Formation of long-lived radicals in radiolysis of oxygen-containing organofluorine compounds

Conclusions In the radiolysis of organofluorine compounds containing a phenoxyl substituent, we have observed the formation of radicals, long-lived in the liquid, in the course of gamma irradiation at 77 K. From an analysis of the EPR spectra of the long-lived radicals, it has been shown that the radiolysis forms two types of long-lived radicals: cyclodienyl and alkoxyl. The lifetimes of these radicals depend on the structure of the compound from which they are obtained by radiolysis.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1255–1260, June, 1988.The authors wish to express their appreciation to L. M. Buchachenko for assistance in this work.     

Low-temperature fluorination of fluoro-containing polymers: EPR studies of polyvinylidenefluoride and the copolymer of tetrafluoroethylene with ethylene

The direct fluorination of polyvinylidenefluoride (PVDF) and the copolymer of tetrafluoroethylene with ethylene (CTE) was studied at 35-300 K. The dependence of radical formation on temperature and reaction time was obtained by use of electron paramagnetic resonance (EPR) spectroscopy. Primary alkyl radicals formed as a result of the reaction of fluorine abstracting a hydrogen from the polymer were detected at 35 K. These radicals rapidly react with molecular oxygen producing long-lived (∼48 h at 300 K) peroxy radicals. The peroxy radicals when subjected to UV-irradiation (λ < 280 nm) give rise to other radicals that are not stable at T > 77 K. The concentration of the radicals produced during fluorination of PVDF at 77-200 K is one order of magnitude less than that formed from CTE under similar conditions. A mechanism based on the abstraction of the H and the energies of the C-H bonds is given. Density functional theory was used to predict the structures and EPR parameters for a number of fluorinated radicals to explain the observed spectra. The FOO radical was detected at low temperatures.     

Strong resistance of Arabidopsis thaliana and Raphanus sativus seeds for ionizing radiation as studied by ESR,ENDOR, ESE spectroscopy and germination measurement: Effect of long-lived and super-long-lived radicals

Resistance of seeds for ionizing radiation effects on Arabidopsis thaliana and Raphanus sativus seeds were investigated by ESR, ENDOR, ESE spectroscopy and germination measurement. Two types of free radicals, such as long-lived (LL) and super-long-lived (SL) radicals, were produced by the γ-irradiation in the seeds. More than 90% of the 1 kGy-irradiated-seeds can germinate probably by decreasing the LL radicals by absorbing water. 10 kGy-irradiated-seeds cannot germinate at all probably due to the existence of significant amounts of the SL radicals even after absorbing water.     

A STUDY OF AQUEOUS SOLUTIONS OF NUCLEIC ACID CONSTITUENTS EXPOSED TO MONOCHROMATIC 160 nm VACUUM-UV LIGHT BY SPIN-TRAPPING METHOD

Spin-trapping technique was employed to detect and identify free radical intermediates produced in aqueous solutions of nucleic acid constituents (pyrimidine bases and pyrimidine nucleosides) after irradiation by monochromatic 160 nm vacuum-UV light from the electron storage ring. Short-lived free radicals produced in the molecules were converted into relatively long-lived free radicals (spin-adducts) by the reaction with MNP used as a spin trap. The resulting nitroxide radicals were subsequently analysed by esr.
The esr measurements were made after irradiating H₂O solution containing sample and the spin trap and once freeze-drying it and re-dissolving the residual powder in D₂O to get the spectrum with a well-resolved hyperfine structure. Thus, clear evidence that most of the radicals were not formed by H-addition but formed by OH-addition at the C5 position of the 5, 6 double bond were obtained for pyrimidine bases. For pyrimidine nucleosides, although the effect of H₂O-D₂O exchange was not recognized on resolution improvement of the hyperfine structure of the esr spectra, careful analysis of the hyperfine structure made it possible to identify the radical structures; OH-addition radicals at the C6 of the double bond of the base moiety in addition to the OH-addition radicals at the C5 position for all except for 2-deoxycytidine. Evidence for the formation of free radicals at the sugar moiety was not clear.     

The role of oxidizing radicals in neptunium speciation in γ-irradiated nitric acid

The irradiation of aqueous nitric acid solutions generates transient, reactive species that are known to oxidize neptunium. However, nitrous acid is also a long-lived product of nitric acid irradiation, which reduces neptunium. When we irradiated nitric acid solutions of neptunium and measured its speciation by UV/Vis spectroscopy, we found that at short irradiation times, oxidation of Np(V) to Np(VI) occurred due to reactions with radicals such as ^?OH, ^?NO₃ and ^?NO₂. However, at higher absorbed doses and after a sufficient amount of nitrous acid was produced, reduction of Np(VI) to Np(V) began to occur, eventually reaching an equilibrium distribution of these species depending on nitric acid concentration. Neptunium(IV) was not produced.     

Nitroxyl radical reactions with 4-pentenyl- and cyclopropylketenes: new routes to 5-hexenyl- and cyclopropylmethyl radicals

4-Pentenylketenes 4a and 9 and cyclopropylketenes 3a, 13, 14 (RCH=C=O) are generated by photochemical Wolff rearrangements and observed by IR as relatively long-lived species at room temperature in hydrocarbon solvents. The reactions of these ketenes with the nitroxyl radicals tetramethylpiperidinyloxyl (TEMPO, TO*) and tetramethylisoindoline-2-oxyl (TMIO, IO*) form carboxy substituted 5-hexenyl and cyclopropylmethyl radicals which are either trapped by a second nitroxyl radical or undergo rearrangements followed by trapping. The rate constant of the reaction of 4a with TEMPO was similar to that of n-BuCH=C=O (1b), while 3a was 4.3 times more reactive, indicating cyclopropyl stabilization of the incipient radical.     

Formation and decay of trifluoromethyl radicals during photodecomposition of the long-lived perfluoro-2,4-dimethyl-3-ethylpent-3-yl radical

UV irradiation of the long-lived radical [(CF₃)₂CF]₂C^·C₂F₅ (1) in a hexafluoropropylene trimer (HFPT) glassy matrix at 77 K and in a HFPT solution at 300 K leads to its decomposition to the ^·CF₃ radical and perfluoroolefin molecule. About 90% of the ^·CF₃ radicals formed recombine at 300 K. The remaining radicals add to the HFPT molecules generating the long-lived radicals [(CF₃)₂CF]₃C^·. Unlike the ^·CF₃ radicals produced by the photodecomposition of radicals 1, the ^·CF₃ radicals formed during radiolysis of HFPT are not stabilized in the glassy HFPT matrix at 77 K.     

Photopolymerization of tetrafluoroethylene in solution of hexafluoropropylene trimer in the presence of long-lived perfluoroallyl radicals

Conclusions In photolysis of tetrafluoroethylene in a solution of the hexafluoropropylene trimer containing long-lived perfluoroallyl radicals, the following processes can take place: 1) phototransformation of these radicals into active perfluoroalkyl radicals and their recombination, and participation in initiation of photopolymerization of tetrafluoroethylene; the formation of the long-lived perfluoroalkyl radical on addition of a molecule of solvent to the propagating radical and the formation of a long-lived radical captured by the propagating chain compete with these processes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2698–2702, December, 1987.     

Participation of an excited monomer in the propagation reaction of the γ-ray induced polymerization of ethylene

In the polymerization of ethylene, the reactivity of the growing radical produced by γ-radiation was compared with that of the radical from 2,2′-azobisisobutyronitrile. The radicals produced in the polymerization at around room temperature were long-lived irrespective of the method of initiation. However, it was found that the radical produced by γ-radiation became unreactive to ethylene when the reaction system was not exposed to γ-rays. Irradiation with γ-rays or ultraviolet light in a region below about 3900 Å was required for its chain growth. On the other hand, the radical from AIBN was always reactive, and the reactivity was little changed by γ-radiation or by the presence of a trace amount of radiolysis products of ethylene. In explaining of these characteristic differences between the nature of these radicals produced by two different methods of initiation, some other information on their reactivity was reviewed, and the participation of an excited ethylene in a dimer form was proposed, as for the propagation reaction of the γ-ray-induced polymerization.     

Formation of long-lived radicals in radiolysis of perfluoro(4-methyl-2-pentene)

Conclusions On the basis of the EPR spectrum of perfluoro(4-methyl-2-pentene) (PMP) that has been exposed to gamma radiation at 77–300 K, three types of long-lived radicals have been observed, with lifetimes greater than two years. With increases in the radiolysis temperature for radiation dose, the concentration of long-lived radicals increases. Their relative concentrations depend on the irradiation conditions and the thawing schedule.Under certain conditions, long-lived radicals of a single type can be obtained from the PMP.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1263–1268, June, 1988.     

The bystander effect is a novel mechanism of UVA-induced melanogenesis

We successfully identified the bystander effect in B16 murine melanoma cells exposed to UVA irradiation. The effect was identified based on melanogenesis following the medium transfer of the B16 cells, which had been cultured for 24 h after being exposed to UVA irradiation, to nonirradiated cells (bystander cells). Our confirmation study of the functional mechanism of bystander cells confirmed the reduced levels of mitochondrial membrane potential 1-4 h after the medium transfer. In addition, we observed increased levels of intracellular oxidation after 9-12 h, and the generation of melanin radicals, including long-lived radicals, 24 h after medium transfer. Further analysis of bystander factors revealed that the administration of EGTA treatment at the time of medium transfer led to an inhibition of melanogenesis and to neutralization of the mitochondrial membrane potential level, as well as to the restoration of intracellular oxidation levels to those of controls. The results demonstrated that the UVA irradiation bystander effect in B16 cells, as indicated by melanogenesis, was induced by the increase in intracellular oxidation due to the mitochondrial activity of calcium ions, which were among the bystander factors involved in the increase.     

FLASH PHOTOLYSIS OF N-ACETYL-L-TRYPTOPHANAMIDE: THE RELATIONSHIP BETWEEN RADICAL YIELDS AND FLUORESCENCE QUENCHING

Abstract— The flash photolysis of N-acetyl-L-tryptophanamide (NATA) in the presence of the fluorescence quenchers, imidazole, acrylamide and trichloroethanol, has been investigated. Imadazole and acrylamide induce a decrease in the NATA radical yield which correlates with their NATA fluorescence quenching action. These observations suggest that the fluorescent state is primarily responsible for the monophotonic photoionization processes. The acrylamide data also suggest that 40–65% of the NATA radicals arise from a long-lived state (τ˜μs) which must originate from the fluorescent state. Unlike imidazole and acrylamide, trichloroethanol enhances the radical yield by reaction with excited state precursors. Mechanisms for the quenching of fluorescence and the long-lived states are discussed.     

Optical spectroscopic study of long-lived radicals formed in radiolysis and fluorination of the trimer of hexafluoropropylene

Optical studies demonstrated the noncorrespondence of the long-lived radicals formed in radiolysis of the trimer of hexafluoropropylene (THFP) to the previously proposed perfluoroallyl radical. The reaction of addition of fluorine at the double bond with the formation of long-lived perfluoroalkyl radicals in the liquid with an optical absorption band at _max < 220 nm takes place in radiolysis and fluorination of THFP. Three types of photosensitive centers which are stable in a liquid and have different lifetime at 300 K were distinguished with the optical absorption spectrum of THFP undergoing radiolysis.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2203–2207, October, 1989.     

Photoinduced electron transfer in naphthalimide-pyridine systems: effect of proton transfer on charge recombination efficiencies

We studied the effect of proton-coupled electron transfer on lifetimes of the charge-separated radicals produced upon light irradiation of the thiomethyl-naphthalimide donor SMe-NI-H in the presence of nitro-cyano-pyridine acceptor (NO(2)-CN-PYR). The dynamics of electron and proton transfer were studied using femtosecond pump-probe spectroscopy in the UV/vis range. We find that the photoinduced electron transfer between excited SMe-NI-H and NO(2)-CN-PYR occurs with a rate of 1.1 × 10(9) s(-1) to produce radical ions SMe-NI-H(?+) and NO(2)-CN-PYR(?-). These initially produced radical ions in a solvent cage do not undergo a proton transfer, possibly due to unfavorable geometry between N-H proton of the naphthalimide and aromatic N-atom of the pyridine. Some of the radical ions in the solvent cage recombine with a rate of 2.3 × 10(10) s(-1), while some escape the solvent cage and recombine at a lower rate (k = 4.27 × 10(8) s(-1)). The radical ions that escape the solvent cage undergo proton transfer to produce neutral radicals SMe-NI(?) and NO(2)-CN-PYR-H(?). Because neutral radicals are not attracted to each other by electrostatic interactions, their recombination is slower that the recombination of the radical ions formed in model compounds that can undergo only electron transfer (SMe-NI-Me and NO(2)-CN-PYR, k = 1.2 × 10(9) s(-1)). The results of our study demonstrate that proton-coupled electron transfer can be used as an efficient method to achieve long-lived charge separation in light-driven processes.     

Effects of ionizing radiation on epoxy,graphite fiber,and epoxy/graphite fiber composites. Part II: Radical types and radical decay behavior

Electron spin resonance (ESR) investigations of line shapes and radical decay behavior have been made on an epoxy based on tetraglycidyl diaminodiphenyl methane (TGDDM)/diaminodiphenyl sulfone (DDS), T-300 graphite fiber, and T-300/5208 (graphite fiber/epoxy) composites after irradiation with Co₆₀ γ-radiation or 0.5 MeV electrons. Two kinetically distinct radical species are found in the irradiated epoxy as the temperature is raised beyond 120 K following irradiation of samples at 77 K with Co₆₀ γ. One has been termed a fast-decaying species and the other a slow-decaying species. The ratio of fast-decaying/slow-decaying radicals increases as the decay temperature rises. The fast-decaying radicals at room temperature are attributed largely to alkyl type radicals residing in regions of relatively low crosslink density, while the long-lived radicals are attributed to radicals residing in the highly crosslinked regions of the epoxy. A large concentration (ca. 10₂₀ to 10₂₁ spins/g) of unpaired electrons was found in unirradiated graphite fiber which masked the ESR spectral change in irradiated composites.     

Theoretical study of single-bubble sonochemistry

Numerical simulations of bubble oscillations in liquid water irradiated by an ultrasonic wave are performed under the experimental condition for single-bubble sonochemistry reported by Didenko and Suslick [Nature (London) 418, 394 (2002)]. The calculated number of OH radicals dissolving into the surrounding liquid from the interior of the bubble agrees sufficiently with the experimental data. OH radicals created inside a bubble at the end of the bubble collapse gradually dissolve into the surrounding liquid during the contraction phase of an ultrasonic wave although about 30% of the total amount of OH radicals that dissolve into the liquid in one acoustic cycle dissolve in 0.1 micros at around the end of the collapse. The calculated results have indicated that the oxidant produced by a bubble is not only OH radical but also O atom and H2O2. It is suggested that an appreciable amount of O atom is produced by bubbles inside a standing-wave-type sonochemical reactor filled with water in which oxygen is dissolved as in the case of air.     

Metabolic activation of chemical carcinogens and binding of metabolites with nucleic acid bases

A free radical was produced enzymatically by incubating benzo(a)pyrene with liver microsomes. This radical, identified as the 6-oxybenzo(a) pyrene radical, was sufficiently reactive to bind covalently with nucleic acid bases. Similar reactive free radicals were produced enzymatically from anthanthrene and 10-aza-benzo(a)pyrene, which are carcinogenic in spite of lacking so-called “bay regions.” Hepatocarcinogens such as 3′-methyl-4-dimethylaminoazobenzene and related compounds, naphthylamines, and 2-acetylaminofluorene yield free radicals after being incubated with liver microsomes. Thus, various kinds of chemical carcinogens are found to be converted to free radicals, suggesting causal significance for the formation of free radicals in chemical carcinogenesis.     

Molecular Engineering of Corrole Radicals by Polycyclic Aromatic Fusion: Towards Open-Shell Near-Infrared Materials for Efficient Photothermal Therapy

Open-shell radicals are promising near-infrared (NIR) photothermal agents (PTAs) owing to their easily accessible narrow band gaps, but their stabilization and functionalization remain challenging. Herein, highly stable π-extended nickel corrole radicals with [4n+1] π systems are synthesized and used to prepare NIR-absorbing PTAs for efficient phototheranostics. The light-harvesting ability of corrole radicals gradually improves as the number of fused benzene rings on β-pyrrolic locations increases radially, with naphthalene- and anthracene-fused radicals and their one-electron oxidized [4n] π cations exhibiting panchromatic visible-to-NIR absorption. The extremely low doublet excited states of corrole radicals promote heat generation via nonradiative decay. By encapsulating naphthocorrole radicals with amphiphilic polymer, water-soluble nanoparticles Na-NPs are produced, which exhibit outstanding photostability and high photothermal conversion efficiency of 71.8 %. In vivo anti-tumor therapy results indicate that Na-NPs enable photoacoustic imaging of tumors and act as biocompatible PTAs for tumor ablation when triggered by 808 nm laser light. The “aromatic-ring fusion” strategy for energy-gap tuning of corrole radicals opens a new platform for developing robust NIR-absorbing photothermal materials.     

CCl2(A1B1和a3B1)被酮类分子猝灭速率常数的测定

对CCl4/Ar混合气体脉冲直流高压放电产生CCl2自由基,经过约110μs后,再用541.52mm激光将电子基态CC2激励到激发态A1B1(0,4,0)振动态K=0能级上,通过检测激发态CCl2(A)时间分辨荧光信号,测得室温下CC2(A1B1和a3B1)被酮类分子猝灭的实验结果,用所提出的三能级模型分析处理实验数据,获得态分辨速率常数KA和Ka值.     

A crossed molecular beam study on the reaction of methylidyne radicals [CH(X(2)Π)] with acetylene [C(2)H(2)(X(1)Σ(g)(+))]-competing C(3)H(2) + H and C(3)H + H(2) channels

We carried out the crossed molecular beam reaction of ground state methylidyne radicals, CH(X(2)Π), with acetylene, C(2)H(2)(X(1)Σ(g)(+)), at a nominal collision energy of 16.8 kJ mol(-1). Under single collision conditions, we identified both the atomic and molecular hydrogen loss pathways forming C(3)H(2) and C(3)H isomers, respectively. A detailed analysis of the experimental data suggested the formation of c-C(3)H(2) (31.5 ± 5.0%), HCCCH/H(2)CCC (59.5 ± 5.0%), and l-HCCC (9.0 ± 2.0%). The reaction proceeded indirectly via complex formation and involved the unimolecular decomposition of long-lived propargyl radicals to form l-HCCC plus molecular hydrogen and HCCCH/H(2)CCC plus atomic hydrogen. The formation of c-C(3)H(2) was suggested to be produced via unimolecular decomposition of the cyclopropenyl radical, which in turn could be accessed via addition of the methylidyne radical to both carbon atoms of the acetylene molecule or after an initial addition to only one acetylenic carbon atom via ring closure. This investigation brings us closer to unraveling of the reaction of important combustion radicals-methylidyne-and the connected unimolecular decomposition of chemically activated propargyl radicals. This also links to the formation of C(3)H and C(3)H(2) in combustion flames and in the interstellar medium.