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Hydrogen peroxide photosynthesis from water and air using a scaled-up 1-m2 flow reactor
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Restricted Partition Function Semiclassical Transition State Theory (RPF-SCTST): Applications to Reactions Involving Hydrogen Cyanide, Hydrogen Peroxide, and Formaldehyde Oxide
We demonstrate the efficiency of the numerical calculation of thermal semiclassical transition state theory (SCTST) rates across several representative chemical reactions using the restricted partition function (RPF)─viz a function that depends only on the imaginary action associated with a reaction. Here, we treat the potential energy surfaces (PESs) through fourth order expansions around the saddle point and integrate the Hamiltonian up to second order in employing vibrational perturbation theory. We apply this formalism to uni- and bimolecular reactions with different─though relatively high─barrier heights to uncover the influence of the barrier properties, minimal energies, and separability of the rotational motion on rate constants and tunneling corrections. Although all modes are coupled within the RPF, we found in our numerical examples that the rotational component in the absence of significant rotational distortions can be separated from the vibrational contributions. Moreover, a classical treatment of the rotational contribution is adequate over the temperature range from approximately 100 to 1000 K. We also found that the choice of DFT basis set can lead to variations in rate constants of up to an order of magnitude. Lastly, different schemes for counting eligible energy levels result in rate constants that differ by no more than a factor of 2, with the remaining discrepancies attributed to the treatment of near-convergent levels in perturbation theory.
Dissolved Oxygen Redox as the Source of Hydrogen Peroxide and Hydroxyl Radical in Sonicated Emulsive Water Microdroplets
Sonicated emulsive water microdroplets (SEWMs) accelerate and enable a variety of catalyst-free chemical transformations. However, significant unanswered questions remain regarding the chemical intermediates they form and their possible redox origin. In this study, we identified dissolved O 2 as the primary originator of reactive oxygen species (ROS) such as OH• and H 2 O 2. We uncovered the role of dissolved O 2 redox by using a combination of microelectrochemical methods to detect H 2 O 2 , isotopic methods to identify the source of H 2 O 2 , and a combination of electron spin resonance and the DMPO spin trap to detect radicals such as OH•. Notably, we found that H 2 O 2 production is correlated with O 2 content via a reduction pathway enabled by a sufficiently large reducing power that can additionally generate H 2 and even perform Pb electroless deposition on Au and Cu metal substrates. Building on our findings, continuous O 2 bubbling of SEWMs showed accumulation of H 2 O 2 up to ∼88 mM in the aqueous phase within 1 h of sonication, demonstrating the scale-up promise of this method. Distinct to sonochemistry of a single phase, this study advances our understanding of the confluence of redox and chemical reaction mechanisms within SEWMs as a biphasic system. This insight paves the way for improving their reaction kinetics, yield, and selectivity, positioning these attractive redox microreactors as alternatives to traditional electrolyzers.
Effect of surface modification on silica supported Ti catalysts for cyclohexene oxidation with vapor-phase hydrogen peroxide
Surface modification via grafting of organic moieties on a Lewis acid catalyst (silica supported Ti catalyst, Ti-SiO 2 ) alters the activation of H 2 O 2 in vapor-phase cyclohexene epoxidation. Grafting a fluorous group (1H,1H-perfluoro-octyl) suppresses activity of Ti-SiO 2 . Conversely, grafting either a nonpolar group (octyl) or a polar aprotic group (triethylene glycol monomethyl ether) enhances rates and shifts selectivity toward trans-1,2-cyclohexanediol.
Enhanced direct air capture by uranyl superoxide: a comprehensive study of the carbonation reaction of uranyl peroxide complexes
Relative humidity dependence on the fate of uranyl triperoxide/uranyl diperoxo superoxide in the presence of atmospheric carbon dioxide.
Expanding Uranyl Peroxide Cluster Chemistry via the Introduction of Superoxide Radicals
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Aging of Uranium Peroxides and Radical Species Investigation by EPR
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Bubble-Free Frontal Polymerization of Acrylates via Redox-Initiated Free Radical Polymerization
Thermal frontal polymerization (FP) of acrylate monomers mixed with conventional peroxide initiators leads to significant bubble formation at the polymerizing front, limiting their practical applications. Redox initiators present a promising alternative to peroxide initiators, as they prevent the formation of gaseous byproducts during initiator decomposition and lower the front temperature, thereby enabling bubble-free FP. In this study, we investigate the FP of acrylate monomers of varying functionalities, including methyl methacrylate (MMA), 1,6-hexanediol diacrylate (HDDA), and trimethylolpropane triacrylate (TMPTA), using N,N -dimethylaniline/benzoyl peroxide (DMA/BPO) redox couple at room temperature and compare their front behavior, pot life, and bubble formation with those of same resin systems mixed with a conventional peroxide initiator, Luperox 231. The use of redox couples in FP of acrylates shows promise for rapid, energy-efficient manufacturing of polyacrylates and can enable new applications such as 3D printing and composite manufacturing.
Reactions of Studtite UO 4 ·4H 2 O in Alkali Hydroxides: Isolation of Single-Crystal Uranate Phases under Mild Hydrothermal Conditions
Uranyl peroxide complexes, particularly studtite (UO 4 ·4H 2 O), are important phases within the nuclear fuel cycle, forming through radiolysis-induced reactions on the surfaces of spent fuel and in waste environments. Studtite has been identified in Hanford’s irradiated fuel storage basins, Chornobyl’s corium lavas, and is anticipated on Fukushima’s fuel debris. While its formation and stability have been extensively studied, the reactivity of UO 4 ·4H 2 O in highly alkaline environments such as those encountered in high-level nuclear waste remains underexplored. These environments contain molar concentrations of [OH - ] and present a chemically dynamic and reactive environment where actinide behavior is not well understood. Reported here are investigations of uranium reactions in concentrated alkali hydroxides under mild hydrothermal conditions (<200 °C) that have resulted in the isolation of the alkali metal uranates Li 2 UO 4 , α-Na 2 UO 4 , γ-Na 2 U 2 O 7 , and K 2 U 2 O 7 . In contrast to conventional solid-state methods (>800 °C) that typically yield polycrystalline powders, our approach enables the isolation of these uranates as single crystals, allowing us to provide single-crystal structure solutions of certain uranates for the first time. Our findings match the results of high-level waste tank sampling, confirming that sodium diuranate (Na 2 U 2 O 7 ) is the most persistent uranium phase, potentially forming through reactions of uranyl peroxide intermediates with NaOH under radiolytic and highly alkaline conditions.
Combined Cyclohexene Dehydrogenation and Oxygenation over VO x /CeO 2 with O 2 and Vaporized H 2 O 2 as Oxidants
Selective oxidation of hydrocarbons over metal oxide catalysts frequently invokes Mars-van Krevelen (MvK) mechanisms involving lattice oxygens. At typical reaction temperatures, oxidative dehydrogenation reactions predominate and compete with total oxidation; production of more useful oxygenates is challenging. However, because regeneration of lattice oxygens has been previously hypothesized to involve transient formation of surface hydroperoxides or peroxides, MvK cycles should be capable of forming products derived from both lattice oxygen and surface (hydro)peroxide species, under certain conditions. In this report, we study the low temperature reaction of cyclohexene with O 2 over CeO 2 -supported VO x catalysts. Below 150°C and for higher VO x loading, cyclohexene converts to benzene and trans-1,2-cyclohexanediol with no appreciable CO 2 formation. Control experiments show similar behavior in other supported vanadia catalysts. Conversely, using vaporized H 2 O 2 as oxidant exclusively gives 1,2-cyclohexanedione, suggesting that MvK reoxidation cycles do not directly involve H 2 O 2 . Furthermore, these findings contribute to the current understanding of selective oxidation mechanisms, and demonstrate routes to the formation of industrially useful oxygenates with O 2 or vaporized H 2 O 2 over metal oxide catalysts.
Spectroscopic Signature of Metal-hydroxo and Peroxo Species in K-edge X-ray Absorption Spectra
Metal-dioxygen species are important intermediates formed during dioxygen activations by metalloenzymes in various biological processes, by catalysts in fuel cells, and prior to O 2 evolution by photosystem II. In this work, we focus on manganese-porphyrin complexes using tetramesitylporphyrin ligand (TMP) to explore changes in Mn K-edge X-ray absorption spectroscopy (XAS) associated with the formation of Mn-hydroxide and Mn–O 2 peroxide species. With limited spectroscopic characterization of these compounds, Mn K β X-ray emission spectroscopy (XES), XAS, density functional theory (DFT), and time-dependent DFT (TD-DFT) analysis will enhance our understanding of their complex electronic structure. We show that the shape of the pre-edge in the K-edge Mn X-ray absorption near-edge structure (XANES) can serve as a spectroscopic signature of the Mn III -peroxo formation and thus can be used to track the presence of the side-on peroxide as an intermediate in time-resolved or in situ experiments. In conclusion, our results will help to further summarize the spectroscopic fingerprints for peroxo and hydroxo species, addressing the challenge of identifying the reactive metal species in catalytic reactions.
Revealing the atomic and electronic mechanism of human manganese superoxide dismutase product inhibition
Human manganese superoxide dismutase (MnSOD) is a crucial oxidoreductase that maintains the vitality of mitochondria by converting superoxide (O 2 •– ) to molecular oxygen (O 2 ) and hydrogen peroxide (H 2 O 2 ) with proton-coupled electron transfers (PCETs). Human MnSOD has evolved to be highly product inhibited to limit the formation of H 2 O 2 , a freely diffusible oxidant and signaling molecule. The product-inhibited complex is thought to be composed of a peroxide (O 2 2– ) or hydroperoxide (HO 2 – ) species bound to Mn ion and formed from an unknown PCET mechanism. PCET mechanisms of proteins are typically not known due to difficulties in detecting the protonation states of specific residues that coincide with the electronic state of the redox center. To shed light on the mechanism, we combine neutron diffraction and X-ray absorption spectroscopy of the product-bound, trivalent, and divalent states of the enzyme to reveal the positions of all the atoms, including hydrogen, and the electronic configuration of the metal ion. The data identifies the product-inhibited complex, and a PCET mechanism of inhibition is constructed.
Electron transfer in polysaccharide monooxygenase catalysis
Polysaccharide monooxygenase (PMO) catalysis involves the chemically difficult hydroxylation of unactivated C–H bonds in carbohydrates. The reaction requires reducing equivalents and will utilize either oxygen or hydrogen peroxide as a cosubstrate. Two key mechanistic questions are addressed here: 1) How does the enzyme regulate the timely and tightly controlled electron delivery to the mononuclear copper active site, especially when bound substrate occludes the active site? and 2) How does this electron delivery differ when utilizing oxygen or hydrogen peroxide as a cosubstrate? Using a computational approach, potential paths of electron transfer (ET) to the active site copper ion were identified in a representative AA9 family PMO from Myceliophthora thermophila ( Mt PMO9E). When Y62, a buried residue 12 Å from the active site, is mutated to F, lower activity is observed with O 2 . However, a WT-level activity is observed with H 2 O 2 as a cosubstrate indicating an important role in ET for O 2 activation. To better understand the structural effects of mutations to Y62 and axial copper ligand Y168, crystal structures were solved of the wild type Mt PMO9E and the variants Y62W, Y62F, and Y168F. A bioinformatic analysis revealed that position 62 is conserved as either Y or W in the AA9 family. The Mt PMO9E Y62W variant has restored activity with O 2 . Overall, the use of redox-active residues to supply electrons for the reaction with O 2 appears to be widespread in the AA9 family. Furthermore, the results provide a molecular framework to understand catalysis with O 2 versus H 2 O 2 .
The mevalonate pathway of isoprenoid biosynthesis supports metabolic flexibility in Mycobacterium marinum
ABSTRACT Isoprenoids are a diverse class of natural products that are essential in all domains of life. Most bacteria synthesize isoprenoids through either the methylerythritol phosphate (MEP) pathway or the mevalonate (MEV) pathway, while a small subset encodes both pathways, including the pathogen Mycobacterium marinum (Mm). It is unclear whether the MEV pathway is functional in Mm, or why Mm encodes seemingly redundant metabolic pathways. Here, we show that the MEP pathway is essential in Mm, while the MEV pathway is dispensable in culture, with the ΔMEV mutant having no growth defect in axenic culture but a competitive growth defect compared to WT Mm. We found that the MEV pathway does not play a role in ex vivo or in vivo acute infection but does play a role in survival of peroxide stress. Metabolite profiling revealed that modulation of the MEV pathway causes compensatory changes in the concentration of MEP intermediates DOXP and CDP-ME, suggesting that the MEV pathway is functional and that the pathways interact at the metabolic level. Finally, the MEV pathway is upregulated early in the shift down to hypoxia, suggesting that it may provide metabolic flexibility to this bacterium. Interestingly, we found that our complemented strains, which vary in copy number of the polyprenyl synthetase idsB2 , responded differently to peroxide and UV stresses, suggesting a role for this gene as a determinant of downstream prenyl phosphate metabolism. Together, these findings suggest that MEV may serve as an anaplerotic pathway to make isoprenoids under stress conditions. IMPORTANCE Organisms from all domains of life utilize isoprenoids to carry out thousands of critical and auxiliary cellular processes, including signaling, maintaining membrane integrity, stress response, and host-pathogen interactions. The common precursor of all isoprenoids is synthesized via one of two biosynthetic pathways. Importantly, some bacteria encode both pathways, including M. marinum . We found that only one pathway is essential in M. marinum , while the nonessential pathway may confer metabolic flexibility to help the bacterium better adapt to various environmental conditions. We also found that the polyprenyl synthetase IdsB2 plays an important role in driving such phenotypes. Further, we demonstrate metabolic interplay between both functional pathways. These insights represent the first characterization of isoprenoid biosynthesis in dual pathway-encoding mycobacteria.
Decreases in polyunsaturated fatty acid content improve heat stress tolerance during flowering and silicle development in pennycress (Thlaspi arvense L.)
Introduction: Pennycress (Thlaspi arvense L.) is an emerging intermediate oilseed crop grown in the offseason between primary summer crops to produce three cash crops in two years. Previous efforts to improve seed oil quality produced Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) genome-edited lines with decreased polyunsaturated fatty acids (PUFAs) levels, through loss of function of the FATTY ACID DESATURASE 2 (FAD2), REDUCED OLEATE DESATURATION1 (ROD1), and FATTY ACID ELONGASE1 (FAE1) genes. While seed oil compositions were previously characterized, it remains unknown how vegetative and reproductive tissue compositions might differ and affect tolerance to high temperature (HT) conditions.Methods: In four growth chamber experiments, we explored HT tolerance during flowering and silicle development. Plants were subjected to a 34 °C day/28 °C night regime and compared to control plants maintained at 20 °C. Pollen grain viability at a range of temperatures, lipid peroxidation and proline content in leaves and silicles following HT, and seed yield were measured.Results: Both fad2 and rod1 mutant lines had relatively higher pollen viability (71% and 54% respectively) under moderately elevated temperature (28 °C) compared to wild-type controls (37%). They also showed smaller decreases in seed yield (0% and 40% for fad2 and rod1 respectively, compared to 61% for wild type), following HT exposure during late flowering and early silicle development. Silicles of fad2 plants experienced 65% less lipid peroxidation under HT and 55% less buildup of proline, signifying less stress.Discussion: The differential results of fad2 and rod1 are likely due to the role of FAD2 in membranes in all tissues, whereas ROD1 predominantly affects triacylglycerol (TAG) composition in oil-accumulating tissues including pollen. Our results indicate that decreasing PUFAs, through gene editing, can increase heat tolerance in reproductive tissues as an auxiliary benefit accompanying improved seed oil quality.