Thiyl radical reaction with oxygen

RSOO are generated by the hydrogen abstraction from thiols by OH radicals, alcohol radicals, etc., producing thiyl radicals (RS ) which on reaction with oxygen produce RSOO [Eqs. (34) and (35)]. Alternatively, reaction of alkylsulfonyl chloride with e q produces alkyl sulfonyl peroxyl radicals [Eq. (36)]. 

The radicals (14) formed may be trapped with, for example, (10) above. Simple alkyl thiyl radicals such as MeS have been detected as reaction intermediates they are highly reactive. Relatively stable oxygen-containing radicals are also known. Thus the phenoxy radical (15),... 

The attack by the thiolate anion on the N-oxide oxygen of 62 produces the intermediate sulfenic acid derivative 65, which, in the presence of thiols, further reacts with the thiolate anion, to give the oxime 66, which has been isolated among the reaction products. By contrast, spontaneous loss of the halide anion from 65 affords the ni-troso intermediate 67 that, by losing NO and the thiyl radical directly, or through 68, produces the a-nitrosoolefm 69. By a Michael type reaction with water this last product immediately yields the final oxime 70, which has been isolated among the reaction products. 

Disulfides (RS-SRJ are not as useful as peroxides in their photochemical reactions, partly because carbon-sulfur bond cleavage (to give R" and RSi) competes more effectively with sulfur-sulfur bond cleavage than does C-O with 0-0 cleavage for peroxides, but largely because thiyl radicals are less reactive than their oxygen analogues. 

Although there is little doubt that the electron transfer reaction (Reaction 2) is involved in the over-all reaction (21), the suggestion that quantitative yields of disulfide (13) arise from the dimerization of thiyl radicals is inconsistent with the observed behavior of other free radicals (24). It seems preferable to suggest that some kind of coordination occurs as a prerequisite to the transfer of electrons (12,15). In this case, metal-thiol complexes should be formed as intermediates in the oxidation, in which the metal acts not only as an electron acceptor but also to locate the resultant thiyl entities in close proximity, thereby favoring dimerization reactions and producing disulfide. The electrons gained by the metal may then be passed on to an oxygen molecule. The over-all reaction may be represented as... 

Hug GL, Bonifacic M, Asmus K-D, Armstrong DA (2000a) Fast decarboxylation of aliphatic amino adds induced by 4-carboxybenzophenone triplets in aqueous solutions. A nanosecond laser flash photolysis study. J Phys Chem B 104 6674-6682 Hug GL, Carmichael I, Fessenden RW (2000b) Direct EPR observation of the aminomethyl radical during the radiolysis of glycine. J Chem Soc Perkin Trans 2 907-908 Hunter EPL, DesrosiersMF, Simic MG (1989) The effect of oxygen, antioxidants and superoxide radical on tyrosine phenoxyl radical dimerization. Free Rad Biol Med 6 581-585 Ito O (1992) Flash photolysis study for reversible addition reactions of thiyl radicals with olefins and acetylenes. Trends Phys Chem 3 245-266... 

Sevilla MD, Becker D, Yan M (1990a) The formation and structure of the sulfoxyl radicals RSO, RSOO, RSCV, and RS0200 from the reaction of cysteine, glutathione and penicillamine thiyl radicals with molecular oxygen. Int J Radiat Biol 57 65-81... 

Sway Ml, Waddington DJ (1982) Reactions of oxygenated radicals in the gas phase, part 11. Reaction of isopropylperoxyl radicals with 2,3-dimethylbut-2-ene. J Chem Soc Perkin Trans 2 999-1003 Tamba M, Simone G, Quintiliani M (1986) Interactions of thiyl free radicals with oxygen a pulse radiolysis study. Int J Radiat Biol 50 595-600... 

There are a number of mechanisms by which ascorbate may facilitate the removal of activated oxygen-derived molecules in vivo-, these include reaction with 0 2, HOj and OH , as well as the reduction of thiyl radicals (see section 5.1),... 

One of the most important implications of these experiments is the conclusion that, due to the high polarity of the C=0 bond, organoelement a-ketones R3MCOR (M = Ge, Sn) are extraordinary effective radical traps. The electronegative oxygen atom of their carbonyl group could be attacked not only by element-centered radicals R3M (M = Ge, Sn), but also by thiyl radicals SR " and phosphorus-centered radicals. Indeed, the experimental estimates of the absolute reaction rate constants of the element-centered radicals with... 

OH free radicals react with almost all amino-acids. For aliphatic residues, rate constants are correlated with the strength of the X-H bond(X = S, C or N) (1). Thus the reaction is relatively slow with glycine (k = 1.7 x 10 mol 1 s ) and fast with the -SH function of cysteine (k = 1.9 x 10 mol M s i). The thiyl radical formed upon oxidation of cysteine, whose spectral properties are in table 3, is formed but a carbon-centered radical is also present (50, 51). In the presence of oxygen, thiyl radical fixes O2 giving peroxy radicals (52). These radicals may photoisomerize into sulfonyl radicals RS02 (53). In small molecules, disulfide groups can also be oxidized. This reaction was not demonstrated in proteins, but cannot be neglected. A disulfide radical cation is formed (54). Final compounds are not known. 

Further thorough studies would be necessary to define in detail the mechanism of these reactions. The limited evidence available is, however, consistent with the reasonable assumption that the reaction proceeds by a radical chain mechanism, probably initiated by heavy metal ions and involving thiyl radicals following a scheme similar to that proposed for the oxidation of mercaptans by molecular oxygen (see section IV). 

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