Major Radical Species Research Articles

The effect of changes in the flame structure on the formation and destruction of soot and NOx in radiating diffusion flames

In this study, soot and NO x production in four counterflow diffusion flames with different flame structures is examined both experimentally and theoretically. The distance between the maximum temperature zone and the stagnation plane is progressively changed by changing the inlet fuel and oxidizer concentrations, thus shifting the flame location from the oxidizer side to the fuel side of the stagnation plane. One flame located at the stagnation plane is also examined. Detailed chemical, thermal, and optical measurements are made to experimentally quantify the flame structure, and supporting numerical calculations with detailed chemistry are also performed by specifying the boundary conditions used in the experiments. Results show that as the radical-rich, high-temperature reaction zone is forced into the sooting zone, several changes occur in the flame structure and appearance. These are the following: (1) The flames become very bright due to enhanced soot-zone temperature. This can cause significant reduction in NO formation due to increased flame radiation. (2) OH concentration is reduced from superequilibrium levels due to soot and soot-precursor oxidation in addition to CO and H 2 oxidation. (3) Soot-precursor oxidation significantly affects soot nucleation on the oxidizer side, while soot nucleation on the fuel side seems to be related to C 2 H 2 concentration. (4) Soot interacts with NO formation through the major radical species produced in the primary reaction zone. It also appears that the Fenimore NO initiation mechanism becomes more important for low-temperature flames and when N 2 is added to the fuel side, due to higher N 2 concentrations in the CH-rich zone.

Proton ENDOR of dimethylnitramine free radicals

Electron nuclear double resonance (ENDOR) spectra of free radicals produced by ultraviolet (UV) photolysis of polycrystalline and single crystal dimethylnitramine (or N-methyl-N-nitromethanamine) [DMN; (CH3)2NNO2 were studied atca. −30°C. Results suggest that multiple radical species are formed during UV photolysis of DMN, perdeutero-DMN-d6, and15N-labeled DMN. Proton ENDOR spectra are consistent with assignment of a cation radical (CH3)2NNO 2 + as the major DMN radical species. Proton hyperfine coupling anisotropy, which is observed from the ENDOR spectra, is attributed to inequivalence of the two DMN methyl groups.

An electron spin resonance study of the radiation chemistry of poly(hydroxybutyrate)

The spectra of the radical species resulting from gamma irradiation of poly(hydroxybutyrate) and its copolymers with hydroxyvalerate have been obtained after radiolysis at 77 and 303 K, and the corresponding radiation chemical yields have been calculated. Photobleaching and annealing experiments have allowed the major radical species to be identified and their reactivities to be assessed. Radical anions, as well as neutral radicals, were observed to be present at 77 K, but the radical anions were found to decay to neutral radicals at temperatures around 140K. Above about 250 K a multi-line component of the spectrum was lost, but the radical associated with this spectral change has not been unequivocally assigned, though it is believed to be a radical resulting from chain scission. Annealing over the temperature range 300–350 K resulted in the loss of a triplet, which accounts for most of the radicals present. This triplet has been assigned to a radical located on the carbon atom adjacent to the carbonyl group. This radical is believed to exist in two possible conformations. All of the radicals decayed at temperatures above 400 K.

Bactericidal activity of alkyl peroxyl radicals generated by heme-iron-catalyzed decomposition of organic peroxides

To clarify the nature of cytocidal molecular species among the radicals generated in the iron-catalyzed reactions of peroxides (ROOH), we examined the cytocidal effects of these radicals against gram-positive and gramnegative bacteria in the presence or absence of various radical scavengers. Three organic peroxides, t-butyl hydroperoxide ( t-BuOOH), methyl ethyl ketone peroxide (MEKOOH), and cumene hydroperoxide, were used. Each radical generated from these peroxides was identified and quantitated by electron spin resonance (ESR) spin trapping with 5,5-dimethyl-1-pyrroline- N-oxide (DMPO). The major cytotoxic radical species generated in the mixtures of various peroxides and heme iron, especially methemoglobin, metmyoglobin, or hemin, was the alkyl peroxyl radical (ROO ·). Strong bactericidal action against gram-positive bacteria was observed in the peroxide-heme iron system, especially in the case of t-BuOOH and MEKOOH. Killing curves for gram-positive bacteria showed an initial lag period, which may indicate the multihit/multitarget kinetics of cell killing. When the diethylenetriamine pentaacetic acid (DTPA)-Fe 2+ complex was used as a catalyst for decomposition of various peroxides, alkyl, alkoxyl, and alkyl peroxyl radicals were identified by spin-trapping analysis. However, study of the time course of alkyl peroxyl radical production in the DTPA-Fe 2+ complex system revealed that radical species generated in this system were very short lived: a maximal level was achieved within 1 min and then declined sharply, and no bactericidal activity was observed after 10 min. In contrast, the alkyl peroxyl radical level generated by the organic peroxide-heme iron system remained high for 30 min or longer. The generation of alkyl peroxyl radicals quantified by ESR correlated quite well with the bactericidal effect of the system of peroxide plus iron. In addition, bactericidal activity was completely inhibited by the addition of the spin trap DMPO, as well as of other various radical scavengers (α-tocopherol and l-ascorbic acid), into the peroxide-heme iron system, but this effect was not observed with superoxide dismutase, β-carotene, dimethyl sulfoxide, diphenylamine, or butylated hydroxyltoluene. In view of these results, it is assumed that alkyl peroxyl radicals are the potent molecular species that are cytotoxic against bacteria, whereas alkoxyl radicals (RO ·) generated in this system do not affect bacterial viability.

Primary reduction and oxidation of thymine derivatives: ESR/ENDOR of thymidine and 1-methylthymine x-irradiated at 10 K

Single crystals of thymidine and 1-methylthymine were x-irradiated at 10 K and studied by using K-band ESR, ENDOR, and FSE spectroscopy. In thymidine, six primary radicals were identified. The major species were the O4-protonated anion (TI), the radical formed by net H addition to the C6 position of the base (TII), the radical formed by net H abstraction from the methyl group (TIII), and the alkoxy radical (TVI). The two minority radicals were one formed by net H addition to the C5 position of the base (TIV) and one in which the unpaired spin was localized on the sugar moiety, thought to be a C1{prime} H abstraction radical (TV). A small, nonexchangeable coupling in radical TI was shown to be due to a coupling to the methyl group. Evidence is presented that the characteristics of this coupling tensor indicates the protonation state of the thymine anion radical. Upon annealing, the anion decayed at about 40 K with no detectable successor. The alkoxy radical decays in the same temperature range. In 1-methylthymine, three major radical species were identified.

Carboxyl radical induced cleavage of disulfide bonds in proteins. A .gamma.-ray and pulse radiolysis mechanistic investigation

Disulfide bond reduction by the CO2.- radical was investigated in aponeocarzinostatin, aporiboflavin-binding protein, and bovine immunoglobulin. Protein-bound cysteine free thiols were formed under gamma-ray irradiation in the course of a pH-dependent and protein concentration dependent chain reaction. The chain efficiency increased upon acidification of the medium, with an apparent pKa around 5, and decreased abruptly below pH 3.6. It decreased also at neutral pH as cysteine accumulated. From pulse radiolysis analysis, CO2.- proved able to induce rapid one-electron oxidation of thiols and of tyrosine phenolic groups in addition to one-electron donation to exposed disulfide bonds. The bulk rate constant of CO2.- uptake by the native proteins was 5- to 10-fold faster at pH 3 than at pH 8, and the protonated form of the disulfide radical anion, [symbol: see text], appeared to be the major protein radical species formed under acidic conditions. The main decay path of [symbol: see text] consisted of the rapid formation of a thiyl radical intermediate [symbol: see text] in equilibrium with the closed, cyclic form. The thiyl radical was subsequently reduced to the sulfhydryl level [symbol: see text] on reaction with formate, generating 1 mol of the CO2.- radical, thus propagating the chain reaction. The disulfide radical anion [symbol: see text] at pH 8 decayed through competing intramolecular and/or intermolecular routes including disproportionation, protein-protein cross-linking, electron transfer with tyrosine residues, and reaction with sulfhydryl groups in prereduced systems. Disproportionation and cross-linking were observed with the riboflavin-binding protein solely. Formation of the disulfide radical cation [symbol: see text], phenoxyl radical Tyr-O. disproportionation, and phenoxyl radical induced oxidation of preformed thiol groups should also be taken into consideration to explain the fate of the oxygen-centered phenoxyl radical.

Thermal and Photo-Induced Reactions of Polymer Radicals in γ-Irradiated Poly(alkyl methacrylate)

Thermal and photo-induced reactions of radical species generated by γ-irradiation of poly(methyl methacrylate) and poly(ethyl methacrylate) have been investigated by means of ESR spectroscopy. The γ-irradiation at cryogenic temperature leads to the formation of three major radical species, side chain –COOCnH2n+1·− anion radicals and –COOĊHCn−1H2n−1 radicals, and main chain –ĊH– radicals. Cation radicals are immediately converted to the neutral radicals by proton transfer to the polymer molecules. Illumination of the polymer anion radicals with visible light leads to electron detachment or the elimination of ester alkyl groups to form alkyl radicals. Illumination with UV light leads to the conversion of the –COOĊHCn−1H2n−1 radicals to acyl-type –Ċ=O radicals. Both the –COOĊHCn−1H2n−1 and the –Ċ=O radicals are the precursor of scission-type ·C(CH3)(COOCnH2n+1)CH2- radicals.

Mechanism of radiation‐induced degradation of poly(methyl methacrylate) as studied by ESR and electron spin echo methods

AbstractThe role of radical species in the degradation of poly(methyl methacrylate) (PMMA) induced by γ‐irradiation has been studied by means of electron spin resonance and electron spin echo spectroscopy. The major radical species generated initially at 77 K are assigned to main chain  CH  and side chain  COOCH2 radicals, and  COOCH anion radical. Only the  COOCH2 radical converts to the scission‐type  CH2  C(CH3)COOCH3 radical on warming the sample of >180 K. A part of the  CH  radical disappears on warming the sample of >265 K. It is concluded that the scission of PMMA main chain occurs by the intramolecular process from the  COOCH2 radical as the precursor state.

Recent ENDOR and Pulsed Electron Paramagnetic Resonance Studies of Cytochrome c Peroxidase ‐ Compound I and Its Site‐Directed Mutants

AbstractEPR, ENDOR and pulsed EPR were applied to the radical signals of yeast cytochrome c peroxidase‐compound I from this protein expressed in E. coli and two variants prepared after site‐directed mutagenesis. The compound I complexes studied were first, that of the parent peroxidase expressed in E. coli, and next, those of the following amino acid mutants as specific probes for the radical in compound I: 1. Tryptophan‐51 → Phenylalanine; 2. Tryptophan‐191 → Phenylalanine. Tryptophan‐51 lies near the heme on the distal (H2O2‐binding) side of cytochrome c peroxidase, while Tryptophan‐191 lies in contact with the proximal heme ligand, Histidine‐175. EPR above 20 K and saturation recovery T1 measurements at liquid He temperatures showed the unusual temperature‐dependent relaxation of compound I's axially symmetric signal (g__ = 2.035, g⟂ = 2.00), which has comprised the major radical species in bakers' yeast wildtype compound I and here in all but the Tryptophan‐191 → Phenylalanine mutant. The ENDOR of the major radical species was characterized by the substantial hyperfine couplings previously proposed to be at a sulfur‐based radical. The mutation Tryptophan‐51 → Phenylalanine does not perturb these ENDOR features. Pulse field‐sweep EPR resolved for the first time distinct proton hyperfine couplings due to the features observed by ENDOR. A narrow, more typical organic free radical signal at g = 2.001 with resolved hyperfine structure was the only species observed from the Tryptophan‐191 → Phenylalanine mutant and was a minor feature in all samples. The Tryptophan‐191 → Phenylalanine mutation does perturb or destabilize the majority radical species of compound I. Thus, Tryptophan‐191 is either intimately involved with the kinetics of radical formation or participates intimately in the majority radical site, despite not being a sulfur‐containing amino acid.

Structure and mobility of electron gain and loss centres in proteins

Radiation damage to proteins is a topic of intense interest to those involved in radiation effects in biology, and also to those involved in radiotherapy. Although it has been widely studied, fundamental processes in protein damage are very hard to specify because of the complexity of the final damage products. But the non-invasive technique of electron-spin resonance is ideally suited to the task of detecting and identifying the primary and secondary products as these are expected to contains unpaired electrons (that is, free-radicals) and such species are uniquely detected by this sensitive form of spectroscopy. Our present study shows that a major radical species formed by electron loss in a range of proteins is the backbone amido radical, -N.(CO)-, characterized by hyperfine coupling to one 14N nucleus. These centres are efficiently trapped in proteins at low temperatures. In contrast, the expected backbone electron-capture centres, -NH(CO.-)-, are not readily trapped and electron transfer occurs until the ejected electron is trapped by some electrophilic centre. Such electron mobility was in fact established in our previous work on oxyhaemoglobin (FeO2----FeO2-), superoxide dismutase (Cu(II)----Cu(I] haemocyanin (Cu(II)O2Cu(I)----Cu(I)O2Cu(II] and various proteins containing S-S bonds (-S-S-)----(-S.-S-) (refs 1-4 respectively). This is strongly supported by our observation that such electrons are captured by DNA molecules, giving T.- centres, when nucleohistones are irradiated, and that Fe(CN)3-(6) ions readily scavenge such electrons from proteins which are devoid of highly electrophilic centres.

The peroxide complex of yeast cytochrome c peroxidase contains two distinct radical species, neither of which resides at methionine 172 or tryptophan 51.

The nature of the free radical species observed in the peroxide complex of yeast cytochrome c peroxidase is described for protein variants containing amino acid substitutions at Met-172 and Trp-51. As was the case with Met-172 mutations (Goodin, D.B., Mauk, A.G., and Smith, M. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 1295-1299), Trp-51 can be substituted to give active enzyme. Phe-51-containing enzyme has a higher turnover rate than the original enzyme and exhibits an altered pH dependence. The properties of the isotropic and axial components (Hoffman, B.M., Roberts, J.E., Kang, C.H., and Margoliash, E. (1981) J. Biol. Chem. 256, 6556-6564; Hori, H., and Yonetani, T. (1985) J. Biol. Chem. 260, 349-355) of the EPR signal of the wild-type enzyme-peroxide complex, studied as a function of H2O2 stoichiometry, support proposals (Goodin et al. (1985) and Hori and Yonetani (1985), see above) that two distinct radical species are formed, and spin quantification shows that the isotropic radical is always formed in substoichiometric amounts. The peroxide complexes for proteins containing amino acid substitutions at either Met-172 or Trp-51 exhibit somewhat larger than normal levels of the isotropic radical signal. In addition, these mutants are unlike wild-type enzyme in that the axial EPR signal associated with the peroxide complex is seen only at 10 K and not at 90 K. Thus, neither amino acid can be considered to be the molecular species responsible for either radical signal, but both mutations appear to affect the physical properties of the axial signal representing the major radical species.

Structural modification of human erythrocyte membranes following gamma-irradiation.

Structural damage to isolated erythrocyte membranes ('ghosts') has been studied following gamma-irradiation under a variety of conditions. For this two fluorescent probes were used; one 1-anilino-8-naphthalene sulphonate probes the lipid-aqueous interface, the other, diphenylhexatriene, was used to probe the membrane fluidity. Irradiation of the membranes caused a decrease in fluorescent intensity of the added probes, and changes in polarization of fluorescence. Oxygen was found to enhance the radiation damage, and scavenger experiments showed the hydroxyl radical was the major radical species involved. The structural modifications are therefore interpreted in terms of preliminary chemical damage involving peroxidation of unsaturated lipids. In addition sensitization and protection was observed in the presence of known dose-modifying chemicals.

Cyclopolymerization. III. Electron Spin Resonance Studies of Diallylamines with Redox Systems

Abstract The reaction of a series of diallylamines and related compounds with free radicals in aqueous acid solution has been studied in a flow system using ESR spectroscopy. The initiation was by radicals generated from titanium trichloride-hydrogen peroxide (hydroxyl radicals) and titanium trichloride-hydroxylamine (amino radicals) systems, respectively. The observed ESR spectra were assigned to five-membered ring radicals as the major radical species present in the system. However, the dimethallyl-amine series gave both five - and six-membered ring radicals.

Gamma-radiolysis of gaseous 1,3-butadiene. Yields of decomposition products

Abstract The radiolysis of gaseous 1,3-butadiene has been investigated using 80Co γ-rays. The yields of C1 to C4 hydrocarbons are reported for both scavenged and unscavenged systems. Ion and electron scavengers have no appreciable effect on the formation of products. The effects of pressure, in the range 7-500 torr, on the yields of major products: ethylene, acetylene, and vinylacetylene are evaluated. These products result mainly from the dissociation of two distinct species of excited 1,3-butadiene molecules, the lower limits of the yields of these species being estimated as 2.5 and 1.2, respectively. The yields of the major radical species: methyl, vinyl, and ethyl radicals are derived from hydrogen sulphide scavenging experiments.

Radical concentrations and reactions in a methane-oxygen flame

A complete set of characteristic profles is presented for a spherical methane-oxygen flame [(CH4)=0.078; (O2)=0.92; P=0.05 atm]. The internsive properties which are presented as a function of radial distance include measured temperature, derived velocity, measured compositions of all of the stable species and measured and derived concentrations of the major free radical species. The new type of information presented is the radical concentrations. Oxygen atoms were measured by a combination of microprobe, sampling and chemical scavening. Hydrogen atom concentrations were derived from the observed rate of oxygen reaction and confirmed by a preliminary scavenger sampling study. Hydroxyl radical concentrations were derived from the measured rate of carbon dioxide appearance and confirmed by direct measurement using the UV absoption. An upper limit has been set for methyl radical concentrations. These data have been used to test various proposed mechanisms in this flame. Two alternative mechanisms are suggested. For the reaction OH+CH4→H2O+CH3 the data suggests an activation energy of 6.5 kilocalories/mole and a frequency factor of 1.4×10−14 cc/moles/sec.