Hydroperoxide Species Research Articles

Identification of the acetaldehyde oxide Criegee intermediate reaction network in the ozone-assisted low-temperature oxidation of trans-2-butene.

Uni- and bi-molecular reactions involving Criegee intermediates (CIs) have been the focus of many studies due to the role these molecules play in atmospheric chemistry. The reactivity of CIs is known to strongly depend on their structure. The reaction network of the second simplest CI, acetaldehyde oxide (CH3CHOO), is investigated in this work in an atmospheric pressure jet-stirred reactor (JSR) during the ozonolysis of trans-2-butene to explore the kinetic pathways relevant to atmospheric chemistry and low-temperature combustion. The mole fraction profiles of reactants, intermediates, and final products are determined by means of molecular-beam mass spectrometry in conjunction with single-photon ionization employing tunable synchrotron-generated vacuum ultraviolet radiation. A network of CI reactions is identified in the temperature region below 600 K, characterized by CI addition to trans-2-butene, water, formaldehyde, formic acid, and methanol. No sequential additions of the CH3CHOO CI are observed, in contrast with the reactivity of the simplest CI (H2COO) and the earlier observation of an extensive reaction network with up to four H2COO sequential additions (Phys. Chem. Chem. Phys., 2019, 21, 7341-7357). Experimental photoionization efficiency scans recorded at 300 K and 425 K and ab initio threshold energy calculations lead to the identification and quantification of previously elusive intermediates, such as ketohydroperoxide and hydroperoxide species. Specifically, the C4H8 + O3 adduct is identified as a ketohydroperoxide (KHP, 3-hydroperoxybutan-2-one, CH3C(O)CH(CH3)OOH), while hydroxyacetaldehyde (glycolaldehyde, HCOCH2OH) formation is attributed to unimolecular isomerization of the CIs. Other hydroperoxide species such as methyl hydroperoxide (CH3OOH), ethyl hydroperoxide (C2H5OOH), butyl hydroperoxide (OOH), hydroperoxyl acetaldehyde (HOOCH2CHO), hydroxyethyl hydroperoxide (CH3CH(OH)OOH), but-1-enyl-3-hydroperoxide, and 4-hydroxy-3-methylpentan-2-one (HOCH(CH3)CH(CH3)C(O)CH3) are also identified. Detection of additional oxygenated species such as methanol, ethanol, ketene, and aldehydes suggests multiple active oxidation routes. These results provide additional evidence that CIs are key intermediates of the ozone-unsaturated hydrocarbon reactions providing critical inputs for improved kinetics models.

Updated thermochemistry for renewable transportation fuels: New groups and group values for acetals and ethers, their radicals, and peroxy species

AbstractWe present new groups and group values for the gas‐phase thermochemistry of ethers, polyethers, and acetals suited for combustion modeling. Our investigation comprises fuel species, their primary radicals, peroxy radicals, and hydroperoxide species. In total, 45 species are used for the parameterization of 14 groups, six of which are newly introduced here. Presently, calculated thermochemistry at the DLPNO‐CCSD(T)/CBS(cc‐pVTZ, cc‐PVQZ) // B3LYP‐D3BJ/def2‐TZVP level of theory is combined with thermochemistry from the literature. Validation of the new group values against the quantum mechanical results gives deviations of 1.22 kcal/mol, 2.36 cal/(molK), and 1.20 cal/(molK) for heats of formation, standard entropies, and heat capacities, respectively. The new thermochemistry is tested with a recent OME2 and OME3 chemical kinetic model, which shows large sensitivities to the updated values in the intermediate temperature regime. This highlights the importance of using updated thermochemistry in future chemical kinetic modeling studies of oxygenated methyl ethers (OMEs) and OME‐related structures.

Photo aging and fragmentation of polypropylene food packaging materials in artificial seawater

Plastic litters in marine environment usually contain varied types and contents of additives that can significantly affect the photochemical aging and fragmentation process of microplastics (MPs). This study investigated the photo aging process of two common polypropylene (PP) food packaging materials (i.e., meal box and tea cup) in artificial seawater within 12 d of ultraviolet (UV) irradiation. Results revealed that the aging of both plastic materials were critically inhibited compared with pure PP, indicating that PP food packaging materials in natural seawater may share longer aging time than pure ones. GC-MS analysis revealed that antioxidant Irgafos 168 (tris (2,4-di-tert-butylphenyl) phosphite) was the dominant additive in these plastic materials. Photo reaction between Irgafos 168 and hydroperoxide species on the surface of MPs to prevent the formation of hydroxyl radical was the possible mechanism for the inhibiting effects. After antioxidant was exhausted, its photo degradation products could become the dominant contributor to influence the aging process of MPs. This is the first work exploring the role of antioxidant on the aging process of PP MPs in simulated ocean environment. The findings could be of great help for unraveling the effect of antioxidants on the aging-related environmental risk of hydrocarbon plastics in ocean environment.

In vivo and in vitro studies on inactivation of selenium containing protein- glutathione peroxidase 3 in mice nephrocytes caused by lead

Glutathione peroxidases (Gpxs) play vital roles in elimination of hydroperoxide and other reactive oxygen species through catalyzing reduced glutathione to protect from oxidative stress caused by heavy metals such as lead. Among the family of Gpxs, Gpx3 is the only extracellular enzyme synthesized in the kidney and actively secreted into the plasma. This study investigated mechanisms of lead-induced GPx3 inactivation both at the animal and molecular levels. Six-week-old mice were randomly divided into 4 groups, and exposed to different lead concentrations (0, 1, 2 and 4 g/L) in their drinking water for 4 weeks. Contents of GPx3 in blood serum were tested by enzyme-linked immunosorbent assay (ELISA) and the mRNA levels of Gpx3 in mice nephrocytes were determined by quantitative real-time PCR (qPCR), both of which showed significantly inhibited at higher lead concentrations accompanied by the decreased Gpx3 activities and the elevated levels of malondialdehyde (MDA) in nephrocytes, which indicated that lead could induce strongly oxidative stress through affecting Gpx3 function. So we further investigated molecular mechanisms of GPx3 inactivation caused by lead with multiple spectroscopic techniques, isothermal titration calorimetry (ITC) and molecular docking studies in vitro. Results showed that lead statically quenched GPx3 fluorescence by tightly binding to the structural domain of GPx3 in a 3:1 ratio with high binding affinity (K = 3.1(±0.087) × 107 mol−1). Further investigation of the conformation of GPx3 by UV–visible spectroscopy and circular dichroism (CD) spectroscopy indicated that lead changed the secondary structure of GPx3 by loosening the GPx3 skeleton and decreasing the hydrophobicity around tryptophan residues. This work proved in vivo and in vitro experiments that lead could induce oxidative stress in mice nephrocytes by interacting with GPx3.

Determination of the mechanism of polymer thermolysis at low temperatures using spin trap electron spin resonance

The thermolysis of polyethylene oxide (PEO) was investigated using the spin trap electron spin resonance technique. Based on the analysis of the formed carbon-center radicals, the PEO thermolysis was initiated at the heating temperature of 60 °C and accelerated from the temperature of 120 °C. Radical production depended on the heating temperature. Observations in this study show that there are two different reaction pathways: below 120 °C, the degradation of PEO was mainly initiated by hydroperoxide (ROOH) species, which may be produced by a high temperature manufacturing process; above 120 °C, the bond in the polymer molecule began to dissociate, leading to decomposition. A comprehensive thermal degradation mechanism of PEO is proposed based on experimental results. The findings from this study are applicable to various polymers produced using a high temperature manufacturing process.

Heme-FeIII Superoxide, Peroxide and Hydroperoxide Thermodynamic Relationships: FeIII-O2•- Complex H-Atom Abstraction Reactivity.

Establishing redox and thermodynamic relationships between metal-ion-bound O2 and its reduced (and protonated) derivatives is critically important for a full understanding of (bio)chemical processes involving dioxygen processing. Here, a ferric heme peroxide complex, [(F8)FeIII-(O22-)]- (P) (F8 = tetrakis(2,6-difluorophenyl)porphyrinate), and a superoxide complex, [(F8)FeIII-(O2•-)] (S), are shown to be redox interconvertible. Using Cr(η-C6H6)2, an equilibrium state where S and P are present is established in tetrahydrofuran (THF) at -80 °C, allowing determination of the reduction potential of S as -1.17 V vs Fc+/0. P could be protonated with 2,6-lutidinium triflate, yielding the low-spin ferric hydroperoxide species, [(F8)FeIII-(OOH)] (HP). Partial conversion of HP back to P using a derivatized phosphazene base gave a P/HP equilibrium mixture, leading to the determination of pKa = 28.8 for HP (THF, -80 °C). With the measured reduction potential and pKa, the O-H bond dissociation free energy (BDFE) of hydroperoxide species HP was calculated to be 73.5 kcal/mol, employing the thermodynamic square scheme and Bordwell relationship. This calculated O-H BDFE of HP, in fact, lines up with an experimental demonstration of the oxidizing ability of S via hydrogen atom transfer (HAT) from TEMPO-H (2,2,6,6-tetramethylpiperdine-N-hydroxide, BDFE = 66.5 kcal/mol in THF), forming the hydroperoxide species HP and TEMPO radical. Kinetic studies carried out with TEMPO-H(D) reveal second-order behavior, kH = 0.5, kD = 0.08 M-1 s-1 (THF, -80 °C); thus, the hydrogen/deuterium kinetic isotope effect (KIE) = 6, consistent with H-atom abstraction by S being the rate-determining step. This appears to be the first case where experimentally derived thermodynamics lead to a ferric heme hydroperoxide OO-H BDFE determination, that FeIII-OOH species being formed via HAT reactivity of the partner ferric heme superoxide complex.

Identification of cadmium-produced lipid hydroperoxides, transcriptomic changes in antioxidant enzymes, xenobiotic transporters, and pro-inflammatory markers in human breast cancer cells (MCF7) and protection with fat-soluble vitamins.

Cadmium (Cd) is a toxic metal that is regarded as a metallohormone with estrogen-like properties. The present study aimed at identification of lipid hydroperoxides produced in human breast cancer (MCF7) exposed to cadmium (Cd) at environmentally relevant levels. Cd induced cytotoxicity and oxidative stress and produced a series of 26 lipid hydroperoxide species including 14 phosphatidylcholine hydroperoxides (PC-OOH), 9 triacylglycerol hydroperoxides (TG-OOH), and 3 cholesteryl ester hydroperoxides (CE-OOH). Among these hydroperoxides, PC34:2-OOH, PC34:3-OOH, TG60:14-OOH, TG48:5-OOH, TG60:15-OOH, and CE20:4-OOH were produced in a dose-dependent manner, suggesting these as possible biomarkers for Cd exposure in MCF7 cells. In addition, Cd led to significant decreases in the gene expressions of antioxidants, detoxification enzymes, and xenobiotic transporters. In a protection trial, co-exposure of MCF7 cells to fat-soluble vitamins including vitamin A, D, and E reduced Cd-induced cytotoxicity, lipid peroxidation, oxidative stress, and inflammatory responses. Fat-soluble vitamins upregulated antioxidant and detoxification enzymes, and xenobiotic transporters. Therefore, dietary supplementation of such micronutrients is recommended for people at risk for exposure to Cd.

2 D Hybrid of Ni-LDH Chips on Carbon Nanosheets as Cathode of Zinc-Air Battery for Electrocatalytic Conversion of O2 into H2 O2.

It remains great challenge to develop precious-metal-free electrocatalysts to implement high-activity electrochemical conversion of O2 into value-added hydroperoxide species (HO2 - ), which are vulnerable when exposed to various transition-metal-based catalysts. A strategy based on steric hindrance and layered nickel-based layered double hydroxide (Ni-LDH) induction has been developed for one-pot inlaying high-density ultrathin 2 D Ni-LDH chips on in situ-grown carbon nanosheets (Ni-LDH C/CNSs). The resulting material exhibits high electrocatalytic selectivity with a faradaic efficiency up to 95 % for oxygen reduction into peroxide and attains a fairly high mass activity of approximately 22.2 A g-1 , outperforming most metal-based catalysts reported previously. Systematic studies demonstrate that the greatly increased defect concentration at Ni edge sites of Ni-LDH chips results in more active sites, which contributes a favorable thermodynamically neutral adsorption of OOH* and adsorbed H2 O2 molecules relatively weakly. Additionally, the modified CNSs effectively suppress H2 O2 decomposition and avoid O-O bond cleavage to produce H2 O by steric effects. The synergistic effect of CNSs and Ni-LDH chips therefore leads to high activity and high selectivity in a two-electron pathway. A proof-of-concept zinc-air fuel cell is proposed and set up to demonstrate the feasibility of green synthesis of peroxide, generating an impressive H2 O2 production rate of 5239.67 mmol h-1 gcat. -1 .

(Invited) Electrochemical Analogues of Dioxygenase and Monooxygenase Activity in Synthetic Iron Porphyrins

Dioxygen adducts of heme proteins are ubiquitous in all O2 activating/reducing heme proteins in nature. While artificial dioxygen adducts of heme are known for six decades, till date these have not been demonstrated to be able to oxidize organic substrates in sharp contrast to their non-heme analogues and naturally occurring enzymes like heme dioxygenases. To address this apparent anomaly an iron porphyrin complex is synthesized which includes a pendant quinol group. The corresponding dioxygen bound iron porphyrin species is demonstrated to perform Hydrogen atom transfer (HAT) from a quinol group appended to the porphyrin ligand. The resultant ferric peroxide, formed by the first HAT, performs a 2nd HAT generating a ferryl species (FeIV=O) and resulting in the 2e-/2H+ oxidation of the pendant hydroquinol to quinone. All the intermediate species are trapped at cryogenic temperatures and spectroscopically characterized using EPR and resonance Raman spectroscopy. DFT calculations reproduce the experimental kinetic parameters and provide insight into the nature of TS involved. These results demonstrate that artificial analogues of both heme ferric superoxide and ferric hydroperoxide species can perform HAT from an organic substrate when appropriately pre-organized to hydrogen bond to the bound superoxide and peroxide species. Siilarly, catalytic oxidation of organic substrates, using a green oxidant like O2, has been a long-term goal of the scientific community. In nature, these oxidations are performed by metalloenzymes which generate highly oxidizing species from O2 which, in turn, can oxidize inert organic substrates e.g. mono/di oxygenases. The same oxidants are produced during O2 reduction during respiration in the mitochondria but are reduced by electron transfer i.e. reductases. Iron porphyrin mimics of the active site of Cytochrome P450 (Cyt P450) are created atop self-assembled monolayer covered electrode. The rate of electron transfer from the electrode to the iron porphyrin site is attenuated to derive monooxygenase reactivity from these constructs which generally show reductase activity and catalytic hydroxylation of inert C-H bonds to alcohol and epoxidation alkenes, using molecular O2, is demonstrated with turnover numbers > 104. Mechanistic investigations suggest that a compound I analogue, formed during O2 reduction, is the primary oxidant and the selectivity is determined by the shape of the distal environment of the catalyst.

Selective Manganese-Catalyzed Oxidation of Hydrosilanes to Silanols under Neutral Reaction Conditions.

The first manganese-catalyzed oxidation of organosilanes to silanols with H2 O2 under neutral reaction conditions has been accomplished. A variety of organosilanes with alkyl, aryl, alknyl, and heterocyclic substituents were tolerated, as well as sterically hindered organosilanes. The oxidation appears to proceed by a concerted process involving a manganese hydroperoxide species. Featuring mild reaction conditions, fast oxidation, and no waste byproducts, the protocol allows a low-cost, eco-benign synthesis of both silanols and silanediols.

Selective Manganese‐Catalyzed Oxidation of Hydrosilanes to Silanols under Neutral Reaction Conditions

AbstractThe first manganese‐catalyzed oxidation of organosilanes to silanols with H2O2 under neutral reaction conditions has been accomplished. A variety of organosilanes with alkyl, aryl, alknyl, and heterocyclic substituents were tolerated, as well as sterically hindered organosilanes. The oxidation appears to proceed by a concerted process involving a manganese hydroperoxide species. Featuring mild reaction conditions, fast oxidation, and no waste byproducts, the protocol allows a low‐cost, eco‐benign synthesis of both silanols and silanediols.

Hydrogen atom abstraction by synthetic heme ferric superoxide and hydroperoxide species.

To date, artificial dioxygen adducts of heme have not been demonstrated to be able to oxidize organic substrates in sharp contrast to their non-heme analogues and naturally occurring enzymes like heme dioxygenases. To address this apparent anomaly, an iron porphyrin complex is synthesized which includes a pendant quinol group. The corresponding dioxygen bound iron porphyrin species is demonstrated to perform hydrogen atom transfer (HAT) from the quinol group appended to the porphyrin ligand. The resultant ferric peroxide, formed by the first HAT, performs a 2nd HAT generating a ferryl species (FeIV[double bond, length as m-dash]O) and resulting in the 2e-/2H+ oxidation of the pendant hydroquinone to quinone.

Low-Temperature Oxidation of Ethylene by Ozone in a Jet-Stirred Reactor.

Ethylene oxidation initiated by ozone addition (ozonolysis) is carried out in a jet-stirred reactor from 300 to 1000 K to explore the kinetic pathways relevant to low-temperature oxidation. The temperature dependencies of species' mole fractions are quantified using molecular-beam mass spectrometry with electron ionization and single-photon ionization employing tunable synchrotron-generated vacuum-ultraviolet radiation. Upon ozone addition, significant ethylene oxidation is found in the low-temperature regime from 300 to 600 K. Here, we provide new insights into the ethylene ozonolysis reaction network via identification and quantification of previously elusive intermediates by combining experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Specifically, the C2H4 + O3 adduct C2H4O3 is identified as a keto-hydroperoxide (hydroperoxy-acetaldehyde, HOOCH2CHO) based on the calculated and experimentally observed ionization energy of 9.80 (±0.05) eV. Quantification using a photoionization cross-section of 5 Mb at 10.5 eV results in 5 ppm at atmospheric conditions, which decreases monotonically with temperature until 550 K. Other hydroperoxide species that contribute in larger amounts to the low-temperature oxidation of C2H4, like H2O2, CH3OOH, and C2H5OOH, are identified and their temperature-dependent mole fractions are reported. The experimental evidence for additional oxygenated species such as methanol, ketene, acetaldehyde, and hydroxy-acetaldehyde suggest multiple active oxidation routes. This experimental investigation closes the gap between ozonolysis at atmospheric and elevated temperature conditions and provides a database for future modeling.

Investigation of the low-temperature oxidation of n-butanal in a jet-stirred reactor

Abstract n-Butanal is a key intermediate formed during the low-temperature oxidation of hydrocarbons and is also a toxic pollutant during the high-temperature oxidation of biofuels. In this work, its low-temperature oxidation chemistry was studied over the temperature range of 500–900 K in a jet-stirred reactor (JSR) at 700 Torr. Synchrotron vacuum ultraviolet photoionization mass spectrometry was employed to identify and quantify the intermediates. A total of 40 species were detected including some reactive peroxy radicals and hydroperoxide species. Speciation measurements obtained in this work can serve as validation targets for new reaction mechanisms, providing new kinetic insights into the n-butanal low-temperature oxidation. Particularly, the O2-addition to the carbonyl radical of n-butanal (n-C3H7CO) proves to be a competing channel with the direct decomposition, which is supported by our experimental results.

The formation of cross‐linking networks in a fluorinated polymer composite system by electron beam irradiation

AbstractIn this article, a cross‐linking network in poly(vinylidene fluoride) (PVDF)/poly(ethylene‐tetrafluoroethylene) copolymer (ETFE) blends was successfully created by electron beam irradiation. Thermal stability, mechanical properties, chemical structure, rheology properties, and crystallization behavior of irradiated samples were investigated. In particular, the PVDF/ETFE (10/90 wt%) blend irradiated with different dose exhibited better thermal stability and mechanical properties than pure ETFE. X‐ray photoelectron spectroscopy results demonstrated that the fluorinated composite system underwent cross‐linking and degradation reactions as a result of HF elimination and the formation of unsaturated structures and hydroperoxide species. Furthermore, the melting property of irradiated samples changed from liquid‐like behavior to pseudo‐solid‐like behavior. The degree of crystallinity of irradiated samples was found to increase at the range of 0–30 kGy and then clearly decreases with further increase in the irradiation dose. In general, our findings suggest that PVDF/ETFE blends treated with electron beam irradiation provide an opportunity to exploit their stability and durability for mechanically challenging applications.

Air Activated Self-Decontaminating Polydicyclopentadiene PolyHIPE Foams for Rapid Decontamination of Chemical Warfare Agents.

The threat of chemical warfare agents (CWA) compels research into novel self-decontaminating materials (SDM) for the continued safety of first-responders, civilians, and active service personnel. The capacity to actively detoxify, as opposed to merely sequester, offending agents under typical environmental conditions defines the added value of SDMs in comparison to traditional adsorptive materials. Porous polymers, synthesized via the high internal phase emulsion (HIPE) templating, provide a facile fabrication method for materials with permeable open cellular structures that may serve in air filtration applications. PolyHIPEs comprising polydicyclopentadiene (polyDCPD) networks form stable hydroperoxide species following activation in air under ambient conditions. The hydroperoxide-containing polyDCPD materials react quickly with CWA simulants, Demeton-S and 2-chloroethyl ethyl sulfide, forming oxidation products as confirmed via gas chromatography mass spectrometry. The simplicity of the detoxification chemistry paired with the porous foam form factor presents an exciting opportunity for the development of self-decontaminating filter media.

Cover Feature: Directing a Non‐Heme Iron(III)‐Hydroperoxide Species on a Trifurcated Reactivity Pathway (Chem. Eur. J. 20/2018)

Cover Feature: Directing a Non‐Heme Iron(III)‐Hydroperoxide Species on a Trifurcated Reactivity Pathway (Chem. Eur. J. 20/2018)

Protective Role of Italian Juglans regia L. nut Ethanolic Extract in Human Keratinocytes under Oxidative and Inflammatory Stress.

In this research, fatty acid profile and polyphenolic content of an ethanolic extract of walnut from Juglans regia L. collected in Central Italy, were characterized. The potential antioxidant and anti-inflammatory effects of the extract were investigated in the human keratinocytes cell line. Fatty acid profile was determined by gas chromatography/mass spectrometry analysis, total phenolic content by Folin-Ciocalteu method and aluminum chloride colorimetric method was used for determination of total flavonoids. Kertatinocytes were exposed to t-butyl hydroperoxide or Tumor Necrosis Factor alfa in the absence or presence of extract. Reduced glutathione was determined by Sedlak method; lipid peroxidation was assessed by measuring thiobarbituric acid reactive substances. t-butyl hydroperoxide and Tumor Necrosis Factor alfa-induced intracellular reactive oxygen species were monitored by fluorescent probes. The expression of some genes related to the inflammatory process (IL-6, IL-8, ikB, and ICAM) were analysed by Real-time PCR. JRE contains a favourable fatty acid profile with low saturated fats (19%) and high-unsaturated fats (81%) with a prevalence of the omega-6 linoleic acid (48%). Also a significant amount of polyphenols was found (5,0052 mg gallic acid equivalent/gdw). Antioxidant and anti-inflammatory properties of JRE were observed on analysed cellular model. JRE antioxidants counteracted ROS production, GSH depletion and lipid peroxidation as well downregulated the expression of some genes related to the inflammatory process. Moreover, polyunsaturated fatty acids exhibited anti-inflammatory properties. The obtained results uphold walnut as dietary adjunct in health promotion and drive towards its development in drug therapy against chronic inflammatory disorders, including inflammatory skin diseases.

Directing a Non-Heme Iron(III)-Hydroperoxide Species on a Trifurcated Reactivity Pathway.

The reactivity of [FeIII (tpena)]2+ (tpena=N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate) as a catalyst for oxidation reactions depends on its ratio to the terminal oxidant H2 O2 and presence or absence of sacrificial substrates. The outcome can be switched between: 1) catalysed H2 O2 disproportionation, 2) selective catalytic oxidation of methanol or benzyl alcohol to the corresponding aldehyde, or 3) oxidative decomposition of the tpena ligand. A common mechanism is proposed involving homolytic O-O cleavage in the detected transient purple low-spin (S=1/2 ) [(tpenaH)FeIII O-OH]2+ . The resultant iron(IV) oxo and hydroxyl radical both participate in controllable hydrogen-atom transfer (HAT) reactions. Consistent with the presence of a weaker σ-donor carboxylate ligand, the most pronounced difference in the spectroscopic properties of [Fe(OOH)(tpenaH)]2+ and its conjugate base, [Fe(OO)(tpenaH)]+ , compared to non-heme iron(III) peroxide analogues supported by neutral multidentate N-only ligands, are slightly blue-shifted maxima of the visible absorption band assigned to ligand-to-metal charge-transfer (LMCT) transitions and, corroborating this, lower FeIII /FeII redox potentials for the pro-catalysts.

Cholesterol Hydroperoxide Generation, Translocation, and Reductive Turnover in Biological Systems.

Cholesterol is like other unsaturated lipids in being susceptible to peroxidative degradation upon exposure to strong oxidants like hydroxyl radical or peroxynitrite generated under conditions of oxidative stress. In the eukaryotic cell plasma membrane, where most of the cellular cholesterol resides, peroxidation leads to membrane structural and functional damage from which pathological states may arise. In low density lipoprotein, cholesterol and phospholipid peroxidation have long been associated with atherogenesis. Among the many intermediates/products of cholesterol oxidation, hydroperoxide species (ChOOHs) have a number of different fates and deserve special attention. These fates include (a) damage-enhancement via iron-catalyzed one-electron reduction, (b) damage containment via two-electron reduction, and (c) inter-membrane, inter-lipoprotein, and membrane-lipoprotein translocation, which allows dissemination of one-electron damage or off-site suppression thereof depending on antioxidant location and capacity. In addition, ChOOHs can serve as reliable and conveniently detected mechanistic reporters of free radical-mediated reactions vs. non-radical (e.g., singlet oxygen)-mediated reactions. Iron-stimulated peroxidation of cholesterol and other lipids underlies a newly discovered form of regulated cell death called ferroptosis. These and other deleterious consequences of radical-mediated lipid peroxidation will be discussed in this review.