, length as m-dash]C bond activation. This work improves our understanding of the dioxygen activation by α-KGD and provides important structural information about the reactive FeIV-oxo species.

Hydrolytic cleavage of both CS2 carbon-sulfur bonds by multinuclear Pd(II) complexes at room temperature.

Developing homogeneous catalysts that convert CS2 and COS pollutants into environmentally benign products is important for both fundamental catalytic research and applied environmental science. Here we report a series of air-stable dimeric Pd complexes that mediate the facile hydrolytic cleavage of both CS2 carbon-sulfur bonds at 25 °C to produce CO2 and trimeric Pd complexes. Oxidation of the trimeric complexes with HNO3 regenerates the dimeric starting complexes with the release of SO2 and NO2. Isotopic labelling confirms that the carbon and oxygen atoms of CO2 originate from CS2 and H2O, respectively, and reaction intermediates were observed by gas-phase and electrospray ionization mass spectrometry, as well as by Fourier transform infrared spectroscopy. We also propose a plausible mechanistic scenario based on the experimentally observed intermediates. The mechanism involves intramolecular attack by a nucleophilic Pd-OH moiety on the carbon atom of coordinated µ-OCS2, which on deprotonation cleaves one C-S bond and simultaneously forms a C-O bond. Coupled C-S cleavage and CO2 release to yield [(bpy)3Pd3(µ3-S)2](NO3)2 (bpy, 2,2'-bipyridine) provides the thermodynamic driving force for the reaction.

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems.

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Because of the significance of energy-related small molecule activation, this review covers recent progress in hydrogen evolution, oxygen evolution, and oxygen reduction reactions catalyzed by porphyrins and corroles.