The free radical species in polyacrylonitrile fibers induced by γ-radiation and their decay behaviors

Free radicals in vacuum, air and oxygen atmospheres were studied using electron spin resonance (ESR). Mainly two types of radicals, namely alkyl radicals and polyimine radicals, are formed in polyacrylonitrile (PAN) fibers after γ-ray irradiation. The G value of the radical formation was calculated to be 2.1 (number of radicals per 100 eV absorbed) in air at room temperature based on the ESR measurements. The radical stability and decay behaviors at room temperature and elevated temperatures were also investigated under different atmospheres. The alkyl radicals were found to be rather stable when stored in vacuum at room temperature, but they decayed via reaction with oxygen when stored in air. The alkyl radicals disappeared completely after a thermal treatment at 110 °C in vacuum, but only 15% of the polyimine radicals decayed; this indicates that polyimine radicals are more stable compared to the alkyl radicals due to their lower mobility.     

Charge‐Tagged DNA Radicals in the Gas Phase Characterized by UV/Vis Photodissociation Action Spectroscopy

Adenosine radicals tagged with a fixed‐charge group were generated in the gas phase and structurally characterized by tandem mass spectrometry, deuterium labeling, and UV/Vis action spectroscopy. Experimental results in combination with Born–Oppenheimer molecular dynamics, ab initio, and excited‐state calculations led to unambiguous assignment of adenosine radicals as N‐7 hydrogen atom adducts. The charge‐tagged radicals were found to be electronically equivalent to natural DNA nucleoside radicals.     

Photooxidative degradation of cellulose: Reactions of the cellulosic free radicals with oxygen

Photooxidative degradation of cellulose resulted in decreases of degree of polymerization (DP) and α-cellulose content, concurrently producing chromophoric groups; namely, carbonyl, carboxyl, and hydroperoxide groups within the polymer. Electron spin resonance (ESR) studies revealed that cellulosic carbon free radicals readily reacted with oxygen molecules at 143–160 K to produce peroxy radicals, whereas cellulosic oxygen free radicals were inert toward oxygen molecules throughout the photooxygenation reactions. At 77 K it is feasible that only photoexcited oxygen molecules reacted with cellulosic carbon free radicals to produce peroxide radicals. These radicals were themselves stabilized at 273 K by abstraction of hydrogen atoms from cellulose to produce polymer hydroperoxides. Simultaneously, new radical sites, which exhibited three-line ESR spectra, were generated in cellulose.     

Competition between mono-and bimolecular reactions of artemisinin alkoxyl radicals

The competition between intramolecular and bimolecular reactions of alkoxyl radicals formed from artemisinin was theoretically analyzed. The enthalpies of these reactions were calculated. The activation energies and rate constants of reactions of intramolecular hydrogen atom transfer, decyclization, and decomposition of alkoxyl radicals of artemisinin and several its derivatives, as well as the activation energies and rate constants of reactions of these radicals with the C-H, S-H, and O-H bonds in biological substrates and their analogs were calculated by the intersecting parabolas method The fastest reactions of artemisinin alkoxyl radicals were identified. The full kinetic scheme of transformation of these radicals was proposed. Artemisinin radicals with the free valence on the carbon atom are predominantly formed due to the transformation of the artemisininoxyl radicals. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1502–1510, September, 2006.     

Electrochemical and ESR studies of aromatic nitroso spin traps

The oxidation and reduction potentials of 23 nitroso compounds among which were included the most important spin traps have been determined by means of cyclic voltammetry and voltammetry at the rotating electrode. The electron transfer generally involved one electron, leading to the corresponding ion radicals. When persistent species resulted from the electron transfer, their ESR spectra were recorded and analyzed with the help of EHT calculations. The observed anion radicals are typical delocalized Π radicals, while the cation radicals are Σ radicals with a SOMO orthogonal to the Π system. Secondary species formed from these primary ion radicals were also observed and identified by e.s.r.     

Esr of pterin and lumazine radicals in solution

Cationic tetrahydrolumazine radicals and cationic tetrahydropterin radicals were detected by electron spin resonance when 5-alkyl-5,6,7,8-tetrahydrolumazines and 5-alkyl-5,6,7,8-tetrahydropterins were oxidized with hydrogen peroxide in formic acid. The hyperfine interactions of both types of radicals are essentially the same.Two consecutive radical species were observed during the oxidation of 3,5,8-trialkyl-5,6,7,8-tetrahydrolumazines in formic acid. They were identified as cationic tetrahydrolumazine radicals and cationic dihydrolumazine radicals.The ESR spectra of neutral trihydro- and monohydro-lumazine radicals, which have not been obtained before, were recorded during the oxidation of 5-alkyl-5,6,7,8-tetrahydrolumazines in chloroform. Starting from 5-butyl-1,3-dimethyl-5,6,7,8-tetrahydrolumazine three different radicals were observed.The spectra were interpreted in terms of hyperfine coupling constants and nuclear spins of the atoms involved.     

Measurements of UV-generated free radicals/reactive oxygen species (ROS) in skin

Free radicals/reactive oxygen species (ROS) generated in skin by UV irradiation were measured by electron spin resonance (ESR). To increase the sensitivity of measurement the short life free radicals/ROS were scavenged and accumulated by using the nitroxyl probe 3-carboxy-2,2,5,5-tetrametylpyrrolidine-1-oxyl (PCA). The spatial distribution of free radicals/ROS measured in pig skin biopsies with ESR imaging after UV irradiation corresponds to the intensity decay of irradiance in the depth of the skin. The main part of free radicals/ROS were generated by UVA (320-400 nm) so that the spatial distribution of free radicals reaches up to the lower side of the dermis. In vivo measurements on human skin were performed with a L-band ESR spectrometer and a surface coil integrating the signal intensities from all skin layers to get a sufficient signal amplitude. Using this experimental arrangement the protection of UVB and UVA/B filter against the generation of free radicals/ROS in skin were measured. The protection against ROS and the repair of damages caused by them can be realized with active antioxidants characterized by a high antioxidative power (AP). The effect of UV filter and antioxidants corresponding to their protection against free radicals/ROS in skin generated by UVAB irradiation can be quantified by the new radical sun protection factor (RSF). The RSF indicates the increase of time for staying in the sun to generate the same number of free radicals/ROS in the skin like for the unprotected skin. Regarding the amount of generated free radicals/ROS in skin as an biophysical endpoint the RSF characterizes both the protection against UVB and UVA radiation.     

Addition-elimination in the reaction of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid and nicotinic acid: example of inner sphere organic electron transfer

Reactions of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid (3,5-PDCA) and nicotinic acid (NA) were studied at appropriate pHs in aqueous solutions by pulse radiolysis technique. At pH 1, CH(3)C*HOH and *CH(2)OH radicals were found to react with 3,5-PDCA by rate constants of 2.2 x 10(9) and 5.1 x 10(8) dm(3) mol(-1) s(-1), respectively, giving radical adduct species. The adduct species formed in the reaction of CH(3)C*HOH radicals with 3,5-PDCA underwent unimolecular decay (k = 9.8 x 10(4) s(-1)) giving pyridinyl radicals. Reaction of (CH(3))(2)C*OH, CH(3)C*HOH, and *CH(2)OH radicals with NA at pH 3.3 gave the adduct species which subsequently decayed to the pyridinyl radicals. At pH 1, wherein NA is present in the protonated form, (CH(3))(2)C*OH radicals directly transfer electrons to NA, whereas CH(3)C*HOH and *CH(2)OH radicals react with higher rate constants compared with those at pH 3.3, initially giving the adduct species which subsequently undergo elimination reaction giving pyridinyl radicals. Reactions of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid and nicotinic acid are found to proceed by an addition-elimination pathway that provides one of the few examples of organic inner sphere electron-transfer reactions. Rate constant for the addition reaction as well as rate of elimination varies with the reduction potential of alpha-hydroxyalkyl radicals.     

EPR STUDY OF WOOL IRRADIATED WITH BLUE LIGHT

Abstract— Wool fibres were exposed to visible light in the blue range. The free radicals created by the illumination were studied using electron paramagnetic resonance. It is shown that by control of the light intensity and frequency it is possible to get considerable insight into the mechanism of the creation and the decay of the free radicals. The radical population under the above conditions appears to consist of an initial stable radical insensitive to radiation, and three other radicals, two of which follow decay kinetics considerably different from that of the remaining free radical. All the latter three radicals are probably caused by N–H or C–H bond scission.
The kinetics of radicals in irradiated proteins is discussed. It is shown that the mechanisms previously suggested in the literature are inadequate to account for the form of the experimental curve describing the number of radicals vs. time in irradiated protein. A new mechanism is suggested in which the decayed radicals form sites immune to further irradiation. Such a mechanism is shown to be consistent with the experimental curves. The kinetics of the decay of radicals after the cessation of illumination is examined in detail. It is found that in the case of irradiated keratin the mechanisms assuming first order decay are inconsistent with the experimental results, while the assumption of second order decay for two different species of radicals yields results consistent with the experimental evidence.     

Antioxidant action of organic sulfites—II. Esters of sulfurous acid as primary antioxidants

The effect of esters of sulfurous acid as primary antioxidants was examined. Different aliphatic, aromatic, open-chain and cyclic sulfites were synthesized. The reactions of organic sulfites with RO₂ and RO radicals, the chain carriers of the autoxidation of hydrocarbons and polymers, were simulated by means of the thermal decomposition of azobisisobutyronitrile (AIBN) in the presence of oxygen and of di-tert-butylperoxalate (DTBPO). The reactivity of organic sulfites with 2-cyanoisopropylperoxyl radicals is low. Only aromatic sulfites are able to trap peroxyl radicals; however, they are not very effective primary antioxidants. The reactions of the organic sulfites with tert-butoxyl radicals generally lead to an increase in the rate of decomposition of DTBPO, as determined from rate constants measured at 50 °C. A decomposition of DTBPO induced by liberated tert-butyl radicals in the presence of alkyl sulfites is very probable. Alkyl sulfites and aromatic sulfites with aliphatic groups act mainly as hydrogen donors in reactions with alkoxyl and peroxyl radicals.     

Electron transfer induced dissociations of 2- and 4-alkyl cyclohexadienones

Several 2- and 4-alkylcyclohexadienones were prepared and shown to accept electrons to produce ketyl radical anions that dissociated rapidly at room temperature to release carbon-centered radicals and an aromatic phenoxide type anion. In the PET process with benzyl-substituted cyclohexadienones, initiated with triethylamine, the benzyl radicals dimerised or abstracted an H-atom from solvent. In electrochemical reductions, and in reductions with alkali metals in liquid ammonia, the benzyl radicals were further reduced to anions.     

光诱导氧自由基与嘧啶碱基及其核苷反应的ESR研究

活性氧在破坏核酸结构,攻击其嘌呤碱基和嘧啶碱基,导致变异的出现和累积方面起重要作用。在生物体内,超氧自由基(O_2)是一个非常重要的活性氧,是氧分子单电子还原时首先生成的产物。由O_2衍生成过氧化氢(H_2O_2),羟基自由基(OH),单线态分子氧(~1O_2),     

Studying the Fundamentals of Radical Polymerization Using ESR in Combination with Controlled Radical Polymerization Methods

Electron spin resonance (ESR) spectroscopy can contribute to understanding both the kinetics and mechanism of radical polymerizations. A series of oligo/poly(meth)acrylates were prepared by atom transfer radical polymerization (ATRP) and purified to provide well defined radical precursors. Model radicals, with given chain lengths, were generated by reaction of the terminal halogens with an organotin compound and the radicals were observed by ESR spectroscopy. This combination of ESR with ATRPs ability to prepare well defined radical precursors provided significant new information on the properties of radicals in radical polymerizations. ESR spectra of the model radicals generated from tert-butyl methacrylate precursors, with various chain lengths, showed clear chain length dependent changes and a possibility of differentiating between the chain lengths of observed propagating radicals by ESR. The ESR spectrum of each dimeric, trimeric, tetrameric, and pentameric tert-butyl acrylate model radicals, observed at various temperatures, provided clear experimental evidence of a 1,5-hydrogen shift.     

On the reactivity of cellulose free radicals in graft copolymerization reactions

The formation and behavior of photo-and mechanoinduced free radicals in cellulose were studied by ESR spectroscopy and the capability of these free radicals to initiate graft copolymerization reactions was demonstrated. Although an 11-line ESR signal was detected from cellulose irradiated with ultraviolet (UV) light, a higher-intensity ESR signal with a five-line pattern was detected from a sample mechanically milled at 77 K. The decay of photoinduced free radicals when heated took place monotonously, whereas mechanoradicals exhibited an anomalous behavior with an increased signal intensity at 150 K before decaying at a higher temperature. Mechanoradicals have been found to react more efficiently and rapidly with oxygen and methyl methacrylate (MMA) than photoinduced free radicals. The peroxy mechanoradicals, however, were mobile and decayed more rapidly than the peroxy photoinduced radicals. Simultaneous graft copolymerizations of MMA to cellulose demonstrated that mechano-and photoinduced free radicals are capable of initiating grafting reactions, but a higher degree of grafting efficiency was obtained from cellulose treated mechanically.     

Magnetic properties of single crystal alpha-benzoin oxime: An EPR study

The electron paramagnetic resonance (EPR) spectra of gamma irradiated single crystals of alpha-benzoinoxime (ABO) have been examined between 120 and 440 K. Considering the dependence on temperature and the orientation of the spectra of single crystals in the magnetic field, we identified two different radicals formed in irradiated ABO single crystals. To theoretically determine the types of radicals, the most stable structure of ABO was obtained by molecular mechanic and B3LYP/6-31G(d,p) calculations. Four possible radicals were modeled and EPR parameters were calculated for the modeled radicals using the B3LYP method and the TZVP basis set. Calculated values of two modeled radicals were in strong agreement with experimental EPR parameters determined from the spectra. Additional simulated spectra of the modeled radicals, where calculated hyperfine coupling constants were used as starting points for simulations, were well matched with experimental spectra.     

Mechanism of the reaction of radicals with peroxides and dimethyl sulfoxide in aqueous solution

The reactions of methyl and methylperoxyl radicals derived from dimethyl sulfoxide (DMSO) with hydrogen peroxide, peroxymonocarbonate (HCO4 (-)), and persulfate were studied. The major reaction observed for the hydroperoxides was the abstraction of the hydrogen atom by the radicals. The radicals interact with a lone pair of electrons on the peroxide to produce methanol and formaldehyde. Furthermore, the results indicate that in RO2H and RO2R', electron-withdrawing groups cause a considerable increase in the reactivity of the peroxides towards the radicals and not only towards nucleophiles. The HO2 (.)/O2 (.-) and CO3 (.-) radicals react with DMSO to produce methyl radicals. Thus, the formation of the (.)CH3 radicals in the presence of DMSO is not proof of the formation of the (.)OH radicals in the system. These reactions must be considered when radical processes, such as in biological and catalytic systems, are studied. Especially, the plausible role of HCO4 (-) ions in biological systems as a source of oxidative stress cannot be overlooked.     

Long-lived polymer radicals. VIII. Easy formation of long-lived propagating radicals of vinyl monomers at room temperature

Poly[acryloyl-L-valine (ALV)] microspheres containing peroxy ester groups were prepared by radical copolymerization of ALV with a small amount of di-tert-butyl peroxyfumarate. When the microspheres were irradiated in the presence of second vinyl monomers, long-lived propagating radicals of the second monomers were formed in the microspheres by the reaction of microsphere polymer radicals with the monomers. The presence of a minute quantity of ethyl alcohol served to soften the microspheres and made the polymer radicals more mobile in the microspheres. As a result, sharper ESR spectra of the propagating radicals were observed although their lifetimes became shorter. This microsphere method also yielded easily the stable propagating radicals of a-methylstyrene and 1,1-diphenylethylene which have no homopolymerizability in usual radical polymerization. When N-n-propyldimethacroylamide and N,N′-dimethyl-N,N′-dimethacroylhydrazine, which undergo cyclopolymerization, were used as second monomer, uncyclized polymer radicals were only observed. Some discussions were given on the propagation mechanism of the cyclopolymerization.     

Synthesis of heteroarenes using cascade radical cyclisation via iminyl radicals

Cascade radical cyclisation involving homolytic aromatic substitution has been used to synthesise new tetracycles. Treatment of vinyl iodide radical precursors with Me(3)Sn. radicals (from hexamethylditin) yielded intermediate vinyl radicals which undergo 5-exo cyclisation onto suitably placed nitrile groups to yield intermediate iminyl radicals. The iminyl radicals undergo aromatic homolytic substitution via 6-endo cyclisation (or 5-exo cyclisation followed by neophyl rearrangement) with loss of hydrogen (H.) in a H-abstraction step. We propose that this abstraction was facilitated by tert-butoxyl (t-BuO.) radicals from di-tert-butyl peroxide or methyl radicals, generated from breakdown of trimethylstannyl radicals (Me(3)Sn.). The biologically active alkaloids mappicine and luotonin A were synthesised using the new methodology. A novel radical conversion of nitriles to primary amides is proposed.     

Organic radicals based on phenalenyl and verdazyl units

Stable phenalenyl (PLY) radicals without sterically bulky substituents need to be synthesized for application as neutral organic molecular conductors. Verdazyl radicals, which have high stability even without sterically bulky substituents, were combined with PLY radicals to produce two novel radicals. The stability of the dimethylthiourea substituted PLY radical is supported by the experimental results and quantum chemical calculations.     

ESR study on free radicals trapped in crosslinked polytetrafluoroethylene (PTFE)

Free radicals in crosslinked PTFE which formed by ⁶⁰Co γ-rays irradiaion at 77 K and at room temperature were studied by electron spin resonance (ESR) spectroscopy. The crosslinked PTFE specimens with different crosslinking density were prepared by electron beam irradiation in the molten state. The ESR spectra observed in the irradiated crosslinked PTFE are much different from those in non-crosslinked PTFE (virgin); a broad singlet component increases with increasing the crosslinking density, G-value of radicals is much higher in crosslinked PTFE than in non-crosslinked one. Free radicals related to the broad component are trapped in the non-crystalline region of crosslinked PTFE and rather stable at room temperature, whereas radicals trapped in amorphous non-crosslinked PTFE are unstable at room temperature. It is thought that most of free radicals trapped in the crosslinked PTFE are formed in the crosslinked amorphous region. The trapped radicals decays around 383 K (110°C) due to the molecular motion of -relaxation.