Spin trapping of polyunsaturated fatty acid-derived peroxyl radicals: reassignment to alkoxyl radical adducts

[What is the Initiating Reaction for the Lipid Radical Chain Reaction System That Can Induce Ferroptotic Cell Death at the Lower Oxygen Content?]

The neural cell death in cerebral infarction is suggested to be ferroptosis-like cell death, involving the participation of 15-lipoxygenase (15-LOx). Ferroptosis is induced by lipid radical species generated through the one-electron reduction of lipid hydroperoxides, and it has been shown to propagate intracellularly and intercellularly. At lower oxygen concentration, it appeared that both regiospecificity and stereospecificity of conjugated diene moiety in lipoxygenase-catalysed lipid hydroperoxidation are drastically lost. As a result, in the reaction with linoleic acid, the linoleate 9-peroxyl radical-ferrous lipoxygenase complex dissolves into the linoleate 9-peroxyl radical and ferrous 15-lipoxygenase. Subsequently, the ferrous 15-lipoxygenase then undergoes one-electron reduction of 13-hydroperoxy octadecadienoic acid, generating an alkoxyl radical (pseudoperoxidase reaction). A part of the produced lipid alkoxyl radicals undergoes cleavage of C-C bonds, liberating small molecular hydrocarbon radicals. Particularly, in ω-3 polyunsaturated fatty acids, which are abundant in the vascular and nervous systems, the liberation of small molecular hydrocarbon radicals was more pronounced compared to ω-6 polyunsaturated fatty acids. The involvement of these small molecular hydrocarbon radicals in the propagation of membrane lipid damage is suggested.

Determining the Oxidation Mechanism through Radical Intermediates in Polysorbates 80 and 20 by Electron Paramagnetic Resonance Spectroscopy

Polysorbates 20 and 80 (PS20 and PS80) are added to many commercial biologic and vaccine pharmaceuticals. It is commonly known that these polysorbates undergo a radical oxidation mechanism; however, the identity of these radical intermediates has not been clearly determined. Furthermore, PS20 and PS80 differ by the presence of a lauric acid instead of an oleic acid, respectively. The oxidation of PS80 is thought to be centered around the double bond of the oleic acid even though PS20 also undergoes oxidation, making the mechanism of oxidation unclear for PS20. Using commercial stocks of PS20 and PS80 alkyl (R•), alkoxyl (C-O•) and peroxyl (C-OO•) radicals were detected by electron paramagnetic resonance spectroscopy likely originating from radical-initiating species already present in the material. When dissolved in water, the peroxyl radicals (C-OO•) originally in the stocks were not detected but poly(ethylene oxide) radicals were. An oxidative pathway for polysorbates was suggested based on the radical species identified in the polysorbate stock material and solutions.

Formation of free acetaldehydes derived from lipid peroxidation in U937 monocyte-like cells

Acetaldehyde can be found in human cells as a byproduct of various metabolic pathways, including oxidative processes such as lipid peroxidation. This secondary product of lipid peroxidation plays a role in various pathological processes, leading to various types of civilization diseases. In this study, the formation of free acetaldehyde induced by oxygen-centred radicals was studied in monocyte-like cell line U937. Exposure of U937 cells to peroxyl/alkoxyl radicals induced by azocompound resulted in the formation of free acetaldehyde. Acetaldehyde is formed by the cleavage of fatty acids, which represents the breakdown of fatty acids into smaller fragments initiated by the cyclization of lipid peroxyl radical and β-scission of lipid alkoxyl radical. The cleavage of fatty acids alters the integrity of the plasma and nuclear membrane, leading to the loss of cell viability. Understanding the pathological processes of acetaldehyde formation is an active area of research with potential implications for preventing and treating various diseases associated with oxidative stress.

Surface-Controlled CdS/Ti3 C2 MXene Schottky Junction for Highly Selective and Active Photocatalytic Dehydrogenation-Reductive Amination

Photocatalytic valorization and selective transformation of biomass-derived platform compounds offer great opportunities for efficient utilization of renewable resources under mild conditions. Here, the novel three-dimensional hierarchical flower-like CdS/Ti3 C2 Schottky junction (MCdS) composed of surface-controlled CdS and pretreated Ti3 C2 MXene is created for photocatalytic dehydrogenation-reductive amination of biomass-derived amino acid production under ambient temperature with unprecedented activity and selectivity. Schottky junction efficiently promotes photoexcited charge migration and separation and inhibits photogenerated electron-hole recombination, which results in a super-high activity. Meanwhile, CdS with the reduced surface energy supplies sufficient hydrogen sources for imine reduction and induces the preferential orientation of alanine, thus contributing superior selectivity. Moreover, a wide range of hydroxyl acids are successfully converted into corresponding amino acids and even one-pot conversion of glucose to alanine is easily achieved over MCdS. This work illustrates the mechanism of crystal orientation control and heterojunction construction in controlling catalytic behavior of photocatalytic nanoreactor, providing a paradigm for construction of MXene-based heterostructure.

Chemical Fate of Oils on Indoor Surfaces: Ozonolysis and Peroxidation

Unsaturated triglycerides found in food and skin oils are reactive in ambient air. However, the chemical fate of such compounds has not been well characterized in genuine indoor environments. Here, we monitored the aging of oil coatings on glass surfaces over a range of environmental conditions, using mass spectrometry, nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) techniques. Upon room air exposure (up to 17 ppb ozone), the characteristic ozonolysis products, secondary ozonides, were observed on surfaces near the cooking area of a commercial kitchen, along with condensed-phase aldehydes. In an office setting, ozonolysis is also the dominant degradation pathway for oil films exposed to air. However, for indoor enclosed spaces such as drawers, the depleted air flow makes lipid autoxidation more favorable after an induction period of a few days. Forming hydroperoxides as the major primary products, this radical-mediated peroxidation behavior is accelerated by indoor direct sunlight, but the initiation step in dark settings is still unclear. These results are in accord with radical measurements, indicating that indoor photooxidation facilitates radical formation on surfaces. Overall, many intermediate and end products observed are reactive oxygen species (ROS) that may induce oxidative stress in human bodies. Given that these species can be widely found on both food and household surfaces, their toxicological properties are worth further attention.

A new bi-radical species formed during the photochemical degradation of synthetic musk tonalide in water: Study of in-situ laser flash photolysis and validation of synthesized standard sample

The ubiquitous presence of synthetic musks is causing serious concern due to the species produced from their transformation and environmental impacts. In this study, tonalide was selected as a representative synthetic musk to evaluate the transformation mechanism and pathway in water under ultraviolet (UV) irradiation. The results showed that tonalide could undergo rapid photochemical degradation through a new pivotal bi-radical, which acts as the initial active species. The bi-radicals with a typical absorption peak at 340 nm was observed by in-situ laser flash photolysis technology, and the absolute decay rate constant was obtained as 3.61 ± 0.01 × 10⁹ M^-1 s^-1 with the life-time of 83.3 ns. The photochemical degradation by-products of tonalide were also identified by high-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry, and the precise structures of key by-products have been validated by our preparative synthesized standard samples confirmed by nuclear magnetic resonance. Thus, the mechanism of tonalide photochemical degradation, continuous photoenolization of the bi-radicals and followed cycloaddition reaction with O₂, was proposed as the predominant pathway. The main degradation by-product, photoenol which has a higher bioconcentration than that of tonalide, was found to form from the bi-radicals photoenolization. This study is the first work to propose a new bi-radical as the photoenol precursors during photochemical degradation of tonalide in water.

Electro-oxidation of tetracycline in ethanol-water mixture using DSA-Cl2 anode and stimulating/monitoring the formation of organic radicals

Recent studies reported a new strategy of electro-oxidation of organic compounds using methanol as solvent. Considering its well-known toxicity, this work sought to evaluate the use of ethanol as an alternative solvent for pollutants degradation. Therefore, thorough analyses were performed in order to evaluate tetracycline (TC) electro-oxidation using DSA-Cl₂ anode in ethanol-H₂O solutions. The effects of solvent mixture, pH and current density on the degradation efficiency were evaluated. TC degradation in methanol-water and ethanol-water media resulted in very close removals of 95% and 90%, respectively, after 15 min of electrolysis at 10 mA cm^-2. In ethanol medium, the increase in current densities from 10 to 25 mA cm^-2 did not lead to significant changes in removal efficiency. The variation of the initial pH of the solution showed that the best removal efficiencies were obtained at neutral pH resulting in TC removals up to 90%, which is actually related to the molecular structure of TC. Through analysis using electron paramagnetic resonance (EPR), the formation of radicals such as hydroxyethyl (CH₃^●CHOH), hydroxyl (^●OH) and ethoxy (CH₃CH₂O^●) were detected, which effectively contributed toward the pollutant oxidation.

Effects of Peroxyl Radicals on the Structural Characteristics and Fatty Acid Composition of High-Density Lipoprotein from Duck Egg Yolk

In this study, high-density lipoprotein (HDL) from duck egg yolk was subjected to oxidation with a system based on 2,2′-azobis (2-amidinopropane) dihydrochloride (AAPH)-derived peroxyl radicals. The effects of peroxyl radicals on the protein carbonyl, free sulfhydryl, secondary/tertiary structure, surface hydrophobicity, solubility, particle size distribution, zeta potential and fatty acid composition of HDL were investigated by using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), Fourier-transform infrared spectroscopy (FTIR), circular dichroism (CD), fluorescence spectroscopy, dynamic light scattering and ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). The results indicated that the content of protein carbonyl was significantly increased, that of free sulfhydryl was obviously reduced, and the ordered secondary structure was also decreased with increasing AAPH concentration. In addition, the surface hydrophobicity and solubility of HDL showed apparent increases due to the exposure of hydrophobic groups and aggregation of protein caused by oxidation. The fatty acid composition of HDL exhibited pronounced changes due to the disrupted protein–lipid interaction and lipid oxidation by AAPH-derived peroxyl radicals. These results may help to elucidate the molecular mechanism for the effect of lipid oxidation products on the oxidation of duck yolk proteins.

Photoactive Fe Catalyst for Light-Triggered Alkyd Paint Curing

Herein, we show that the photoactive complexes [(Cp)Fe(arene)]⁺ (Cp = cyclopentadienyl; arene = C₆H₆, C₆H₅Me) act as latent catalysts that allow for photochemical control over the onset of alkyd paint curing, without the need for antiskinning agents such as the volatile 2-butanone oxime normally used to prevent curing during paint storage. The highly soluble neutral complexes [(Cp)Fe(Ch)] and [(Cp)Fe(Ch')] (Ch = cyclohexadienyl, Ch' = methylcyclohexadienyl) readily convert to the photoactive complexes [(Cp)Fe(arene)]⁺ upon oxidation in alkyd, allowing the latter to be dosed in a wide range of concentrations. Infrared and Raman studies show similar spectral changes of the alkyd paint matrix as have been observed in alkyd curing mediated by well-known, industrially applied cobalt- and manganese-based catalyst Co(neodecanoate)₂ and [(Me₃TACN)₂Mn₂(μ-OOCR)₃](OOCR). The [(Cp)Fe(Ch)]/[(Cp)Fe(arene)]⁺ system performs equally well as these cobalt- and manganese-based catalysts in terms of drying time and outperform the manganese catalyst by showing a hardness development (increase) similar to that of the cobalt-based catalyst. Based on electron paramagnetic resonance and light-activity studies, we propose that photolysis of [(Cp)Fe(arene)]⁺ generates short-lived active Fe^II species, explaining the desired latency. The [(Cp)Fe(Ch)]/[(Cp)Fe(arene)]⁺ alkyd curing systems presented herein are unique examples of intrinsically latent paint curing catalysts that (1) are based on an abundant and harmless transition metal (Fe), (2) do not require any antiskinning agents, and (3) show favorable performance in terms of drying times and hardness development.

Electron paramagnetic resonance and spin trapping to detect free radicals from allergenic hydroperoxides in contact with the skin: from the molecule to the tissue

A major research topic consists of revealing the contribution of radical-mediated reactions in dermatological diseases related to xenobiotic-induced stress, to succeed risk assessment procedures protecting producers and consumers. Allergic contact dermatitis is the clinically relevant consequence of skin sensitization, one of the most critical occupational and environmental health issues related to xenobiotics exposure. The first key event identified for the skin sensitization process to a chemical is its aptitude to react with epidermal proteins and form antigenic structures that will further trigger the immune response. Many chemical sensitizers are suspected to react through mechanisms involving radical intermediates. This review focuses on recent progress we have accomplished over the last few years studying radical intermediates derived from skin sensitizing chemicals by electron paramagnetic resonance in combination with the spin trapping technique. Our work is carried out "from the molecule", performing studies in solution, "to the tissue", by the development of a methodology on a reconstructed human epidermis model, very close in terms of histology and metabolic/enzymatic activity to real human epidermis, that can be used as suitable biological tissue model. The benefits are to test chemicals under conditions close to human use and real-life sensitization exposures and benefit from the 3D microenvironment. This article is protected by copyright. All rights reserved.

Interplay between antioxidants in response to photooxidative stress in Arabidopsis

Tocochromanols (tocopherols, tocotrienols and plastochromanol-8), isoprenoid quinone (plastoquinone-9 and plastoquinol-9) and carotenoids (carotenes and xanthophylls), are lipid-soluble antioxidants in the chloroplasts, which play an important defensive role against photooxidative stress in plants. In this study, the interplay between the antioxidant activities of those compounds in excess light stress was analyzed in wild-type (WT) Arabidopsis thaliana and in a tocopherol cyclase mutant (vte1), a homogentisate phytyl transferase mutant (vte2) and a tocopherol cyclase overexpressor (VTE1oex). The results reveal a strategy of cooperation and replacement between α-tocopherol, plastochromanol-8, plastoquinone-9/plastoquinol-9 and zeaxanthin. In the first line of defense (non-radical mechanism), singlet oxygen is either physically or chemically quenched by α-tocopherol; however, when α-tocopherol is consumed, zeaxanthin and plastoquinone-9/plastoquinol-9 can provide alternative protection against singlet oxygen toxicity by functional replacement of α-tocopherol either by zeaxanthin for the physical quenching or by plastoquinone-9/plastoquinol-9 for the chemical quenching. When singlet oxygen escapes this first line of defense, it oxidizes lipids and forms lipid hydroperoxides, which are oxidized to lipid peroxyl radicals by ferric iron. In the second line of defense (radical mechanism), lipid peroxyl radicals are scavenged by α-tocopherol. After its consumption, plastochromanol-8 overtakes this function. We provide a comprehensive description of the reaction pathways underlying the non-radical and radical antioxidant activities of α-tocopherol, carotenoids, plastoquinone-9/plastoquinol-9 and plastochromanol-8. The interplay between the different plastid lipid-soluble antioxidants in the non-radical and the radical mechanism provides step by step insights into protection against photooxidative stress in higher plants.

Visible-light-driven amino acids production from biomass-based feedstocks over ultrathin CdS nanosheets

Chemical synthesis of amino acids from renewable sources is an alternative route to the current processes based on fermentation. Here, we report visible-light-driven amination of biomass-derived α-hydroxyl acids and glucose into amino acids using NH₃ at 50 °C. Ultrathin CdS nanosheets are identified as an efficient and stable catalyst, exhibiting an order of magnitude higher activity towards alanine production from lactic acid compared to commercial CdS as well as CdS nanoobjects bearing other morphologies. Its unique catalytic property is attributed mainly to the preferential formation of oxygen-centered radicals to promote α-hydroxyl acids conversion to α-keto acids, and partially to the poor H₂ evolution which is an undesired side reaction. Encouragingly, a number of amino acids are prepared using the current protocol, and one-pot photocatalytic conversion of glucose to alanine is also achieved. This work offers an effective catalytic system for amino acid synthesis from biomass feedstocks under mild conditions.

Biomimetic Aerobic Epoxidation of Alkenes Catalyzed by Cobalt Porphyrin under Ambient Conditions in the Presence of Sunflower Seeds Oil as a Co-Substrate

In this work, a mild and sustainable catalytic aerobic epoxidation of alkenes catalyzed by cobalt porphyrin was performed in the presence of sunflower seeds oil. Under ambient conditions, the conversion rate of trans-stilbene reached 99%, and selectivity toward epoxide formation was 88%. The kinetic studies showed that the aerobic epoxidation followed the Michaelis-Menten kinetics. Mass spectroscopy and in situ electron spin resonance indicated that linoleic acid was converted to fatty aldehydes via hydroperoxide intermediates. A plausible mechanism of epoxidation of alkenes was accordingly proposed.

Immuno-spin trapping of macromolecules free radicals in vitro and in vivo - One stop shopping for free radical detection

The only general technique that allows the unambiguous detection of free radicals is electron spin resonance (ESR). However, ESR spin trapping has severe limitations especially in biological systems. The greatest limitation of ESR is poor sensitivity relative to the low steady-state concentration of free radical adducts, which in cells and in vivo is much lower than the best sensitivity of ESR. Limitations of ESR have led to an almost desperate search for alternatives to investigate free radicals in biological systems. Here we explore the use of the immuno-spin trapping technique, which combine the specificity of the spin trapping to the high sensitivity and universal use of immunological techniques. All of the immunological techniques based on antibody binding have become available for free radical detection in a wide variety of biological systems.

Development of a Functional Contrast Agent for Targeting Lipid-derived Radicals

Lipid derived radicals and their metabolic products are closely involved in the pathogenesis of oxidative stress diseases, such as inflammation and angiogenesis, through the formation of a protein or DNA complex. The starting point of lipid peroxide generation is lipid-derived radicals, which increase explosively via radical chain reaction. Therefore, the trapping of lipid-derived radicals is useful in understanding the mechanism of the formation of oxidative stress diseases, and in suppressing the following chain reaction. On the other hand, nitroxides with a stable unpaired electron allow for spin trapping with carbon-centered radicals. Hence, we focused on the following points to develop lipid radical detection methods. 1) Fluorescence will be quenched through interaction with nitroxide's unpaired electron. 2) Nitroxide can react with lipid-derived radicals via radical-radical reaction. 3) Fluorescence will recover from the loss of an unpaired electron in nitroxide, after reaction with the lipid-derived radicals, by using a profluorescent nitroxide. In this paper, I will discuss the development of a lipid-derived detection method using profluorescent nitroxide switching methods, and discuss its application to cell imaging.

Imaging free radicals in organelles, cells, tissue, and in vivo with immuno-spin trapping

The accurate and sensitive detection of biological free radicals in a reliable manner is required to define the mechanistic roles of such species in biochemistry, medicine and toxicology. Most of the techniques currently available are either not appropriate to detect free radicals in cells and tissues due to sensitivity limitations (electron spin resonance, ESR) or subject to artifacts that make the validity of the results questionable (fluorescent probe-based analysis). The development of the immuno-spin trapping technique overcomes all these difficulties. This technique is based on the reaction of amino acid- and DNA base-derived radicals with the spin trap 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) to form protein- and DNA-DMPO nitroxide radical adducts, respectively. These adducts have limited stability and decay to produce the very stable macromolecule-DMPO-nitrone product. This stable product can be detected by mass spectrometry, NMR or immunochemistry by the use of anti-DMPO nitrone antibodies. The formation of macromolecule-DMPO-nitrone adducts is based on the selective reaction of free radical addition to the spin trap and is thus not subject to artifacts frequently encountered with other methods for free radical detection. The selectivity of spin trapping for free radicals in biological systems has been proven by ESR. Immuno-spin trapping is proving to be a potent, sensitive (a million times higher sensitivity than ESR), and easy (not quantum mechanical) method to detect low levels of macromolecule-derived radicals produced in vitro and in vivo. Anti-DMPO antibodies have been used to determine the distribution of free radicals in cells and tissues and even in living animals. In summary, the invention of the immuno-spin trapping technique has had a major impact on the ability to accurately and sensitively detect biological free radicals and, subsequently, on our understanding of the role of free radicals in biochemistry, medicine and toxicology.

A novel DPP-4 inhibitor teneligliptin scavenges hydroxyl radicals: In vitro study evaluated by electron spin resonance spectroscopy and in vivo study using DPP-4 deficient rats

AIMS

Recently various dipeptidyl peptidase-4 (DPP-4) inhibitors have emerged because of their high effectiveness and safety. In spite of their common effect of DPP-4 inhibition, the chemical structures are diverse. Here we show that the structure of teneligliptin, a novel DPP-4 inhibitor, has a scavenging activity on hydroxyl radical (·OH).

METHODS

·OH and superoxide (O2(-)) were detected by electron spin resonance (ESR) spectroscopy. ·OH and O2(-) were generated in vitro by the Fenton reaction and a hypoxanthine-xanthine oxidase system, respectively. The level of free radicals was estimated from the ESR signal intensity. The product via teneligliptin and ·OH reaction was identified by thin layer chromatography and mass spectrometry analysis. In vivo effect was also evaluated using DPP-4 deficient rats with streptozotocin-induced diabetes.

RESULTS

ESR spectroscopy analysis showed that teneligliptin did not scavenge O2(-), but scavenged ·OH in a dose dependent manner. Its activity was greater than that of glutathione. The reaction product appeared to have an oxygen-atom added structure to that of teneligliptin, which was identical to the most abundant metabolite of teneligliptin in human plasma. Furthermore, using DPP-4 deficient rat, teneligliptin did not affect plasma glucose levels or body weight, but normalized increased levels of 8-hydroxy-2'-deoxyguanosine in urine, kidney and aorta of diabetic rats, supporting that teneligliptin may have a ·OH scavenging activity in vivo independently of DPP-4 inhibition.

CONCLUSIONS

Teneligliptin is not only effective as DPP-4 inhibitor, but may also be beneficial as ·OH scavenger, which may be useful in the prevention of diabetic complications.

Universal degradation performance of a high-efficiency AgBr/Ag2CO3 photocatalyst under visible light and an insight into the reaction mechanism

A AgBr/Ag₂CO₃ photocatalyst exhibited excellent photocatalytic activity and stability under visible light and a two-stage photocatalytic mechanism was proposed.

Absence of an effect of vitamin E on protein and lipid radical formation during lipoperoxidation of LDL by lipoxygenase

Low-density lipoprotein (LDL) oxidation is the primary event in atherosclerosis, and LDL lipoperoxidation leads to modifications in apolipoprotein B-100 (apo B-100) and lipids. Intermediate species of lipoperoxidation are known to be able to generate amino acid-centered radicals. Thus, we hypothesized that lipoperoxidation intermediates induce protein-derived free radical formation during LDL oxidation. Using DMPO and immuno-spin trapping, we detected the formation of protein free radicals on LDL incubated with Cu(2+) or the soybean lipoxidase (LPOx)/phospholipase A2 (PLA2). With low concentrations of DMPO (1mM), Cu(2+) dose-dependently induced oxidation of LDL and easily detected apo B-100 radicals. Protein radical formation in LDL incubated with Cu(2+) showed maximum yields after 30 min. In contrast, the yields of apo B-100 radicals formed by LPOx/PLA2 followed a typical enzyme-catalyzed kinetics that was unaffected by DMPO concentrations of up to 50mM. Furthermore, when we analyzed the effect of antioxidants on protein radical formation during LDL oxidation, we found that ascorbate, urate, and Trolox dose-dependently reduced apo B-100 free radical formation in LDL exposed to Cu(2+). In contrast, Trolox was the only antioxidant that even partially protected LDL from LPOx/PLA2. We also examined the kinetics of lipid radical formation and protein radical formation induced by Cu(2+) or LPOx/PLA2 for LDL supplemented with α-tocopherol. In contrast to the potent antioxidant effect of α-tocopherol on the delay of LDL oxidation induced by Cu(2+), when we used the oxidizing system LPOx/PLA2, no significant protection was detected. The lack of protection of α-tocopherol on the apo B-100 and lipid free radical formation by LPOx may explain the failure of vitamin E as a cardiovascular protective agent for humans.

In vivo detection of free radicals using molecular MRI and immuno-spin trapping in a mouse model for amyotrophic lateral sclerosis

Free radicals associated with oxidative stress play a major role in amyotrophic lateral sclerosis (ALS). By combining immuno-spin trapping and molecular magnetic resonance imaging, in vivo trapped radical adducts were detected in the spinal cords of SOD1(G93A)-transgenic (Tg) mice, a model for ALS. For this study, the nitrone spin trap DMPO (5,5-dimethyl-1-pyrroline N-oxide) was administered (ip) over 5 days before administration (iv) of an anti-DMPO probe (anti-DMPO antibody covalently bound to an albumin-gadolinium-diethylenetriamine pentaacetic acid-biotin MRI contrast agent) to trap free radicals. MRI was used to detect the presence of the anti-DMPO radical adducts by a significant sustained increase in MR signal intensities (p < 0.05) or anti-DMPO probe concentrations measured from T₁ relaxations (p < 0.01). The biotin moiety of the anti-DMPO probe was targeted with fluorescence-labeled streptavidin to locate the probe in excised tissues. Negative controls included either Tg ALS mice initially administered saline rather than DMPO followed by the anti-DMPO probe or non-Tg mice initially administered DMPO and then the anti-DMPO probe. The anti-DMPO probe was found to bind to neurons via colocalization fluorescence microscopy. DMPO adducts were also confirmed in diseased/nondiseased tissues from animals administered DMPO. Apparent diffusion coefficients from diffusion-weighted images of spinal cords from Tg mice were significantly elevated (p < 0.001) compared to wild-type controls. This is the first report regarding the detection of in vivo trapped radical adducts in an ALS model. This novel, noninvasive, in vivo diagnostic method can be applied to investigate the involvement of free radical mechanisms in ALS rodent models.

Pro- and Antioxidative Effect of α-Tocopherol on Edible Oils, Triglycerides and Fatty Acids

Using advanced electron paramagnetic resonance techniques (EPR), oxidation of crude vegetable oils and their components (fatty acids and triglycerides) by radicals generated from hydrogen peroxide was investigated. The correlation rotational times were determined allowing us to characterize radicals formed during edible oils oxidation. Additionally ¹H- and ¹⁴N-hyperfine coupling constants differentiate the fatty acids dependently on their unsaturation. The acids with a higher number of unsaturated bonds exhibit higher A_N values of PBN/·lipid adduct. The waste oil with high free fatty acids content underwent the oxidation reaction more efficiently, however due to saturation and the high content of the fatty acids the carbon-centered radicals formed (upon hydrogen peroxide radicals) and their PBN (N-tert-butyl-α-phenylnitrone) adducts were less stable. The antioxidant effect was dependent on the amount of α-tocopherol added. In small amounts of up to 0.35 mg/1 g of fatty acid or triglyceride, it inhibited the creation of PBN/·lipid adducts while with higher amounts it intensified adduct formation. The α-tocopherol (AT) addition influence was also studied as spin scavenging dependence and indicated that any addition of the antioxidant in the investigated samples led to free radical scavenging and the effect increased with the increase in AT content.

Potential implication of the chemical properties and bioactivity of nitrone spin traps for therapeutics

Nitrone therapeutics has been employed in the treatment of oxidative stress-related diseases such as neurodegeneration, cardiovascular disease and cancer. The nitrone-based compound NXY-059, which is the first drug to reach clinical trials for the treatment of acute ischemic stroke, has provided promise for the development of more robust pharmacological agents. However, the specific mechanism of nitrone bioactivity remains unclear. In this review, we present a variety of nitrone chemistry and biological activity that could be implicated for the nitrone's pharmacological activity. The chemistries of spin trapping and spin adduct reveal insights on the possible roles of nitrones for altering cellular redox status through radical scavenging or nitric oxide donation, and their biological effects are presented. An interdisciplinary approach towards the development of novel synthetic antioxidants with improved pharmacological properties encompassing theoretical, synthetic, biochemical and in vitro/in vivo studies is covered.

In vivo imaging of immuno-spin trapped radicals with molecular magnetic resonance imaging in a diabetic mouse model

Oxidative stress plays a major role in diabetes. In vivo levels of membrane-bound radicals (MBRs) in a streptozotocin-induced diabetic mouse model were uniquely detected by combining molecular magnetic resonance imaging (mMRI) and immunotrapping techniques. An anti-DMPO (5,5-dimethyl-1-pyrroline N-oxide) antibody (Ab) covalently bound to an albumin (BSA)-Gd (gadolinium)-DTPA (diethylene triamine penta acetic acid)-biotin MRI contrast agent (anti-DMPO probe), and mMRI, were used to detect in vivo levels of DMPO-MBR adducts in kidneys, livers, and lungs of diabetic mice, after DMPO administration. Magnetic resonance signal intensities, which increase in the presence of a Gd-based molecular probe, were significantly higher within the livers, kidneys, and lungs of diabetic animals administered the anti-DMPO probe compared with controls. Fluorescence images validated the location of the anti-DMPO probe in excised tissues via conjugation of streptavidin-Cy3, which targeted the probe biotin moiety, and immunohistochemistry was used to validate the presence of DMPO adducts in diabetic mouse livers. This is the first report of noninvasively imaging in vivo levels of MBRs within any disease model. This method can be specifically applied toward diabetes models for in vivo assessment of free radical levels, providing an avenue to more fully understand the role of free radicals in diabetes.

Low temperature oxidation of linseed oil: a review

This review analyses and summarises the previous investigations on the oxidation of linseed oil and the self-heating of cotton and other materials impregnated with the oil. It discusses the composition and chemical structure of linseed oil, including its drying properties. The review describes several experimental methods used to test the propensity of the oil to induce spontaneous heating and ignition of lignocellulosic materials soaked with the oil. It covers the thermal ignition of the lignocellulosic substrates impregnated with the oil and it critically evaluates the analytical methods applied to investigate the oxidation reactions of linseed oil.

Initiation of radical chains by singlet oxygen (1Δg), and their propagation underpin the mechanism of oxidation of linseed oil, leading to the self-heating and formation of volatile organic species and higher molecular weight compounds. The review also discusses the role of metal complexes of cobalt, iron and manganese in catalysing the oxidative drying of linseed oil, summarising some kinetic parameters such as the rate constants of the peroxidation reactions.

With respect to fire safety, the classical theory of self-ignition does not account for radical and catalytic reactions and appears to offer limited insights into the autoignition of lignocellulosic materials soaked with linseed oil. New theoretical and numerical treatments of oxidation of such materials need to be developed. The self-ignition induced by linseed oil is predicated on the presence of both a metal catalyst and a lignocellulosic substrate, and the absence of any prior thermal treatment of the oil, which destroys both peroxy radicals and singlet O2 sensitisers. An overview of peroxyl chemistry included in the article will be useful to those working in areas of fire science, paint drying, indoor air quality, biofuels and lipid oxidation.

Measurement of intracellular biomolecular oxidation in liver ischemia-reperfusion injury via immuno-spin trapping

Hepatic ischemia-reperfusion (I/R) can lead to liver failure in association with remote organ damage, both of which have significant rates of morbidity and mortality. In this study, novel spin trapping and histopathological techniques have been used to investigate in vivo free radical formation in a rat model of warm liver I/R injury. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was administered to rats via intraperitoneal injection at a single dose of 1.5g of pure DMPO/kg body wt 2h before the initiation of liver ischemia. Blood vessels supplying the median and left lateral hepatic lobes were occluded with an arterial clamp for 60min, followed by 60min reperfusion. The effects of DMPO on I/R injury were evaluated by assessing the hepatic ultrastructure via transmission electron microscopy and by histopathological scoring. Immunoelectron microscopy was performed to determine the cellular localization of DMPO nitrone adducts. Levels of nitrone adducts were also measured to determine in situ scavenging of protein and DNA radicals. Total histopathological scoring of cellular damage was significantly decreased in hepatic I/R injury after DMPO treatment. DMPO treatment significantly decreased the hepatic conversion of xanthine oxidase and 4-hydroxynonenal formation in I/R injury compared to the untreated I/R group. The distribution of gold-nanoparticle-labeled DMPO nitrone adducts was observed in mitochondria, cytoplasm, and nucleus of hepatocytes. The formation of protein- and DNA-nitrone adducts was increased in DMPO-treated I/R livers compared to DMPO controls, indicating increased in situ protein and DNA radical formation and scavenging by DMPO. These results suggest that DMPO reduces I/R damage via protection against oxidative injury.

Synthesis and characterization of 5-alkoxycarbonyl-4-hydroxymethyl-5-alkyl-pyrroline N-oxide derivatives

The syntheses, analytical properties, and spin trapping behavior of four novel EMPO derivatives, namely 5-ethoxycarbonyl-4-hydroxymethyl-5-methyl-pyrroline N-oxide (EHMPO), 5-ethoxycarbonyl-5-ethyl-4-hydroxymethyl-pyrroline N-oxide (EEHPO), 4-hydroxymethyl-5-methyl-5-propoxycarbonyl-pyrroline N-oxide (HMPPO), and 4-hydroxymethyl-5-methyl-5-iso-propoxycarbonyl-pyrroline N-oxide (HMiPPO), towards different oxygen- and carbon-centered radicals are described.

Oxidative changes of lipids monitored by MALDI MS

Oxidation processes of lipids are of paramount interest from many viewpoints. For instance, oxidation processes are highly important under in vivo conditions because molecules with regulatory functions are generated by oxidation of lipids or free fatty acids. Additionally, many inflammatory diseases are accompanied by lipid oxidation and, therefore, oxidation products are also useful disease (bio)markers. Thus, there is also considerable interest in methods of (oxidized) lipid analysis. Nowadays, soft ionization mass spectrometric (MS) methods are regularly used to study oxidative lipid modifications due to their high sensitivities and the extreme mass resolution. Although electrospray ionization (ESI) MS is so far most popular, applications of matrix-assisted laser desorption and ionization (MALDI) MS are increasing. This review aims to summarize the so far available data on MALDI analyses of oxidized lipids. In addition to model systems, special attention will be paid to the monitoring of oxidized lipids under in vivo conditions, particularly the oxidation of (human) lipoproteins. It is not the aim of this review to praise MALDI as the "best" method but to provide a critical survey of the advantages and drawbacks of this method.

Synthesis and characterization of 5-hydroxymethyl-5-methyl-pyrroline N-oxide and its derivatives

Synthesis and spin trapping behavior of three novel DMPO derivatives, namely 5-hydroxymethyl-5-methyl-pyrroline N-oxide (HMMPO), 5-(2-furanyl)-oxymethyl-5-methyl-pyrroline N-oxide (FMMPO), and 5-(2-pyranyl)-oxymethyl-5-methyl-pyrroline N-oxide (PMMPO) towards different oxygen- and carbon-centered radicals are described. The stabilizing effect of a series of cyclodextrins on the superoxide adducts was tested.

Light-induced oxidation of unsaturated lipids as sensitized by flavins

Triplet-excited riboflavin ((3)RF*) was found by laser flash photolysis to be quenched by polyunsaturated fatty acid methyl esters in tert-butanol/water (7:3, v/v) in a second-order reaction with k approximately 3.0 x 10(5) L mol(-1) s(-1) at 25 degrees C for methyl linoleate and 3.1 x 10(6) L mol(-1) s(-1), with DeltaH(double dagger) = 22.6 kJ mol(-1) and DeltaS(double dagger) = -62.3 J K(-1) mol(-1), for methyl linolenate in acetonitrile/water (8:2, v/v). For methyl oleate, k was <10(4) L mol(-1) s(-1). For comparison, beta-casein was found to have a rate constant k approximately 4.9 x 10(8) L mol(-1) s(-1). Singlet-excited flavin was not quenched by the esters as evidenced by insensitivity of steady-state fluorescence to their presence. Density functional theory (DFT) calculations showed that electron transfer from unsaturated fatty acid esters to triplet-excited flavins is endergonic, while a formal hydrogen atom transfer is exergonic (DeltaG(o)(HAT) = -114.3, -151.2, and -151.2 kJ mol(-1) for oleate, linoleate, and linolenate, respectively, in acetonitrile). The reaction is driven by acidity of the lipid cation radical for which a pK(a) approximately -0.12 was estimated by DFT calculations. Absence of electrochemical activity in acetonitrile during cyclic voltammetry up to 2.0 V versus NHE confirmed that DeltaG(o)(ET) > 0 for electron transfer. Interaction of methyl esters with (3)RF* is considered as initiation of the radical chain, which is subsequently propagated by combination reactions with residual oxygen. In this respect, carbon-centered and alkoxyl radicals were detected using the spin trapping technique in combination with electron paramagnetic resonance spectroscopy. Moreover, quenching of (3)RF* yields, directly or indirectly, radical species which are capable of initiating oxidation in unsaturated fatty acid methyl esters. Still, deactivation of triplet-excited flavins by lipid derivatives was slower than by proteins (factor up to 10(4)), which react preferentially by electron transfer. Depending on the reaction environment in biological systems (including food), protein radicals are expected to interfere in the mechanism of light-induced lipid oxidation.

Visualizing chemical reactions and crossover processes in a fuel cell inserted in the ESR resonator: detection by spin trapping of oxygen radicals, nafion-derived fragments, and hydrogen and deuterium atoms

We present experiments in an in situ fuel cell (FC) inserted in the resonator of the ESR spectrometer that offered the ability to observe separately processes at anode and cathode sides and to monitor the formation of HO and HOO radicals, H and D atoms, and radical fragments derived from the Nafion membrane. The presence of the radicals was determined by spin-trapping electron spin resonance (ESR) with 5,5-dimethylpyrroline N-oxide (DMPO) as a spin trap. The in situ FC was operated at 300 K with a membrane-electrode assembly (MEA) based on Nafion 117 and Pt as catalyst, at closed and open circuit voltage conditions, CCV and OCV, respectively. Experiments with H(2) or D(2) at the anode and O(2) at the cathode were performed. The DMPO/OH adduct was detected only at the cathode for CCV operation, suggesting generation of hydroxyl radicals from H(2)O(2) formed electrochemically via the two-electron reduction of oxygen. The DMPO/OOH adduct, detected in this study for the first time in a FC, appeared at the cathode and anode for OCV operation, and at the cathode after CCV FC operation of >or=2 h. These results were interpreted in terms of electrochemical generation of HOO at the cathode (HO + H(2)O(2) --> H(2)O + HOO) and its chemical generation at the anode from hydrogen atoms and crossover oxygen (H + O(2) --> HOO). DMPO/H and DMPO/D adducts were detected at the anode and cathode sides, for CCV and OCV operation; H and D are aggressive radicals capable of abstracting fluorine from the tertiary carbon in the polymer membrane chain and of leading to chain fragmentation. Carbon-centered radical (CCR) adducts were detected at the cathode after CCV FC operation; weak CCR signals were also detected at the anode. CCRs can originate only from the Nafion membranes, and their presence indicates membrane fragmentation. Taken together, this study has demonstrated that FC operation involves processes such as gas crossover, reactions at the catalyst surface, and possible attack of the membrane by reactive H or D that do not occur in ex situ experiments in the laboratory, thus implying different mechanistic pathways in the two types of experiments.

Identification of free radicals of glycerophosphatidylcholines containing ω-6 fatty acids using spin trapping coupled with tandem mass spectrometry

Metal-catalysed radical oxidation of diacyl-glycerophosphatidylcholines (GPC) with omega-6 acyl polyunsaturated fatty acids (PAPC, palmitoyl-arachidonoyl-glycerophosphatidylcholine and PLPC, palmitoyl-lineloyl-glycerophosphatidylcholine) was studied. Free radical oxidation products were trapped by spin trapping with 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) and identified by electrospray mass spectrometry (ES-MS). The spin adducts of oxidised GPC containing one and two oxygen atoms and one and two DMPO molecules were observed as doubly charged ions. Structural characterisation by tandem mass spectrometry (MS/MS) of these ions revealed product ions corresponding to loss of the acyl chains (sn-1-palmitoyl and sn-2-oxidised spin adduct of lineloyl or arachidonoyl), loss of the spin trap (DMPO) and product ions attributed to oxidised sn-2 fatty acid spin adduct (lineloyl and arachidonoyl). Product ions formed by homolytic cleavages near the spin trap and also from 1,4 hydrogen elimination cleavages involving the hydroxy group in the sn-2 fatty acid spin adduct allowed to infer the nature of the radical. Altogether, the presence of GPC hydroxy-alkyl/DMPO and hydroxy-alkoxyl/DMPO spin adducts was proposed.

Identification of free radicals by spin trapping with DEPMPO and MCPIO using tandem mass spectrometry

This study evaluates the use of a pyrroline (DEPMPO) and an imidazole (MCPIO) spin trap for the detection of hydroxyl and biomolecule (a peptide and a phospholipid) free radical adducts by Electrospray Ionization Mass Spectrometry (ESI-MS). The hydroxyl and biomolecule free radical adducts were detected using a QTOF and a linear ion trap (LIT) mass spectrometers. In the presence of hydroxyl radical, the mass spectrum obtained for each of the spin traps, DEPMPO and MCPIO, showed the presence of ions that could be attributed to hydroxyl and peroxyl radicals. Further characterisation by tandem mass spectrometry (ESI-MS/MS) revealed also the presence of hydroxy-hydroxyl adducts. Based on the results here described, we show that DEPMPO is a better spin trap for free radicals trapping and detection by mass spectrometry mainly because adducts show increased signal intensity. The ESI-MS spectra obtained for DEPMPO and MCPIO in the presence of biomolecule radicals (peptide and phospholipid) show molecular ions of DEPMPO and MCPIO adducts, which were characterised by tandem mass spectrometry. Both carbon centered radicals and oxygen centered radicals were efficiently trapped by the two spin traps and analysis of QTOF-MS/MS mass spectra allowed the location of the radical position in either the peptide or in the phospholipid fatty acyl chain. However, the tandem mass spectra of MCPIO adducts were more informative than DEPMPO adducts. The LIT-MS/MS spectra only shows typical peptide and phospholipid fragmentation, which difficult the structural characterisation of the spin adduct. In this study, the DEPMPO and MCPIO adducts were identified either in the nitrone or in the hydroxylamine form, which are ESR silent forms. The results described here show that both spin traps coupled with detection by mass spectrometry are valuable tools for trapping radicals of biomolecules. Furthermore, the acquired data provide valuable information on the presence of adducts (hydroxyl and biomolecule) that are Electron Spin Resonance (ESR) silent. This is especially important considering the complexity of the radical species in biological environment and the presence of reducing compounds that convert the spin adducts to silent ESR forms.