ELECTRON SPIN RESONANCE STUDIES OF ULTRAVIOLET IRRADIATED KERATIN AND RELATED PROTEINS

Abstract— The formation of free radicals in keratin and related proteins by exposure to u.v. light has been examined by electron spin resonance spectroscopy. From a comparison of the spectra obtained and the effects of different wavelengths and the stabilities of the radicals produced, it has been shown that several species of free radicals are produced by the u.v. irradiation of keratin. However it has been possible to identify only those free radicals associated with the cystine residues. The free radicals produced in keratin by exposure to shorter wavelength irradiation (below 3250 Å) were found to be quite different to those produced at longer wavelengths.     

ESR spectra of radicals at a low-temperature X-radiolysis of phosphates

ESR spectroscopy at 77 K has been used for the investigation of radicals formed in the low-temperature X-radiolysis of tri-n-butylphosphate /TBP/, di-n-butylphosphate /DBP/, di /2-ethylhexyl/phosphoric acid /DEHPA/, tri-p-tolylphosphate /TTP/, di-n-butyl-n-butylphosphonate /DBBPN/. Besides the phosphate radicals also hydrogen atom stabilization in the matrix has been observed. In the presence of oxygen peroxy radicals occur as well. The structure of the radicals and reaction mechanisms are discussed.     

Matrix isolation of intermediates on the activated silica surface: mechanisms and efficiencies of photochemical reactions of peroxy radicals

The technique of matrix isolation on the activated silica surface for the study of mechanisms and determination of efficiencies of photochemical reactions of alkylperoxy radicals has been used. The direct experimental evidence for photochemical dissociation of peroxy radicals via two parallel pathways by the rupture of O-O and C-O bonds under light of wavelength 313 nm has been obtained. It has been found that activated silica surface is not "inert" support in studies of transformations of surface-grafted peroxy radicals.     

A systematic scheme of qualitative analysis for anions.IV

A comprehensive investigation has been undertaken to ascertain how the basic radicals that pass into the sodium carbonate filtrate arc distributed among the various groups for the detection of anions. Methods for the detection of these basic radicals in the presence of the anions have been worked out in detail. It was found that the scheme for the detection of anions put forward previously remains unaffected by the presence of these basic radicals. It has been shown that the scheme for the detection of anions can be integrated with the comprehensive schemes for the detection of basic radicals and insoluble substances.     

HEMOLYSIS OF HUMAN ERYTHROCYTES BY ACTIVATED OXYGEN SPECIES

Abstract— The hemolysis of human erythrocytes by irradiation at 254 nm has been studied. Neither superoxide radicals nor singlet oxygen play a significant rôle and it is likely that the major species involved are hydroxyl radicals and, indirectly, carbonate anion or formate radicals. Similarly, when erythrocytes are treated with a system commonly used as source of superoxide radicals (photoreduction of riboflavin) it has been demonstrated that O^-₂ does not participate in lysis, but that singlet oxygen (possibly with hydroxyl radicals) is a major oxygen species involved in destruction of the cell membrane.     

Chain-breaking antioxidant activity of phosphite esters

The inhibition of the autoxidation of hydrocarbons and polypropylene by aliphatic, aromatic, sterically hindered and cyclic phosphites has been studied by means of volumetric and ³¹P-NMR techniques. The antioxidant activity of phosphites depends on the rate of their reactions with peroxyl radicals and on the way they react with alkoxyl radicals. Only those phosphites which react by substitution to give free aryloxyl radicals are effective as chain-breaking antioxidants.The reaction modes of various phosphites with various peroxyl and alkoxyl radicals have been studied in some model reactions and the relationship between structure, reaction mechanism and antioxidant activity has been elucidated.     

Addition of radicals to quinones—I. ESR study of addition products from chloroquinones and free radicals

The mechanism of addition reactions of radicals formed during thermal decomposition of benzoyl peroxide with various chloro-substituted p-benzoquinones has been studied by the ESR technique in various solvents. The ESR spectra of the intermediate radicals show that addition occurs at the carbonyl oxygen. The important role of charge-transfer complexes in the reaction has been established. For strong CT complexes, the quinone molecule reacts with radicals derived from the solvent.     

ELECTRON SPIN RESONANCE OF MELANIN FROM HAIR. EFFECTS OF TEMPERATURE, pH AND LIGHT IRRADIATION

Abstract— The variation with temperature, pH and light of the ESR signal of hydrated melanin powders from Japanese black hair has been studied. An explanation of the results is proposed on the basis of quinhydrone type complexes and of acid-base equilibria of melanin and its semiquinone radicals. During exposure to light of wavelengths 254–600 nm, both stable and unstable radicals have been observed. The action spectrum for the formation of stable melanin radicals has been determined.     

Spectroscopic properties of anion radicals studied using pulse radiolysis

The anion radicals of benzophenone, amino derivatives of benzophenone, fluorenone, furil and antharcene have been generated by pulse radiolysis in acetonitrile solution and their spectroscopic and kinetic properties have been characterized. The G-value for generation of anion radicals in acetonitrile has been measured to be 1.01 ± 0.1 per 100 eV. We have also investigated the electron-transfer reactions from anion radicals of one kind of solutes to the ground state of another solute. This study has demonstrated that pulse radiolysis can be a useful technique for generation and characterization of anion radicals and for studying electron-transfer reactions using acetonitrile as solvent.     

Evidence of formation of radicals in the polystyrene thermodegradation

The formation of radicals in the thermal degradation of polystyrene (PS) in tubes sealed under vacuum has been evidenced by using ESR spectroscopy. The radicals were found to be stable for weeks at room temperature. The rate coefficients of the formation of radicals have been evaluated in the temperature range 350–420°C. The reaction kinetically followed has been related to the initiation step of the multi-step degradation process of PS. The activation energy of this reaction resulted 185 kJ/mol.     

H-D exchange in γ-irradiated glycine

H-D exchange has been observed in certain radicals by using ENDOR. The protons at any position in the radicals participate in the exchange. The structure of the radicals is affected by the exchange.     

PHOTOINDUCED GENERATION OF HYDROGEN PEROXIDE AND HYDROXYL RADICALS IN MELANINS

The hydrogen peroxide produced during photolysis of melanin pigments has been measured using an oxidase electrode. The photooxidation has been shown to occur via the superoxide intermediate. In the presence of superoxide dismutase the rate of photo-induced production of hydrogen peroxide is increased, reflecting the ability of melanin to scavenge superoxide radicals. Evidence for metal-ion dependent formation of hydroxyl radicals during photooxidation of melanin pigments was obtained using electron spin resonance-spin trapping procedures. Superoxide dismutase increased the rate of formation of hydroxyl radicals in the system. Mechanisms of metal ion-induced production of hydroxyl radicals during photolysis of melanin pigments are discussed.     

Temperature dependent reaction pathways in the addition of silyl, germyl, stannyl and plumbyl radicals to maleic anhydride and related compounds: an ESR study

The addition of Group IVB organometallic radicals to maleic anhydride, maleic thioanhydride, maleimide and N-methyl maleimide has been investigated by ESR spectroscopy. For Si and Ge centred radicals the reaction pathway has been found to be temp dependent, the addition of the organometallic radical occurring at the CC double bond at lower temps at either of the two CO groups at higher temps. For Sn and Pb centred radicals only the latter spin adducts could be detected even at low temp.     

Strained heterocycles in radical chemistry

Over the last few decades the use of radicals in synthesis has witnessed an explosive growth through introduction of efficient chain and electron-transfer reactions. Strained heterocycles, in particular, have emerged as a highly versatile and readily available class of radical precursors. The generation of carbinyl radicals of heterocycles has resulted in many elegant applications of heteroatom-centered radicals, such as beta fragmentations, cyclizations, and intramolecular hydrogen atom abstractions. Direct electron transfer to strained heterocycles has been realized through the use of arene radical anions. The method combines the virtues of radical and organometallic chemistry to yield useful functionalized organolithium compounds. Epoxides have been opened with high regioselectivity by titanocene(III) reagents in either stoichiometric or catalytic quantities to yield beta-titanoxy radicals. This development has resulted in many new applications in natural product synthesis.     

The fluorescence lifetimes of the methyl-substituted benzyl,triphenylmethyl and diphenylmethyl radicals

Time resolved fluorescence of some methyl-substituted benzyl, triphenylmethyl, and diphenylmethyl radicals trapped in rigid solvents at low temperature has been observed. These radicals were excited by pulsed N₂ laser at 337 nm. It is found that these radicals exhibit very long fluorescence lifetime. The long lifetimes of these radicals seem to indicate that the first doublet-doublet electronic transitions of the radicals have a forbidden character.     

Electrochemical behaviour of organolead compounds: Part I. Triphenyl-lead acetate

The electrochemical behaviour of triphenyl-lead acetate in 50% (v/v) ethanol has been investigated using various electrochemical techniques including polarography, cyclic voltammetry and controlled potential coulometry. It has been found to give three/four polarographic waves. The first normal wave involves an one-electron irreversible reduction of triphenyl-lead ions giving triphenyl-lead free radicals which are strongly adsorbed at the DME giving rise to an adsorption prewave. The triphenyl-lead free radicals produced in the normal reduction step immediately react with mercury of the DME producing phenylmercury radicals and metallic lead. Wave II has been ascribed to the simultaneous reduction of triphenyl-lead free radicals and phenylmercury free radicals. The metallic lead produced in these processes undergoes oxidation at the electrode and “distorts” the “normal” adsorption prewave of step I. A mechanism of reduction is proposed and polarographic methods have been developed for determining triphenyl-lead compounds at ordinary level and down to submicromolar level.     

Solvent effect in free radical reactions on protonated heteroaromatic bases

The influence of reaction medium on the relative reactivity of various carbon radicals towards 4-CH₃-and 4-CN- quinolines has been investigated. A relevant solvent effect for α-amidoalkyl and α-oxyalkyl radicals has been demonstrated and a solvent-radical interaction is proposed to explain the phenomenon.     

High-conversion polymerization. I. Theory and application to methyl methacrylate

The concept of polymer entanglements has been applied in conjunction with classical free-radical kinetics to describe vinyl polymerizations carried to high conversion. A kinetic model has been developed on the assumption that two populations of radicals exist in a high-conversion polymerization system: those radicals whose chain lengths are long enough to become entangled with neighboring molecules and have, therefore, a restricted mobility; and those shorter radicals whose mobilities are not strongly affected by diffusional effects. It has also been assumed that the kinetic rate constant for the termination step between entangled radicals is inversely proportional to the mean entanglement density. The model contains only two parameters in addition to the kinetic rate constants required to describe low-conversion polymerizations. One of these parameters can be determined, at least in principle, from measurements of solution properties of the polymer-monomer mixtures. The model so developed has been tested against experimental data obtained from the literature on the bulk polymerization of methyl methacrylate. The agreement between predicted and experimental monomer conversions and molecular weight averages is found to be satisfactory.     

An electron spin resonance study of azaallyl radicals

The ESR spectrum of the 2-aza-allyl radical, Me2CNCMe₂, has been recorded. A series of 1-aza-allyl radicals have been generated by reaction of 2-alkyl-4,4-dimethyloxazolines with t-butoxyl radicals and their ESR spectra studied. In this reaction 2-alkyl-4,4-dimethyloxazolin-5-yl radicals are also formed.     

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),