The thiocyanate dosimeter (10–2 mol dm–3 SCN– in O2-saturated water) has been standardised against the super-Fricke dosimeter (10–2 mol dm–3 FeII in O2-saturated 0.4 mol dm–3 H2SO4) using the hexacyanoferrate(II) dosimeter [5 × 10–3 mol dm–3 Fe(CN)64– in O2-saturated water] as a secondary standard. On the basis that G(FeIII)= 1.67 × 10–6 mol J–1 and IµFeIII= 220.4 m2 mol–1 at 304 nm and 25 °C in the super-Fricke dosimeter, we obtain GIµ[Fe(CN)63–]=(3.47 ± 0.06)× 10–5 m2 J–1 at 420 nm and GIµ(SCN)2˙–=(2.59 ± 0.05)× 10–4 m2 J–1 at 475 nm. These values remain unchanged when the solutions are saturated with air instead of O2 and are doubled in N2O-saturated solution.

Re-evaluation of the thiocyanate dosimeter for pulse radiolysis

This review summarizes direct and indirect analytical methods for the detection and quantification of the reactive oxygen species (ROS): 1O2, O2·−/HOO·, H2O2, HO·, and CO3·− in aqueous solution. Each section briefly describes the chemical properties of a specific ROS followed by a table (organized alphabetically by detection method, i.e., absorbance, chemiluminescence, etc.) summarizing the nature of the observable (associated analytical signal) for each method, limit of detection, application notes, and reaction of the probe molecule with the particular ROS.

Methods for reactive oxygen species (ROS) detection in aqueous environments

This study reevaluates the role of peroxymonosulfate anion radical (-O3SOO• or SO5•-) intermediate during radical-induced chain oxidation of HSO3-/SO32- in oxygenated aqueous solution. The SO5•- radical absorption band in the UV is weak:  e = 1065 ± 80 M-1 cm-1 at λmax (260−265 nm). The SO5•- radical takes part in two radical−radical and four radical−solute reactions, partially producing the other chain carrier, the SO4•- radical, in either case. In this study, employing the pulse-radiolysis technique but adopting a new approach, these two types of reactions of the SO5•- radical have been separately quantified (at room temperature). For example, over pH 3.5−12, the branching ratio of (SO5•- + SO5•-) reactions giving rise to either the SO4•- radical or S2O82- is found to remain ∼1. The respective reaction rate constants for I → 0 are (2.2 ± 0.3) and (2.1 ± 0.3) × 108 M-1 s-1. The (SO5•- + HSO3-) reactions in acid pH follow two paths, forming the SO4•- radical in one and regenerating the SO3•- radical in th...

Reactivity and Role of SO5•- Radical in Aqueous Medium Chain Oxidation of Sulfite to Sulfate and Atmospheric Sulfuric Acid Generation

Scavenging of nitrogen dioxide, thiyl, and sulfonyl free radicals by the nutritional antioxidant beta-carotene.

The pathogenesis of Alzheimer's disease is strongly associated with the formation and deposition of β-amyloid peptide (βAP) in the brain. This peptide contains a methionine (Met) residue in the C-terminal domain, which is important for its neurotoxicity and its propensity to reduce transition metals and to form reactive oxygen species. Theoretical studies have proposed the formation of βAP Met radical cations as intermediates, but no experimental evidence with regard to formation and reactivity of these species in βAP is available, largely due to the insolubility of the peptide. To define the potential reactions of Met radical cations in βAP, we have performed time-resolved UV spectroscopic and conductivity studies with small model peptides, which show for the first time that (i) Met radical cations in peptides can be stabilized through bond formation with either the oxygen or the nitrogen atoms of adjacent peptide bonds; (ii) the formation of sulfur−oxygen bonds is kinetically preferred, but on longer ti...

Free Radical Reactions of Methionine in Peptides: Mechanisms Relevant to β-Amyloid Oxidation and Alzheimer's Disease

The radiation chemical yield for oxidation of ferrocyanide by OH produced in the fast electron radiolysis of N/sub 2/O-saturated aqueous solutions is found to increase from a low concentration limiting yield of 5.2 to values in excess of 6.7 for ferrocyanide concentrations above 0.1 M. This observed dependence reflects competition between the scavenging reaction and track recombination processes involving OH radicals which occurs on a time scale greater than 10/sup -9/s. By extrapolation an additional yield of -- 1.5 is estimated for OH radicals which recombine at still shorter times. These scavenging studies show that significant amounts of recombination occur within the track even at times as long as 10/sup -6/s. Secondary radicals produced within the track are also subject to recombination processes as is illustrated by the low concentration limit which shows that most OH radicals produced from N/sub 2/O within the track do not escape from the track. In general, for concentrations of reactive solutes greater than -- 10/sup -4/ M, i.e., for k(S) > 10/sup 6/ s/sup -1/, scavenging of OH from the track will contribute significantly to the observed yields and this effect must be taken into account in comparing the results from different radiolytic systems.

Track effects in radiation chemistry. Concentration dependence for the scavenging of hydroxyl by ferrocyanide in nitrous oxide-saturated aqueous solutions

Amy L. Hartzell

Papers

Track effects in radiation chemistry. Concentration dependence for the scavenging of hydroxyl by ferrocyanide in nitrous oxide-saturated aqueous solutions