Rate constants have been compiled for reactions of various inorganic radicals produced by radiolysis or photolysis, as well as by other chemical means in aqueous solutions. Data are included for the reactions of ⋅CO2 −, ⋅CO3⋅−, O3, ⋅N3, ⋅NH2, ⋅NO2, NO3⋅, ⋅PO32−, PO4⋅2−, SO2⋅ −, ⋅SO3 −, SO4⋅−, ⋅SO5⋅−, ⋅SeO3⋅ −, ⋅(SCN)2⋅ −, ⋅CL2⋅−, ⋅Br2⋅ −, ⋅I2⋅ −, ⋅ClO2⋅, ⋅BrO2⋅, and miscellaneous related radicals, with inorganic and organic compounds.

Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution

The high intensity of third-generation X-ray sources, along with the development of cryo-cooling of protein crystals at temperatures around 100 K, have made it possible to extend the diffraction limit of crystals and to reduce their size. However, even with cryo-cooled crystals, radiation damage becomes a limiting factor. So far, the radiation damage has manifested itself in the form of a loss of overall diffracted intensity and an increase in the temperature factor. The structure of a protein (myrosinase) after exposure to different doses of X-rays in the region of 20 × 1015 photons mm−2 has been studied. The changes in the structure owing to radiation damage were analysed using Fourier difference maps and occupancy refinement for the first time. Damage was obvious in the form of breakage of disulfide bonds, decarboxylation of aspartate and glutamate residues, a loss of hydroxyl groups from tyrosine and of the methylthio group of methionine. The susceptibility to radiation damage of individual groups of the same kind varies within the protein. The quality of the model resulting from structure determination might be compromised owing to the presence of radiolysis in the crystal after an excessive radiation dose. Radiation-induced structural changes may interfere with the interpretation of ligand-binding studies or MAD data. The experiments reported here suggest that there is an intrinsic limit to the amount of data which can be extracted from a sample of a given size.

Structural changes in a cryo-cooled protein crystal owing to radiation damage

The sulfate radical anion (SO4•–) based oxidation of trace organic contaminants (TrOCs) has recently received great attention due to its high reactivity and low selectivity In this study, a meta-analysis was conducted to better understand the role of functional groups on the reactivity between SO4•– and TrOCs The results indicate that compounds in which electron transfer and addition channels dominate tend to exhibit a faster second-order rate constants (kSO4•–) than that of H–atom abstraction, corroborating the SO4•– reactivity and mechanisms observed in the individual studies Then, a quantitative structure activity relationship (QSAR) model was developed using a sequential approach with constitutional, geometrical, electrostatic, and quantum chemical descriptors Two descriptors, ELUMO and EHOMO energy gap (ELUMO–EHOMO) and the ratio of oxygen atoms to carbon atoms (#O:C), were found to mechanistically and statistically affect kSO4•– to a great extent with the standardized QSAR model: ln kSO4•– = 268–397 × #O:C – 0746 × (ELUMO–EHOMO) In addition, the correlation analysis indicates that there is no dominant reaction channel for SO4•– reactions with various structurally diverse compounds Our QSAR model provides a robust predictive tool for estimating emerging micropollutants removal using SO4•– during wastewater treatment processes

Quantitative Structure-Activity Relationship (QSAR) for the Oxidation of Trace Organic Contaminants by Sulfate Radical

Publisher Summary This chapter discusses the formation and essential physicochemical properties of radical species from the three major classes of organic sulfur compounds—namely, thiols, disulfides, and monosulfides. Electron spin resonance (ESR) is often hampered for sulfur-centered radicals owing to the lack of nuclear spin in the main sulfur isotope, 32S. This practically prevents any information on structural aspects. Most of the data available nowadays on sulfur-centered radicals stem from radiation chemical studies, particularly those based on the technique of pulse radiolysis. The latter essentially relies on a time-resolved detection of optical or other physical properties of the radicals. The variety of sulfur-centered radicals seems to exceed that of oxygen-centered radicals. Also, the spectrum of their chemical reactions is at least as manifold as for the oxygen analogs. Among the significant aspects emerging from the available data is the pronounced tendency of sulfur-entered radicals to coordinate with electron-rich centers. Specific electronic structures, such as the 2σ/1σ* three-electron bonds, provide a rationale for many physical and chemical characteristics of the radical species. Another important finding is the frequent involvement of sulfur-centered radicals in thermodynamic equilibria. This appears to be of particular significance for the role of thiyl radicals, because it could explain, for example, some of the insufficiencies in the repair and protection mechanism of thiols. In fact, the equilibria open the possibility that thiyl radicals indirectly could even contribute to biological damage.

K. D. Asmus

Papers

Free radical oxidation of organic disulfides