John E. Packer

Reactions of the carbon tetrachloride-related peroxy free radical (CCl3O2.) with amino acids : Pulse radiolysis evidence

Abstract Pulse radiolysis studies indicate that the free radical CCl3O2. but not the radical CCl3 reacts rapidly with tryptophan, tyrosine, phenol or promethazine to yield transient products with strong visible absorption spectra.

Free radicals and singlet oxygen scavengers: Reaction of a peroxy-radical with β-carotene, diphenyl furan and 1,4-diazobicyclo(2,2,2)-octane

The ‘singlet oxygen scavengers’. 1,4-diazobicyclo(2,2,2)-octane (DABCO), diphenyl furan and β-carotene react rapidly with the organic peroxyradical CCl3O2•. The absolute reaction rate constants k = 1.2 ± 0.2 × 107, 6 ± 2 × 107 at 1.5 ± 0.2 × 109 M−1s−1 respectively have been determined by pulse radiolysis. Comparison with other data suggest that other free radicals are also likely to react with these compounds; in the case of the hydroxyl radical and DABCO k = 1.25 × 109 M−1s−1 has been determined.

Reaction of the trichloromethyl and halothane-derived peroxy radicals with unsaturated fatty acids: A pulse radiolysis study

The absolute rates of reaction of the trichloromethylperoxy radical, CCl3OO., derived from carbon tetrachloride and the halothane peroxy radical, CF3CHClOO., with oleic, linoleic, linolenic and arachidonic acids have been determined using the fast reaction technique of pulse radiolysis. In general, the rates of reaction of the radical derived from carbon tetrachloride are approximately five times greater than those for the halothane related radical. In both cases the rate constant increases with increasing unsaturation of the fatty acid in agreement with the known greater susceptibility of polyunsaturated fatty acids to peroxidative decomposition.

Nitrogen dioxide and related free radicals: electron-transfer reactions with organic compounds in solutions containing nitrite or nitrate

The absolute rate constants for the reaction of NO32–˙(formed by one-electron reduction of nitrate) with oxygen, benzoquinone, and methyl viologen have been determined by pulse radiolysis. Experiments have shown that such reactions can occur in competition with the hydrolysis reaction leading to NO2˙ and that the formation of the latter can be catalysed by the presence of hydrogen or ammonium ions. Absolute rate constants for the oxidation of the phenothiazine derivative metiazinic acid, of 2,2′-azinobis-(3-ethyl-2,3-dihydrobenzothiazole-6-sulphonate), of ascorbate, and of dihydroxyfumarate by NO2˙ have also been measured.

Electron transfer reactions of halogenated aliphatic peroxyl radicals: measurement of absolute rate constants by pulse radiolysis

The peroxyl radicals Cl3COO·, Cl2CHOO·, ClCH2OO·, CH3OO; ·OOCCl2CO2–, ·OOCHClCO2–, ·OOCH2CO2–, ·OOCF2CO2–, and (CH3)2C(OH)CH2OO· have been generated in aqueous alcohol solutions by pulse radiolysis using the Brunel and Hahn-Meitner accelerators. The absolute rate constants of the one-electron transfer reactions of related aliphatic peroxyl radicals with ascorbate, promethazine, phenol, and tyrosine have been found to increase with increasing substitution of chlorine atoms. For the reaction of the radical Cl3CO2· and promethazine, at pH 6, k 6.0 ± 0.4 × 108 l mol–1 s–1 has been measured. For the analogous reaction of the radical ClCH2O2·, k 3.3 ± 0.2 × 107 l mol–1 S–1 has been determined.

Reactions of the Trichloromethylperoxy Free Radical (Cl3COO.) with Tryptophan, Tryptophanyl-tyrosine and Lysozyme

The trichloromethyl peroxy radical Cl3COO reacts with tryptophan, tryptophanyl-tyrosine and with lysozyme to form products whose overall absorption spectrum is different from those observed following the reaction of hydroxyl, bromide, thiocyanate or azide radicals. Two spectral components have been identified: a minor component attributed to the neutral tryptophanyl radical which can react with ascorbate and intramolecularly with tyrosine residues and a major component which does not undergo either of these reactions and is probably a radical adduct.

Kinetics of the reaction of hydrogen peroxide with cysteine and cysteamine

The rate of the reaction between hydrogen peroxide and cysteine or cysteamine is proportional to [H2O2] and [NH3+CHXCH2S–](X = H or CO2–) consistent with nucleophilic attack by the thiolate ions on peroxide oxygen. The rate decreases at higher pH where loss of the NH3+ proton occurs, and it is suggested that hydrogen bonding between this group and hydrogen peroxide facilitates the reaction.

Aminopyrine and antipyrine free radical-cations: pulse radiolysis studies of one-electron transfer reactions

Absolute rate constants for the reaction of a variety of electrophilic free radicals with the pyrazoline derivatives aminopyrine and antipyrine have been measured by pulse radiolysis. In the case of aminopyrine the resulting radical cation is a particularly stable species (Iµ325 5.35 × 103 dm3 mol–1 cm–1). Both compounds are readily oxidised to their respective radical-cations with the one-electron oxidation potential of antipyrine (E° 1.1–1.6 V) being higher than that of aminopyrine (E° 0.26–0.5 V). Studies of the reaction of the radical-cations with reducing agents suggest that aminopyrine in particular may prove to be a useful reference compound in studies of free radical one-electron oxidations.

A reinvestigation of the oxidation of cysteine by Br2–˙ and I2–˙. Evidence for CyS ∴ Br– and CyS ∴ I–

The existence of the species CyS ∴ Br– and CyS ∴ I–, which may be regarded as complexes between the thiyl radical from cysteine and bromide or iodide ions, is reported, and their properties discussed in relation to other sulphur- and halogen-containing species which also have a three-electron half-order bond. The oxidation of cysteine by I2–˙ is shown to be much more complex than hitherto reported, and pH-dependent equilibria involving iodine atoms, iodide ions, cysteine, and cysteinyl radicals are described. The first measured rate constant for the reaction of iodine (as I3–) with a thiol is also reported.

Free-radical reductions of arenediazonium ions in aqueous solution. Part II. Kinetics of reactions with formate, cysteine, methanol, hypophosphite, and phosphite as reducing agents

Two classes of kinetics are exhibited in the γ-radiation-induced radical-chain reduction of toluene-p-diazonium ions with the above reducing agents in aqueous oxygen-free solution, the overall reaction being ArN2++ RH2→ ArH + N2+ R + H+. In class A, G(–ArN2+) is proportional to [RH2]/[ArN2+] and independent of dose rate; and in class B, proportional to [ArN2+]/(dose rate)1/2 and independent of [RH2]. A reaction sequence to account for these observations is proposed, the relative rates of the reactions involved determining the class of kinetics exhibited. By altering the concentration of ArN2+ it is possible to change the behaviour of the system from one class to the other. The rate of hydrogen abstraction by the p-tolyl radical from RH2 is in the order hypophosphite and phosphite > formate > methanol > formic acid. Addition of acid decreased G(–ArN2+) in the case of the oxyanions, but not for methanol.