Thiyl radicals (RS) formed by the reaction of radiolytically generated OH radicals with thiols, e.g. 1,4-dithiothreitol (DTT), react with cis- and trans-2,5-dimethyltetrahydrofuran by abstracting an H atom in the alpha-position to the ether function (k approximately equal to 5 X 10(3) dm3 mol-1 s-1). The so-formed planar ether radical is 'repaired' by the thiol (k = 6 X 10(8) dm3 mol-1 s-1) thereby regenerating a cis- or trans-2,5-dimethyltetrahydrofuran molecule. In this reaction a thiyl radical is reproduced. Thus trans-2,5-Me2THF from cis-2,5-Me2THF and vice versa are formed in a chain reaction: at a dose rate of 2.8 X 10(-3) Gys-1 and a trans-2,5-Me2THF concentration of 1 X 10(-2) mol dm-3 using DTT as the thiol, G(cis-2,5-Me2THF) = 160 has been found. The chain reaction is very sensitive to impurities and also to disulphides such as those radiolytically formed. 2,5-Me2THF can be regarded as a model for the sugar moiety of DNA where the C(4')-radical is known to lead to DNA strand breakage. The possible role of cellular thiols in the repair of the C(4') DNA radical, and also the conceivable role of thiyl radicals inducing DNA strand breakage, are discussed.
The Arrhenius parameters and rates of reaction of three hydroxyradicals, methyl radical, and the hindered primary C- centred radical from t-butyl alcohol with dithiothreitol were measured by pulse radiolysis in water. The bimolecular rate constants were found to be in the order: C-.(CH3)(2)OH > (CH)- C-.(CH3)OH > (CH2OH)-C-. > (.)CH3 > (CH2C)-C-.(CH3)(2)OH. The reaction of three of these, C-.(CH3)(2)OH, (CH2OH)-C-., and (CH3)-C-., with methanethiol were examined at the ab initio B3LYP/6311+G(d,p) level, coupled with transition state theory, both in the gas phase and in solution. The solvent effects are evaluated by two different continuum models (SCIPCM, CPCM), coupled with a novel approach to the calculation of the solution phase entropy. The reaction is discussed in terms of the charge and spin polarization in the transition state, as determined by AIM analysis, and in terms of orbital interaction theory. Rate constants, calculated by transition state theory are in good agreement with the experimental data
Oxygenated aqueous solutions of alginic acid, a model compound for polyuronic acids contained in surface waters, were photolyzed in UV light ( = 254 nm), treated with ozone, or reacted with radiolytically generated hydroxyl radicals. The average molecular weight decrease upon such treatment was measured by viscosimetry. At a fluence of 250 J m~2, which is generally considered sufficient to disinfect drinking water, 0.0005 strand breaks per macromolecule are effected. Alginic acid is capable of complexing ferric ions. Their presence increases photolytic strand-break formation. At an iron concentration of 10"6 mol dm"3, such as may prevail after flocculation with iron salts, 0.004 strand breaks per macromolecule are detected at the above fluence. Hydroxyl radicals, produced by subjecting the N20/02-saturated aqueous alginic acid solution to ionizing radiation from a 60Co y source, cause strand breakage with an efficiency of 22%, while superoxide radicals are released from the polymer peroxyl radicals with an efficiency of 71%. The efficiency of ozone in producing a strand break is 18%, relative to the total of the ozone consumed. The destruction of the alginic acid by ozone is mainly caused by the intermediate hydroxyl radicals. The polysaccharide peroxyl radicals that are formed by OH attack and subsequent addition of 02 eliminate superoxide radicals, which in turn stimulate further hydroxyl radical production by reacting rapidly with the ozone.
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