Johan Lind

Pulse radiolysis study on the reactivity of Trolox C phenoxyl radical with superoxide anion

The reaction between the phenoxyl radical of Trolox C, a water‐soluble vitamin E analogue, and superoxide anion radical was examined by using the pulse radiolysis technique. The results indicate that the Trolox C phenoxyl radical may undergo a rapid one‐electron transfer from superoxide radical [k=(4.5±0.5) × 108 M−1·s−1] to its reduced form. This finding indicates that superoxide radical might play a role in the repair of vitamin E phenoxyl radical.

THE REACTIVITY OF SUPEROXIDE (O2‐) and ITS ABILITY TO INDUCE CHEMILUMINESCENCE WITH LUMINOL

Abstract— The quantum yield and the kinetics of O‐induced luminol chemiluminescence was investigated in a broad pH interval at varying luminol and concentrations. It is suggested that the weak chemiluminescence observed is mediated via a luminol‐superoxide‐adduct proposed to be an a‐hydroxyperoxyl radical. At pH 7 the maximum quantum yield of chemiluminescence per initial percent was determined to be 4 times 10‐8. The degree of involvement in phagocytosis and related processes should be viewed against this maximum limit.

One- and two-electron reduction potentials of peroxyl radicals and related species

Utilising gaseous and aqueous thermodynamic quantities, estimates (in water versus NHE) have been made of the one-electron reduction potentials of alkyl peroxyl radicals including CCl3OO˙, percarboxyl and carboxyl radicals, and alkoxyl radicals, including CCl3O˙ and two-electron reduction potentials of alkyl hydroperoxides including CCl3OOH, alkyl peroxyl radicals including CCl3OO˙ and of percarboxyl radicals.

The reaction of ozone with the hydroxide ion: mechanistic considerations based on thermokinetic and quantum chemical calculations and the role of HO4- in superoxide dismutation.

The reaction of OH(-) with O(3) eventually leads to the formation of *OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol. 1982, 16, 676-681), it was suggested that the first step occurred by O transfer: OH(-)+O(3)-->HO(2)(-)+O(2) and that *OH was generated in the subsequent reaction(s) of HO(2)(-) with O(3) (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one-step O transfer such as that mentioned above would require the release of O(2) in its excited singlet state ((1)O(2), O(2)((1)Delta(g))); this state lies 95.5 kJ mol(-1) above the triplet ground state ((3)O(2), O(2)((3)Sigma(g)(-))). The low experimental rate constant of 70 M(-1) s(-1) is not incompatible with such a reaction. However, according to our calculations, the reaction of OH(-) with O(3) to form an adduct (OH(-)+O(3)-->HO(4)(-); DeltaG=3.5 kJ mol(-1)) is a much better candidate for the rate-determining step as compared with the significantly more endergonic O transfer (DeltaG=26.7 kJ mol(-1)). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO(4)(-) have been probed: HO(4)(-)-->HO(2)(-)+(1)O(2) is spin allowed, but markedly endergonic (DeltaG=23.2 kJ mol(-1)). HO(4)(-)-->HO(2)(-)+(3)O(2) is spin forbidden (DeltaG=-73.3 kJ mol(-1)). The decay into radicals, HO(4)(-)-->HO(2)*+O(2)(*-), is spin allowed and less endergonic (DeltaG=14.8 kJ mol(-1)) than HO(4)(-)-->HO(2)(-)+(1)O(2). It is thus HO(4)(-)-->HO(2)*+O(2)(*-) by which HO(4)(-) decays. It is noted that a large contribution of the reverse of this reaction, HO(2)*+O(2)(*-)-->HO(4)(-), followed by HO(4)(-)-->HO(2)(-)+(3)O(2), now explains why the measured rate of the bimolecular decay of HO(2)* and O(2)(*-) into HO(2)(-)+O(2) (k=1 x 10(8) M(-1) s(-1)) is below diffusion controlled. Because k for the process HO(4)(-)-->HO(2)*+O(2)(*-) is much larger than k for the reverse of OH(-)+O(3)-->HO(4)(-), the forward reaction OH(-)+O(3)-->HO(4)(-) is practically irreversible.

Significance of the intramolecular transformation of glutathione thiyl radicals to α-aminoalkyl radicals. Thermochemical and biological implications

Product studies have been undertaken on the OH˙ radical-induced oxidation of glutathione in N2O- saturated aqueous solutions. Ammonia has been found to be a prominent product with G values around 2.5–2.9 × 10-7 J mol-1 from pH 6 to 10.5. The ammonia is considered to be a product of the disproportionation reaction of the α-amino carbon-centred radicals, formed via the intramolecular transformation of glutathione thiyl radicals. At pH ca. 4–6, the ammonia yield decreases due to the fact that the transformation reaction slows down with decreasing pH and eventually comes into competition with bimolecular recombination. From the pH dependence of the ammonia yield curve, the equilibrium constant between the glutathione thiyl radical and the α-amino carbon-centred radical is deduced to be >104. The strength of the C–H bond α to the NH2 and CO2- groups is thus <343 kJ mol-1. The corresponding bond energy of the C–H bond α to the NH2 and CO2H groups is estimated to be <329 kJ mol-1. Based on the ammonia formation, consumption of free SH groups and the HPLC chromatograms obtained at different pH values after γ-irradiation of N2O-saturated glutathione solutions, the overall reaction mechanism concerning the fate of glutathione thiyl radicals is proposed. This mechanism and its kinetics indicate that the intramolecular transformation is one of the principal pathways of self-removal of glutathione thiyl radicals, which is formed in various repair processes, in both anaerobic and aerobic conditions.

N–H bond dissociation energies, reduction potentials and pKas of multisubstituted anilines and aniline radical cations

The one-electron reduction potential and the pKa of 10 ortho-, meta- and para-substituted aniline radical cations have been determined by means of pulse radiolysis. The N–H bond dissociation energies of the corresponding anilines were also determined using a thermodynamic cycle. All three properties of the aniline radical cations and the corresponding anilines were shown to be linearly dependent on the sum of the Brown substituent constants, Σσ+. A conditional scale for ortho substituents was also derived (σo+= 0.73σp+). The results from this work along with previously published results have been used to derive a linear free energy relationship between one-electron reduction potentials of benzene radical cations and the substituent pattern. In addition an equation for the calculation of X–Y bond dissociation energies of arbitrarily substituted molecules with the general formula Ph–X–Y is proposed.

Chemiluminescence of 5-aminophthalazine-1,4-dione in the presence of hydrogen peroxide

In aqueous solutions containing luminol, H2O2 and ClO–2, chlorine dioxide (ClO.2) is generated by pulse radiolysis. ClO.2 oxidizes luminol to 5-aminophthalazine-1,4-dione (azaquinone). The latter reacts with HO–2 to yield an intense chemiluminescence. The results obtained from detailed kinetic study at various pH values and from product analysis are consistent with the following conclusions:The azaquinone combines with HO–2 to form an open carbon-centred peroxide. The latter decomposes via a short-lived endoperoxide in two parallel reactions. The first of these yields oxygen and luminol while the second one generates nitrogen and ultimately excited 3-aminophthalate with an efficiency of 4%. The kinetics of the present system is compared with that of other similar systems reported. Based on observed differences, the presence of several peroxide intermediates is implied. In addition evidence is presented that 3-aminophthalate may not be the sole emitter in every chemiluminescent system based on luminol.

REDOX PROPERTIES OF 4-SUBSTITUTED ARYL METHYL CHALCOGENIDES IN WATER

The one-electron reduction potentials of 14 4-substituted aryl methyl chalcogenide radical cations in water have been measured by pulse radiolysis. The reduction potentials were plotted against the Brown substituent constant of the 4-substituent giving straight lines with slopes close to zero. Aryl methyl chalcogenides with strongly electron-donating substituents deviated markedly from the general trend indicating a different nature of the radical cation. The results are compared with electrochemical results in organic solvents and with gas-phase ionization energies. This comparison shows that the substituent effect on the potential decreases with increasing polarity of the solvent. In addition, the redox properties of diaryl, dimethyl and aryl methyl sulfides are compared and discussed.

Oxidation of luminol by chlorine dioxide. Formation of 5-aminophthalazine-1,4-dione

The pH-dependent reaction rate between luminol and chlorine dioxide has been determined. The net oxidation reaction proceeds in two steps. The first of these is the formation of a luminol–ClO2 adduct. This adduct, which is unreactive towards oxygen, reacts rapidly with itself or with a second ClO2 molecule. The final product has a broad absorbance between 450 and 650 nm. An identical absorbance is produced when luminol is oxidized by HOCl. The species displaying this absorbance is very reactive towards OH– and undissociated luminol. In the presence of hydrogen peroxide and at pH > 7 the absorbance disappears. At the same time an intense chemiluminescence is observed, the decay of which is quantitatively in agreement with the decay of the 450–650 nm absorbance. From these findings it is concluded that the oxidation product of luminol with ClO2 is 5-aminophthalazine-1,4-dione.

Determination of the chemiluminescence quantum yield of luminol in rapid chemical reactions

Abstract By use of the “Hastings standard” the chemiluminescence quantum yield φ cl of aqueous hemin-catalyzed luminol-H 2 O 2 solutions at pH = 11.6 was determined to be (1.28 ± 0.15) X 10 −2 . The same φ cl (1.23 × 10 −2 ) was found in the reaction of 5-aminophthalazine-1,4-dione (azaquinons) with HO BM 2 when the yield was based on the 3-aminophthalate (or N 2 ) produced.