Trapping of free radicals formed by gamma-irradiation of organic compounds

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.

Insights into the Antioxidant Mechanism of Newly Synthesized Benzoxazinic Nitrones: In Vitro and In Silico Studies with DPPH Model Radical

Synthetic nitrone spin-traps are being explored as therapeutic agents for the treatment of a wide range of oxidative stress-related pathologies, including but not limited to stroke, cancer, cardiovascular, and neurodegenerative diseases. In this context, increasing efforts are currently being made to the design and synthesis of new nitrone-based compounds with enhanced efficacy. The most researched nitrones are surely the ones related to α-phenyl-tert-butylnitrone (PBN) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) derivatives, which have shown to possess potent biological activity in many experimental animal models. However, more recently, nitrones with a benzoxazinic structure (3-aryl-2H-benzo[1,4]oxazin-N-oxides) have been demonstrated to have superior antioxidant activity compared to PBN. In this study, two new benzoxazinic nitrones bearing an electron-withdrawing methoxycarbonyl group on the benzo moiety (in para and meta positions respect to the nitronyl function) were synthesized. Their in vitro antioxidant activity was evaluated by two cellular-based assays (inhibition of AAPH-induced human erythrocyte hemolysis and cell death in human retinal pigmented epithelium (ARPE-19) cells) and a chemical approach by means of the α,α-diphenyl-β-picrylhydrazyl (DPPH) scavenging assay, using both electron paramagnetic resonance (EPR) spectroscopy and UV spectrophotometry. A computational approach was also used to investigate their potential primary mechanism of antioxidant action, as well as to rationalize the effect of functionalization on the nitrones reactivity toward DPPH, chosen as model radical in this study. Further insights were also gathered by exploring the nitrone electrochemical properties via cyclic voltammetry and by studying their kinetic behavior by means of EPR spectroscopy. Results showed that the introduction of an electron-withdrawing group in the phenyl moiety in the para position significantly increased the antioxidant capacity of benzoxazinic nitrones both in cell and cell-free systems. From the mechanistic point of view, the calculated results closely matched the experimental findings, strongly suggesting that the H-atom transfer (HAT) is likely to be the primary mechanism in the DPPH quenching.

Amidinoquinoxaline-Based Nitrones as Lipophilic Antioxidants

The potential of nitrones (N-oxides) as therapeutic antioxidants is due to their ability to counteract oxidative stress, mainly attributed to their action as radical scavengers toward C- and O-centered radicals. Among them, nitrones from the amidinoquinoxaline series resulted in interesting derivatives, due to the ease with which it is possible to introduce proper substituents within their structure in order to modulate their lipophilicity. The goal is to obtain lipophilic antioxidants that are able to interact with cell membranes and, at the same time, enough hydrophilic to neutralize those radicals present in a water compartment. In this work, the antioxidant efficacy of a series of amidinoquinoxaline nitrones has been evaluated regarding the oxidation of 2-deoxyribose and lipid peroxidation. The results have been rationalized on the basis of the different possible mechanisms involved, depending on some of their properties, such as lipophilicity, the ability to scavenge free radicals, and to undergo single electron transfer (SET) reactions.

Embedding cyclic nitrone in mesoporous silica particles for EPR spin trapping of superoxide and other radicals

Mesoporous silica functionalised with a cyclic spin trap enabled the identification of a wide range of radicals in organic and aqueous media, including superoxide radical anion.

An Electron Paramagnetic Resonance (EPR) spectroscopy study on the γ-irradiation sterilization of the pharmaceutical excipient l-histidine: Regeneration of the radicals in solution

The effects of γ-radiation sterilization on the parenteral excipient l-histidine were analysed by means of EPR spectroscopy. The irradiation process was found to induce the formation of a deamination radical which was persistent in the solid state. The nature and reactivity of the radicals following dissolution in water was evaluated using spin-trapping EPR experiments. The deamination radical was found to regenerate in solution in the presence of trace metals, potentially leading to radical induced degradation reactions occurring up to an hour after the dissolution process. Understanding this process is significant for the improved design of parental pharmaceutical formulations in which unwanted radical reactions after γ-radiation sterilization could lead to degradation of active ingredients.

Amidinoquinoxaline N-oxides: spin trapping of O- and C-centered radicals

Amidinoquinoxaline N-oxides represent a novel family of heterocyclic spin traps. In this work, their ability to trap O- and C-centered radicals was tested using selected derivatives with different structural modifications. All the studied nitrones were able to trap radicals forming persistent spin adducts, also in the case of OH and OOH radicals which are of wide biological interest as examples of ROS. The stability of the adducts was mainly attributed to the wide delocalization of the unpaired electron over the whole quinoxaline moiety. The nitroxide spectral parameters (hfccs and g-factors) were analyzed and the results were supported by DFT calculations. The N-19 hfccs and g-factors were characteristic of each aminoxyl and could aid in the identification of the trapped radical. The enhanced stability of the OH adducts under the employed reaction conditions could be ascribed to their possible stabilization by IHBs with two different acceptors: the N-O˙ moiety or the amidine functionality. DFT calculations indicate that the preferred IHB is strongly conditioned by the amidine ring size. While five membered homologues show a clear preference for the IHB with the N-O˙ group, in six membered derivatives this stabilizing interaction is preferentially established with the amidine nitrogen as an IHB acceptor.

DFT calculations as a powerful tool for ESR spin trapping experiments

A combined electron spin resonance (ESR) and density functional theory study has been carried out to obtain deeper insight in the reaction mechanism of the photorearrangement of 3-hydroxyindolic nitrones. Also, in this case, the combination of these techniques constitutes a powerful tool when discriminating between different reaction pathways. In particular, density functional theory calculations played an important role for unraveling the overlapping of signals coming from different species in the ESR spin trapping experiments. In the present study, the computed energies and the corresponding ESR parameters of each possible isomer hypothesized have been considered, taking also into account their different possibilities of intramolecular H-bond formation. However, the results obtained indicate that the possibility of intramolecular H-bonding did not play a determinant role in this case. Copyright © 2016 John Wiley & Sons, Ltd.

Amidinoquinoxaline N-oxides as novel spin traps

A series of nitrones were synthesized and tested as novel spin traps. The adducts generated by CH₃ addition showed remarkably persistent signals. Their EPR features and kinetics were rationalised by DFT and MP2 calculations.

Post-trapping derivatization of radical-derived EPR-silent adducts: application to free radical detection by HPLC/UV in chemical, biochemical, and biological systems and comparison with EPR spectroscopy

Free radicals are conventionally detected by electron paramagnetic resonance (EPR) spectroscopy after being trapped as spin adducts. Albeit this technique has demonstrated utmost efficacy in studying free radicals, its application to biological settings is intrinsically hampered by the inevitable bioreduction of radical-derived paramagnetic adducts. Herein, we describe a reliable technique to detect and quantify free radical metabolites, wherein reduced alkyl- and phenyl-5,5-dimethyl-1-pyrroline N-oxide (DMPO) adducts are converted into ultrastable N-naphthoate esters. To mimic the ubiquitous in vivo microenvironment, bioreductants, exogenous thiols, and sodium borohydride were studied. Nitroxyl reduction was confirmed using EPR and triphenyltetrazolium chloride. The formation of the N-naphthoyloxy derivatives was established by liquid chromatography/mass spectrometry (LC/MS). The derivatives were chromatographed using a binary eluent. HPLC and internal standards were synthesized using Grignard addition. The labeled DMPO adduct is (1) fluorescent, (2) stable as opposed to nitroxyl radical adducts, (3) biologically relevant, and (4) excellently chromatographed. Applications encompassed chemical, biochemical, and biological model systems generating C-centered radicals. Different levels of phenyl radicals produced in situ from whole blood were successfully determined. The method is readily applicable to the detection of hydroxyl radical. Analogously, DMPO, the spin trap, could be detected with extreme sensitivity suitable for in vivo applications. The developed method proved to be a viable alternative to EPR, where for the first time the reductive loss of paramagnetic signals of DMPO-trapped free radicals is transformed into fluorescence emission. We believe the proposed methodology could represent a valuable tool to probe free radical metabolites in vivo using DMPO, the least toxic spin trap.

Hydrazyl-nitrones, novel hybrid molecules in free radical research

This work describes the synthesis and characterisation of some novel hybrid molecules which contains in the same molecule a free radical moiety of hydrazyl type and a spin-trap moiety of nitrone type. The new compounds synthesized have multiple and easy to follow spectroscopic properties, making them useful as sensors or probes in radical chemistry. The new class of hydrazyl-nitrone molecules can act, in a single step process, as both generator and spin-trap of short-lived radicals. The hybrid molecules can be also involved in acid-base or redox processes, and the chemical processes can be easily monitored by visible or electron paramagnetic resonance spectroscopy. The excellent generator and trap properties recommend them as valuable sensors and probes in radical chemistry.

The Barton Reaction: Can a More Tenable Pathway Be Hypothesized for the Formation of Nitrosoalkyl Derivatives?

Herein, we report that in the formation of nitrosoalkyl derivatives during the photolysis of alkyl nitrites, the formation of the intermediate alkyl alkoxy nitroxide, due to the trapping of alkyl radicals by the starting nitrite, is the key step of the entire process. In fact, these nitroxides, detectable by EPR spectroscopy, decay to the final nitroso derivatives under thermodynamic control. In light of this, the Barton reaction mechanism has been reviewed. The nitrosoalkyl derivatives, or the hydroxamic acids when steroids are involved, have now to be considered as the ending products of the entire process and not, unless a very high concentration of NO is present in the medium, the result of a direct reaction of NO with the alkyl radical, as is commonly accepted.

Transient radicals in heterogeneous systems: detection by spin trapping

The application of spin trapping to the detection of transient radicals generated in heterogeneous systems is discussed. The major part of this review focuses on the interaction of light with particles in dispersion and the resultant radicals which are produced. This includes photoactive pigments such as CdS, TiO2, ZnO and phthalocyanine in a variety of solvents as well as micelles, vesicles and microemulsions containing molecules such as phthalocyanine or chlorophyll. The utility of spin trapping in heterogeneous systems which are not a function of illumination (i.e. electrochemical reactions, catalysis and biological systems) is also discussed. It is demonstrated that reaction mechanisms can be better understood by the indirect detection of the radical intermediates which are involved in the various processes. This impacts studies in solar energy utilization, electrophotography, catalysis, electrochemistry, photosynthesis and the disease process.

Hydroxyl free radical reactions with amino acids and proteins studied by electron spin resonance spectroscopy and spin-trapping

It has recently been shown that Fe(I) complexes of ADP or ATP generate OH radicals with H2O2 in a Fenton-type reaction. The OH radicals can be detected by using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap in electron spin resonance spectroscopy. All the biologically occurring amino acids, some related compounds and several proteins (histone, bovine serum albumin, collagen) were tested as OH radical scavengers against DMPO. The tested compounds competed with DMPO in trapping OH radicals to various extents as shown by the decrease of signal intensity of DMPO-OH spin-adduct. The tested compounds did not oxidize Fe(II) itself, with the only exception being tyrosine, as revealed by properly designed ferrozine reaction. Some of the amino acids reacted also with the DMPO-OH spin-adduct to a certain extent, whereas others did not. The formation of carbon centered organic radicals of the amino acids could be detected under the influence of OH radicals by using the spin traps phenyl-tert-butylnitrone (PBN) and alpha-pyridyl-1-oxide-N-tert-butylnitrone (4-POBN). The proteins, however, did not react with these spin traps. One can conclude that the amino acids and proteins can be targets of OH radical damage even in vivo, and such phenomena may be of importance in the deterioration of the conformation of proteins, e.g., during aging or in some pathological processes.

E.s.r. of spin-trapped radicals in gamma-irradiated polycrystalline DL-alanine. A quantitative determination of radical yield

The quantitative aspects of determining free radicals in polycrystalline amino acids gamma-irradiated at room temperature and subsequently dissolved in spin-trap solutions were investigated. The deamination radical in DL-alanine was used for detailed studies and 2-methyl-2-nitrosopropane (MNP) was employed as the spin-trap. The spin-trapping efficiency (the number of radicals spin-trapped in solution divided by the number of radicals initially present in the gamma-irradiated solid) was found to be in the range 1 to 10 per cent for aqueous solutions depending on the experimental conditions. The effects of dose, particle size, pH, spin-trap concentration, age of spin-trap solution, MNP monomer to dimer ratio and the presence of organic solvents were investigated. Several reactions were found to decrease the spin-trapping efficiency; radical-radical recombination, the competition between the spin-adduct and the spin-trap for radicals and the reaction of radicals with the MNP dimer. The reaction of intact DL-alanine molecules with deamination radicals to produce H-abstraction radicals which are not spin-trapped does not significantly lower the spin-trapping efficiency. The results obtained with compounds such as glycine, glycylglycine, L-valine and L-proline suggest that the low spin-trapping efficiency found for DL-alanine may be representative of polycrystalline amino acids.

E.S.R. of spin-trapped radicals in gamma-irradiated polycrystalline amino acids. Chromatographic separation of radicals

The free radicals produced by gamma-radiolysis of polycrystalline amino acids (L-valine, L-leucine, L-isoleucine and L-proline) at room temperature in the absence of air were investigated by spin trapping with 2-methyl-2-nitrosopropane (MNP). The spin adducts produced by dissolving the irradiated solids in aqueous MNP solutions were separated by high-performance liquid chromatography and then identified by e.s.r. Deamination (ring-opening reaction for L-proline) was observed for all amino acid. For L-valine and L-leucine, H-abstraction from the tertiary carbon in the side chains occurred. For isoleucine, H-abstractions from the alpha-carbon of the amino acid and from a non-terminal carbon in the side chain were found.

Hydrogen-deuterium exchange in gamma-irradiated polycrystalline DL-alanine: a spin-trapping and e.s.r. study

The hydrogen-deuterium exchange reactions in gamma-irradiated DL-alanine in the solid state were investigated by spin-trapping and electron spin resonance (e.s.r) using selectively deuterated DL-alanine. Subsequent to gamma-radiolysis at 30 degrees C, polycrystalline DL-alanine was dissolved in aqueous solutions of 2-methyl-2-nitrosopropane and the extent of H-D exchange of the deamination radicals was followed by e.s.r. After formation of the deamination radicals, four exchange reactions were found to occur between the radicals and the surrounding undamaged molecules. The first reaction, which occurs between the hydrogens of the C-2 carbon of the radicals and those of the methyl groups of the neighbouring molecules, can be followed at room temperature. The three other reactions could be conveniently monitored in gamma-irradiated polycrystalline alanine at 110 degrees C. The first of the other three reactions takes place between the methyl hydrogens of the radicals and the C-2 hydrogens of nearby molecules, while the remaining processes involve exchange between the hydrogen atoms of the amino group and those on the C-2 and C-3 carbon atoms of the deamination radical.

E.S.R. of spin-trapped radicals in gamma-irradiated polycrystalline nucleic acid constituents and their halogenated derivatives

Free radicals in gamma-irradiated polycrystalline nucleic acid constituents and their 5-halogenated derivatives have been studied by e.s.r. and spin-trapping. After gamma-irradiation at room temperature, the polycrystalline samples were dissolved in aqueous solutions of t-nitrosobutane (tNB) in the absence or presence of oxygen. For many of the nucleic acid constituents, two types of radicals, -C(5)RH-C(6)H- and -C(5)R-C(6)H2-, formed by H-addition to the double bond [-C(5)R=C(6)H-] of the base, were observed, where R is -CH3 or -H. In addition, radicals formed on the sugar moiety were found for some nucleosides. When oxygen was present in the tNB solution, the relative stability of trapped radicals was changed, and thus the presence of more than one radical species could be established. For halogenated bases, the radical produced by H-abstraction from N(1) was observed, and an additional radical species formed by H-addition to the C(6) position was found for 5-fluorouracil. For halogenated nucleosides, the same spectrum was observed in all compounds except the 5-fluoroderivatives, and was assigned to the radicals produced on the sugar moiety. For 5-fluorodeoxyuridine and 5-fluorouridine, the radical formed by H-addition to the C(6) position of the base was observed. In general, the present results are in good agreement with those of previous single crystal studies, but in the case of halogenated compounds other than the 5-fluoroderivatives, it was not possible to spin-trap the alpha-halo radicals which were the most prominent radicals formed from gamma-irradiation of single crystals at room temperature.

E.s.r. of spin-trapped radicals in gamma-irradiated polycrystalline amino acids, N-acetyl amino acids and dipeptides

The radicals produced in several polycrystalline amino acids, N-acetyl amino acids and dipeptides by gamma-radiolysis at room temperature were investigated by spin-trapping. After irradiation in the solid state, the samples were dissolved in aqueous solutions f t-nitrosobutane and the trapped radicals identified by e.s.r. For alpha-amino acids, deamination radicals were found, and in some cases H-abstraction radicals were also observed. No decarboxylation radicals could be detected. For N-acetyl amino acids, except for N-acetylglycine, the major radical was the decarboxylation radical. For N-acetyglycine the H-abstraction radical from the glycine residue was observed. For dipeptides of the x-glycine, the radical formed by removal of H from the alpha-carbon of the carboxyl-terminal residue was always spin-trapped. Some primary deamination radicals and minor amounts of decarboxylation radicals could also be observed. For dipeptides of the type x-alanine, glycine-x and alanine-x, the decarboxylation radical was always the major spin-trapped radical. Some primary and secondary deamination radicals were also detected.

E.s.r. of spin-trapped radicals in aqueous solutions of amino acids. Reactions of the hydroxyl radical

The radicals produced by reactions of hydroxyl radicals with amino acids in aqueous solutions have been investigated. Hydroxyl radicals were formed by U.V.-photolysis of hydrogen peroxide and the short-lived amino acid radicals were spin-trapped by tert-nitrosobutane and identified by electron spin resonance spectroscopy. Nineteen amino acids were studied, and several radicals were identified which have not been observed previously by other methods. Only side-chain radicals were identified for alanine, threonine, aspartic acid, asparagine, lysine, phenylalanine, tyrosine, proline and hydroxyproline; whereas for glycine the C(2) carbon radical was spin-trapped. Both C(2) carbon radicals and side-chain radicals were assigned to valine, leucine, isoleucine, serine, glutamic acid, glutamine, arginine and methionine.