Oxygen-centered Radicals Research Articles

Characteristics and potential inhalation exposure risks of PM2.5–bound environmental persistent free radicals in Nanjing, a mega–city in China

PM2.5–bound toxic elements and organic pollutants have been extensively investigated, while limited information is available for environmental persistent free radicals (EPFRs) associated with PM2.5, which may lead to oxidative stress in the human lung when exposed to PM2.5. In this study, the levels and types of PM2.5–bound EPFRs present in Nanjing, a mega–city in China, were analyzed. PM2.5–bound EPFRs were found to mainly be a mixture of carbon– and oxygen–centered radicals. The concentration of PM2.5–bound EPFRs ranged from 2.78 × 1012 to 1.72 × 1013 spins m−3, with an average value of 7.61 × 1012 spins m−3. The half–life of the PM2.5–bound EPFRs was calculated to be an average of 83.5 days when stored at room temperature, with only weak correlations observed between EPFRs and conventional air pollutants (NO2, O3, CO and PM2.5)/PM2.5–bound transition metals (Cu, Zn, Cr, Mn, V, Cd, and Ni) and significant correlations between EPFRs and SO2/PM2.5–bound Fe. PM2.5–bound EPFRs can induce the formation of reactive oxygen species (ROS) in both water and a H2O2 solution, which are used to simulate lung solution of a healthy person and patient, respectively. Therefore, PM2.5–bound EPFRs can lead to potential oxidative stress in humans. Overall, PM2.5–bound EPFRs show an obvious temporal variation and can pose potential health risks to humans via the induction of ROS in the lung solution.

Evolution and stabilization of environmental persistent free radicals during the decomposition of lignin by laccase

Soil microbial enzymes may induce lignin decomposition, accompanied by generation of free radicals. The evolution of environmentally persistent free radicals (EPFRs) and reactive oxygen species (ROS) during laccase-catalyzed lignin decomposition remains unclear. Characterization by electron paramagnetic resonance spectroscopy revealed gradually increased concentration of EPFRs, with maximum levels within 6 h that remained constant, accompanied by the increase in g-factor from 2.0037 to 2.0041. The results suggested the generation of oxygen-centered radicals on lignin. The EPFRs produced on solid samples slowly decreased by 17.2% over 17 d. ROS were also detected to have a similar trend as that of the evolution of EPFRs. Scanning electron microscopy, attenuated total reflectance-Fourier transform infrared spectroscopy, gel permeation chromatography and nuclear magnetic resonance analyses suggested the demethylation and oxidation of lignin. We clarify the biogeochemical transformation of lignin and potential contributions to the generation of EPFRs and ROS in soil.

Radiation-Induced Transformations of Tryptophan and Its Derivatives in Oxygenated Ethanol

The interaction of tryptophan, tryptamine, and 3-indolepropionic acid with peroxyl radicals formed during the radiolysis of oxygen-saturated ethanol has been studied using steady-state radiolysis. It has been found that 3-indolepropionic acid has a low reactivity toward peroxyl radicals. At the same time, both tryptamine and, to a lesser extent, tryptophan inhibit the radiation-induced processes of ethanol oxidation due to electron transfer from the lone pair of the amino group to oxygen-centered radicals. The tryptophan and tryptamine radical cations formed in these reactions are predominantly regenerated to the parent compounds upon interaction with $${\text{O}}_{2}^{{\bullet - }}$$ and $${\text{HO}}_{2}^{\bullet },$$ as evidenced by the low radiation-chemical yields of decomposition of the test compounds.

An EPR characterisation of stable and transient reactive oxygen species formed under radiative and non-radiative conditions

Electron paramagnetic resonance (EPR) spectroscopy is the ideal method of choice when detecting and studying the wide variety of paramagnetic oxygen-centred radicals. For simple diatomic radicals, such as the superoxide (O2−) or peroxy ({text{ROO}}^{bullet}) species, the CW EPR profile (in particular the g-values) of these species can appear similar and indeed indistinguishable in some cases. Experiments using 17O-enriched oxygen, revealing a rich 17O hyperfine pattern, are therefore essential to distinguish between the two species. However, in many cases, particularly involving TiO2 photocatalysis, the peroxy-type ({text{ROO}}^{bullet}) radicals or other intermediate species such as the [O2−…organic]-type adducts can be transient in nature and once again can produce similar g-values. In general terms, these reactive oxygen species (ROS) are formed and detected at low-temperature conditions. Hence, the application of EPR spectroscopy to studies of surface-stabilised oxygen-centred radicals must be performed under carefully selected conditions in order to confidently distinguish between the differing types of diatomic radicals, such as O2−, {text{ROO}}^{bullet} or [O2−…organic].

How Does Pyridoxamine Inhibit the Formation of Advanced Glycation End Products? The Role of Its Primary Antioxidant Activity.

Pyridoxamine, one of the natural forms of vitamin B6, is known to be an effective inhibitor of the formation of advanced glycation end products (AGEs), which are closely related to various human diseases. Pyridoxamine forms stable complexes with metal ions that catalyze the oxidative reactions taking place in the advanced stages of the protein glycation cascade. It also reacts with reactive carbonyl compounds generated as byproducts of protein glycation, thereby preventing further protein damage. We applied Density Functional Theory to study the primary antioxidant activity of pyridoxamine towards three oxygen-centered radicals (•OOH, •OOCH3 and •OCH3) to find out whether this activity may also play a crucial role in the context of protein glycation inhibition. Our results show that, at physiological pH, pyridoxamine can trap the •OCH3 radical, in both aqueous and lipidic media, with rate constants in the diffusion limit (>1.0 × 108 M s). The quickest pathways involve the transfer of the hydrogen atoms from the protonated pyridine nitrogen, the protonated amino group or the phenolic group. Its reactivity towards •OOH and •OOCH3 is smaller, but pyridoxamine can still scavenge them with moderate rate constants in aqueous media. Since reactive oxygen species are also involved in the formation of AGEs, these results highlight that the antioxidant capacity of pyridoxamine is also relevant to explain its inhibitory role on the glycation process.

Molecular oxygenates from the thermal degradation of tobacco and material characterization of tobacco char

Abstract This contribution investigates the formation of selected oxygenated molecular compounds with smoking temperatures, particulates and free radicals from the burning of tobacco selected from two cigarettes (SM1 and ES1), in a poor oxygen environment - an oxidative atmosphere (5% oxygen in N2), in the temperature region 200–700 °C at 2 s contact time. Molecular carbonyls were determined using a Gas chromatography (GC) hyphenated to a mass selective detector (MSD). Fourier transform infrared (FTIR) spectroscopy afforded the determination of functional groups adsorbed on tobacco particulates. Free radicals in tobacco smoke particulates were investigated using an electron paramagnetic resonance spectrometer (EPR) whereas particulate size of tobacco smoke was estimated using image J. Aztec (UK) software coupled to an Oxford detector (UK) facilitated elemental analysis of tobacco char. At ∼ 400 °C, 2-methylcyclopentenone gave a maximum yield of 7.95 μ g while 2-methyl-2-propanol had a maximum yield of 14.75 μ g . The absorption band at 1740 cm−1 confirms the presence of carbonyl groups ( − C = O ) adsorbed on tobacco smoke particulates. Tobacco particulates generated at 600 °C showed an isotropic g-value of 2.0028 and a particle size distribution between 18 and 30 nm. It was noted that SM1 cigarette had very high radical, oxygen and nitrogen content and therefore considerably more toxic than ES1 cigarette. Tobacco char was largely carbon (∼66–69%) – Energy dispersive X-ray (EDX) data hence the major reason carbon based radicals dominated the oxygen centered radicals at elevated temperatures. Generally, tobacco free radicals are very reactive intermediates and therefore precursors for detrimental ailments associated with cigarette smoking. These findings support the fact that research based policy framework is essential in minimizing the burden associated with tobacco abuse.

Characterization of Protein Radicals in Arabidopsis.

Oxidative modification of proteins in photosystem II (PSII) exposed to high light has been studied for a few decades, but the characterization of protein radicals formed by protein oxidation is largely unknown. Protein oxidation is induced by the direct reaction of proteins with reactive oxygen species known to form highly reactive protein radicals comprising carbon-centered (alkyl) and oxygen-centered (peroxyl and alkoxyl) radicals. In this study, protein radicals were monitored in Arabidopsis exposed to high light by immuno-spin trapping technique based on the detection of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) nitrone adducts using the anti-DMPO antibody. Protein radicals were imaged in Arabidopsis leaves and chloroplasts by confocal laser scanning microscopy using fluorescein conjugated with the anti-DMPO antibody. Characterization of protein radicals by standard blotting techniques using PSII protein specific antibodies shows that protein radicals are formed on D1, D2, CP43, CP47, and Lhcb3 proteins. Protein oxidation reflected by the appearance/disappearance of the protein bands reveals that formation of protein radicals was associated with protein fragmentation (cleavage of the D1 peptide bonds) and aggregation (cross-linking with another PSII subunits). Characterization of protein radical formation is important for better understating of the mechanism of oxidative modification of PSII proteins under high light.

Formation and Evolution of Solvent-Extracted and Nonextractable Environmentally Persistent Free Radicals in Fly Ash of Municipal Solid Waste Incinerators.

Environmentally persistent free radicals (EPFRs) are emerging contaminants occurring in combustion-borne particulates and atmospheric particulate matter, but information on their formation and behavior on fly ash from municipal solid waste (MSW) incinerators is scarce. Here, we have found that MSW-associated fly ash samples contain an EPFR concentration of 3-10 × 1015 spins g-1, a line width (ΔHp-p) of ∼8.6 G, and a g-factor of 2.0032-2.0038. These EPFRs are proposed to be mixtures of carbon-centered and oxygen-centered free radicals. Fractionation of the fly ash-associated EPFRs into solvent-extracted and nonextractable radicals suggests that the solvent-extracted part accounts for ∼45-73% of the total amount of EPFRs. Spin densities of solvent-extracted EPFRs correlate positively with the concentrations of Fe, Cu, Mn, Ti, and Zn, whereas similar correlations are comparatively insignificant for nonextractable EPFRs. Under natural conditions, these two types of EPFRs exhibit different stabilization that solvent-extracted EPFRs are relatively unstable, whereas the nonextractable fraction possesses a long life span. Significant correlations between concentrations of solvent-extracted EPFRs and generation of hydroxyl and superoxide radicals are found. Overall, our results suggest that the fractionated solvent-extracted and nonextractable EPFRs may experience different formation and stabilization processes and health effects.

Interaction of benzo[a]pyrene with Cu(II)-montmorillonite: Generation and toxicity of environmentally persistent free radicals and reactive oxygen species

This paper presents the interaction of benzo[a]pyrene (B[a]P) with Cu(II)-montmorillonite to investigate the formation, evolution and potential toxicity of environmentally persistent free radicals (EPFRs) under dark and visible light irradiation conditions. Degradation of B[a]P and the generated transformative products on clay mineral are monitored by gas chromatography–mass spectrometry (GC–MS) technique. Hydroxyl-B[a]P and B[a]P-diones are observed during the transformation of B[a]P under dark condition. B[a]P-3,6-dione and B[a]P-6,12-dione are the main products under visible light irradiation. B[a]P transformation is accompanied by the formation of EPFRs, which are quantified by electron paramagnetic resonance (EPR) spectroscopy. With increasing reaction time, the concentrations of the produced EPFRs are initially increased and then gradually decrease to an undetectable level. The deconvolution results of EPR spectra reveal formation of three types of organic radicals (carbon-centered radicals, oxygen-centered radicals, and carbon-centered radicals with a conjugated oxygen), which also co-exist. Correspondingly, visible-light irradiation promotes the formation and the decay of these EPFRs. The produced B[a]P-type EPFRs induce the generation of reactive oxygen species (ROS), such as superoxide (O2–) and hydroxide radicals (OH), which may cause oxidative stress to cells and tissues of organisms. The toxicity of degradation products is evaluated by the livability of human gastric epithelial GES-1cells. The toxicity is initially increased and then decreases with the elapsed reaction time, which correlates with the evolution of EPFRs concentrations. The present work provides direct evidence that the formation of EPFRs in interaction of PAHs with metal-contaminated clays may result in negative effects to human health.

Graphene oxide mediated reduction of silver ions to silver nanoparticles under environmentally relevant conditions: Kinetics and mechanisms

We systematically investigated the reduction mechanisms and reduction kinetics of silver ions (Ag ions) by graphene oxide (GO) under ambient condition. UV–vis spectroscopy, transmission electron microscopy, and electron diffraction results revealed that silver nanoparticles (Ag NPs) could be formed from aqueous Ag ions in the presence of GO at pH 8 under light. Formation of Ag NPs increased with increasing pH (7.4, 8, and 9) and temperature (from 30 to 90); however, the increasing ionic strength and dissolved oxygen reduced the Ag NPs yield. The Ag ions reduction by GO followed pseudo-first-order kinetics under both dark and light, and light irradiation significantly accelerated the Ag NPs formation induced by GO. The phenolic-OH on GO was the dominating electron donator for Ag ion reduction in dark. Exposure to light increased the concentration of phenolic-OH on the GO surface, thereby stimulating the reduction rate of Ag ions by GO. In addition, the light induced electron-hole pairs on GO surface and light activated oxygen-centered radicals on GO surface promoted the reduction of adsorbed Ag ions by GO. Our findings provide important information for the role of GO in reducing Ag ions to Ag NPs in aquatic environments, and shed light on understanding the environmental fate and risk of both Ag ions and GO materials.

Visible-light-responsive surface-modified TiO2 powder with 4-chlorophenol: A combined experimental and DFT study

The visible-light-responsive inorganic-organic hybrid was prepared by surface modification of commercial TiO2 powder (Degussa P25) with 4-chlorophenol (4-CP). The optical absorption of the hybrid material is red-shifted compared to unmodified TiO2 powder due to the surface charge transfer complex (CTC) formation. The experimental results are supported by the density functional theory (DFT) calculations of the corresponding model cluster. The calculated electronic excitation spectrum is in agreement with the measured reflection spectrum of surface-modified TiO2 powder with 4-CP. The paramagnetic species, generated in the unmodified and surface-modified TiO2 powders upon excitation with ultraviolet and visible light, were identified using low-temperature electron paramagnetic resonance (EPR) spectroscopy. The formation of trapped electrons (Ti(III) centers) and the persistent oxygen-centered organic radicals indicated the photoinduced electron transfer from the chemisorbed 4-chlorophenol to the conduction band of TiO2.

Reductants for polyperoxides to accelerate degradation at elevated temperatures

The alternating copolymer of a conjugated diene monomer and oxygen, i.e., polyperoxide, is a main-chain degradable polymer and undergoes radical chain degradation by heating. During the degradation process, oxygen-centered radicals are formed and thus side reactions such as hydrogen abstraction from the polymer by the oxygen-centered radicals and subsequent coupling, which prevent the degradation, sometimes take place. In this study, in order to promote the degradation of polyperoxides only at elevated temperatures while maintaining their stability at ambient temperature, appropriate organic reductants, which do not significantly react with the polyperoxides at 30 °C but react at 100 °C, were explored. N,N-Dimethylaniline, which is a conventional reductant for redox initiation systems, initiated the redox degradation of cyclopentadiene-based polyperoxides even at 30 °C due to the very high nucleophilicity. The introduction of an electron donating group at the para position, i.e., 4-dimethylaminoacetophenone, 4-dimethylaminobenzaldehyde, and 4-dimethylaminobenzonitrile, successfully maintained the stability of the polyperoxide at 30 °C and accelerated the degradation at 100 °C due to the reduced nucleophilicity. Similar results were obtained by using 1-acetyl-2-phenylhydrazine and 1-phenyl-3-pyrazolidone (PhP) as a reductant, and PhP resulted in the most significant acceleration effect at 100 °C. The activation energy for the overall degradation of the polyperoxide in the presence of PhP was much lower than that of the polyperoxide in the absence of a reductant. In the case of sorbic ester-based polyperoxides, their stabilities at 30 °C were not maintained in the presence of the reductants and the stability was improved by increasing the 2,5 structure regarding the diene moiety in the main-chain.

Hydrogen- and oxygen-atom transfers in the thermal activation of benzene mediated by Cu2O2+ cations.

The mass-selected copper oxide cluster cations Cu2O2+ are successfully prepared by laser ablation and reacted with benzene in a linear ion trap reactor. The cluster reaction is characterized by reflectron mass spectrometry in conjunction with density functional theory calculations. Four types of reaction channels are observed: (1) Cu2OH+ + C6H5O˙, (2) Cu2C6H6+ + O2, (3) Cu(C6H6)2+ + Cu and (4) Cu2O2C6H6+, in which the first one is the major product. Observation of the products Cu2OH+ indicates that oxygen atom transfer (OAT) accompanying hydrogen atom transfer (HAT) occurs, and the oxygen-centered radical (O-˙) in Cu2O2+ is crucial to the cleavages of C-H and Cu-O bonds. It is interesting that HAT and OAT occur in a single reaction channel to form C6H5O˙, which has not been reported in the reactions of gas-phase ions with benzene. The calculated results are in agreement with the experimental observations, and no two-state reactivity scenario is present in the potential energy surface of channel 1. This work can provide useful insights into the nature of active sites over copper oxides and reaction mechanisms in the corresponding heterogeneous reactions.

Roaming-like Mechanism for Dehydration of Diol Radicals.

Diol radicals (DRs) are important intermediates in biocatalysis, atmospheric chemistry, and biomass combustion. They are particularly generated from photolysis of halogenated diols and addition of hydroxyl radical to a double bond of unsaturated alcohols, such as lignols. The energized DRs further isomerize/decompose to form products, including water. Aqueous-phase dehydration in radiolytic and biomimetic systems typically occurs at low temperatures, with or without catalysis, whereas the gas-phase dehydration is usually considered energetically unfavorable. In the present study, we propose a new low-energy, roaming-like mechanism based on a detailed dispersion-corrected DFT and ab initio level analysis of the gas-phase dehydration of DRs obtained from the combination of OH radicals with allyl alcohol (AA, CH2═CHCH2OH)-the simplest relevant model of the unsaturated alcohols. The roaming pathways involve a nearly dissociated OH-group, which subsequently abstracts an H atom of the remaining fragment to form water and [C3H5O] radical via a transition state (TS) with energy close to the C-O bond fission asymptote. Two types of roaming-like first-order saddle points (SP) are identified for unimolecular dehydration of 1,2- and 1,3-DR radical adducts involving either both hydroxyl groups of diol radicals to generate an oxygen-centered radical, or β-OH group and a skeletal α-hydrogen atom of the 1,2-DR to form a resonantly stabilized hydroxyallyl radical. Two higher energy conventional (tight) transition states, along with the pathways to 1,2-OH-migration, as well as direct H-abstraction, are also identified and analyzed. Most of the traditional density functional theory methods that have been successfully employed in the literature to locate so-far-known roaming SPs were also able to identify the new mechanism, in accord with dispersion-corrected double hybrid B2PLYP-D3(BJ) and mPW2PLYPD methods involving MP2-correlation corrections. However, the MP2 method itself failed to locate any of them, which seems to be typical for MP2 method for loose TS structures, confirmed here for a flat region of PES connecting direct and roaming saddle points. However, MP2 method correctly locates an identical roaming SP for a larger p-coumaryl alcohol model involving hydroxyphenyl substituent at Cγ atom of AA. Two types of interfragmental interactions are identified that stabilize the roaming SPs: (a) H-bonding of the leaving OH radical either with the H atom of the remaining OH group, or with π-cloud of the double bond; (b) direct interaction of π-electrons with the lone-pair electrons of the heteroatom in the leaving OH group through the TS-ring. The alternative TSs are qualitatively characterized by "collinearity" angle of the OH radical attack on the O-H/C-H bonds of the substrate in abstraction-like O-H-O geometry, attributed to the improved orbital overlaps. The proposed mechanism presents broader implications to signify, particularly, a larger role in atmospheric and combustion processes, especially biomass pyrolysis.

The leishmanicidal activity of artemisinin is mediated by cleavage of the endoperoxide bridge and mitochondrial dysfunction.

Endoperoxides kill malaria parasites via cleavage of their endoperoxide bridge by haem or iron, leading to generation of cytotoxic oxygen-centred radicals. In view of the Leishmania parasites having a relatively compromised anti-oxidant defense and high iron content, this study aims to establish the underlying mechanism(s) accounting for the apoptotic-like death of Leishmania promastigotes by artemisinin, an endoperoxide. The formation of reactive oxygen species was confirmed by flow cytometry and was accompanied by inhibition of mitochondrial complexes I-III and II-III. However, this did not translate into a generation of mitochondrial superoxide or decrease in oxygen consumption, indicating minimal impairment of the electron transport chain. Artemisinin caused depolarization of the mitochondrial membrane along with a substantial depletion of adenosine triphosphatase (ATP), but it was not accompanied by enhancement of ATP hydrolysis. Collectively, the endoperoxide-mediated radical formation by artemisinin in Leishmania promastigotes was the key step for triggering its antileishmanial activity, leading secondarily to mitochondrial dysfunction indicating that endoperoxides represent a promising therapeutic strategy against Leishmania worthy of pharmacological consideration.

Transformation of functional groups and environmentally persistent free radicals in hydrothermal carbonisation of lignin

In this study, lignin was selected as the main component of waste biomass to synthesise carbonaceous adsorbents with environmentally persistent free radicals (EPFRs) through hydrothermal carbonisation (HTC) under different conditions. The HTC method with Fe(III) doping was superior for promoting dehydration and decarboxylation of lignin hydrochar than that with HTC Cu(II) doping. The deconstruction of oxygen-containing groups of lignin was reduced while esterification was enhanced by Fe(III). The Fe(III) addition was conducive to the formation of EPFRs as carbon-centred radicals with an adjacent oxygen atom and oxygen-centred radicals. A comparison of lignin with cellulose and d-xylose suggests that the formed EPFRs on lignin hydrochar were derived from their phenolic hydroxyl groups. A high removal efficiency of bisphenol A by lignin hydrochar was observed owing to oxygen-centred radicals, which activated H2O2 to produce OH. These results will facilitate the design and application of novel hydrochar materials with EPFRs.

The Quest for Selectivity in Hydrogen Atom Transfer Based Aliphatic C-H Bond Oxygenation.

Aliphatic C-H bond functionalization is at the frontline of research because it can provide straightforward access to simplified and cost-effective synthetic procedures. A number of these methodologies are based on hydrogen atom transfer (HAT), which, as a consequence of the inert character of C-H bonds, often represents the most challenging step of the overall process. Because the majority of organic molecules contain multiple nonequivalent C-H bonds that display similar chemical properties, differentiating between these bonds with high levels of selectivity represents one of the most challenging issues. Clarification of the factors that govern the relative reactivity of C-H bonds toward HAT reagents is thus of primary importance in order to develop selective functionalization procedures. In this Account we describe, through the combination of kinetic studies employing a genuine HAT reagent such as the cumyloxyl radical, along with oxidations performed with H2O2 and iron or manganese catalysts, our contribution toward the development of selective C-H functionalization methodologies. Despite the different nature of these reagents, an oxygen-centered radical and a metal-oxo species, congruent reactivity and selectivity patterns have emerged, providing strong evidence that both reactions proceed via HAT. Consequently, selectivity in this class of metal catalyzed C-H oxidations can be reasonably predicted and synthetically exploited. Amides have been identified as preferential functional groups for governing selectivity on the basis of electronic, steric, and stereoelectronic effects. Torsional effects have proven moreover to be particularly important C-H directing factors in the oxidation of cyclohexane scaffolds where a delicate balance of these effects, in synergistic combination with catalyst design, enables highly chemoselective and enantioselective oxidations. Medium effects have been also shown to govern the relative HAT reactivity of C-H bonds in proximity to polar, hydrogen bond acceptor (HBA) functional groups. By engaging in hydrogen bonding with these groups, fluorinated alcohols strongly deactivate proximal C-H bonds toward HAT-based oxidation. As a result, alcohols, ethers, amines, and amides, which are electron rich and effective proximal C-H activating groups toward HAT reagents in conventional solvents, become oxidatively robust deactivating functionalities that direct C-H oxidation toward remote positions. These deactivating effects enable moreover the accomplishment of product chemoselective methylenic hydroxylations. Overall, clarification of the factors that govern HAT-based reactions has served to provide unique examples of catalytic methodologies for chemoselective and enantioselective oxidation of nonactivated aliphatic C-H bonds of potential utility in organic synthesis.

Dominant Fraction of EPFRs from Nonsolvent-Extractable Organic Matter in Fine Particulates over Xi'an, China.

To understand the nature and possible sources of environmentally persistent free radicals (EPFRs) in atmospheric aerosols, the present study used a solvent extraction method to fractionate aerosol components with different polarities and solvent resistance in fine particulate matter (PM2.5) from Xi'an, China. The characteristics of EPFRs, that is., their concentration, type and lifetime, were obtained based on their electron paramagnetic resonance spectra. The results showed that the EPFRs in the PM2.5 samples were carbon-centered with a nearby heteroatom ( g = 2.0031) and had a long half-life of more than 3 years. Nearly all of the extractable EPFRs were detected in the water-insoluble organic fraction and showed characteristics indicating that may contain oxygen-centered radical ( g = 2.0038). Most of the total EPFRs in the PM2.5 were derived from solvent-resistant organic matter (88%), which likely consisted of graphene oxide analogues. The results suggest that previous studies may have missed the major proportion of EPFRs in atmospheric particulates if they only focused on solvent-extractable or metallic oxide-formed EPFRs. Our results showed that the EPFR concentration was significantly and positively correlated with the elemental carbon and NO2 concentrations, suggesting that traffic emissions may be an important source of EPFRs in PM2.5 over Xi'an.

Mechanisms of triacylglycerides oxidation in artificial lipid droplets

Lipid droplets (LDs) are inducible cellular organelles that have a pivotal role in physiological processes. Depending on their size and host cell type they serve functions ranging from lipid storage (e.g. in adipocytes), cytoprotection (in non-adipose tissue) and inflammation (in macrophages and leukocytes).[1] LD are micelles consisting of a neutral lipid core (triglycerides - TAG, cholesteryl esters - CE and fatty acids - FA) surrounded by a monolayer of phospholipids (PL). Under conditions of oxidative stress (e.g. cancer) lipid droplets have been found to contain increased levels of oxidized TAG (oxTAG). [2] Oxidized lipid species have been shown to be involved in the disturbance of cellular processes by various mechanisms but are also known to serve regulatory functions [3]. Non-enzymatic lipid oxidation is based on an oxygen dependent radical-propagation mechanism. Due to the highly unpolar milieu in the LDs core, polar oxygen-centered radicals (e.g. hydroxyl radical) cannot penetrate through the outer LDs phospholipid monolayer indicating that other mechanisms of radical reactions might govern generation of oxidized lipids in LDs, either involving longer lived, hydrophobic organic radicals or radical propagation from PL monolayer into the LD core. LDs of specific composition (molar ratio: PC/PE 3:1; PL/TAG 1:20) and defined size (140 nm) were prepared by differential ultracentrifugation steps. Size and composition of generated micellar structures was elucidated by nanotracking light scattering microscopy and fluorescence spectrophotometry. Generated micelles were oxidized by either Fenton chemistry (hydroxyl radicals) or by AAPH (organic radicals) and further analyzed by LC-MS using RPC-ESI-QTOF or ESI-LTQ-Orbitrap.

The Nature of Stable Char Radicals: An ESR and DFT Study of Structural and Hydrogen Bonding Requirements.

Pyrolysis is a promising way to convert biomass into fuels and chemicals. This reaction is complex and inevitably involves a cascade of radical reactions that lead to char formation, in which some radicals become trapped and stabilized. Their nature is difficult to characterize, and in this respect computational chemistry can be a strong supplementary tool to electron spin resonance spectroscopy and other experimental methods. Here biomass char radicals and oxidation reactivity are studied experimentally, and density functional theory is used to predict the thermodynamic stability and g-values of carbon- and oxygen-centered radicals of polyaromatic char models including defect structures. Hydroxylated and especially certain dihydroxylated structures provide exceptional stabilization of oxygen-centered radicals. Hydrogen bonding plays a crucial role, and it is proposed that hydrogen atom transfer couples radical localizations. This is a new proposal on the structural requirements for stabilization of char radicals, which impacts our understanding of pyrolysis mechanisms and char reactivity.