PERTHIYL RADICALS, TRISULFIDE RADICAL IONS, AND SULFATE FORMATION - A COMBINED PHOTOLYSIS AND RADIOLYSIS STUDY ON REDOX PROCESSES WITH ORGANIC DISULFIDES AND TRISULFIDES

The formation of perthiyl and thiyl radicals has been studied in a combined photochemical and radiation chemical investigation of the di- and trisulfides of penicillamine and cysteine, together with glutathione disulfide, cystamine dihydrochloride, and dithiodiglycolic acid in aqueous solution. The highest RSS. yields are found from the trisulfides with little difference between PenSSSPen and CySSSCy in the case of photolysis, but with a (4-5):1 ratio in favor in PenSS. over CySS. upon radiation chemically induced processes. Thiyl radicals are the other major species formed. The RSS./RS. ratio changes significantly depending on the nature of the di- or trisulfide and the method of radical generation employed. Perthiyl radicals are found to be moderately good oxidants, weaker though than thiyl radicals, and readily react with molecular oxygen in an addition process. The following absolute rate constants have been measured: k(PenSS. + ascorbate) = (4.1 +/- 1.0) x 10(6) M-1 s-1 and k(PenSS. + O2) = (5.1 +/- 1.0) x 10(6) M-1 s-1. Probably the most interesting finding about perthiyl radicals is that inorganic SO42- is formed in their reaction with molecular oxygen. The mechanism of sulfate formation is suggested to proceed via the perthioperoxyl radical, RSSOO., structural rearrangement of this transient into the corresponding sulfonyl-type radical, RSSO2., addition of oxygen to the latter to yield a sulfonyl peroxyl, RSSO2OO., followed by a bimolecular radical-radical reaction of these peroxyl radicals (in analogy to the fate of peroxyl radicals in general) leading to RSSO3.. Cleavage of SO3 from RSSO3. and hydrolysis would then result in SO42-. Reduction of trisulfides yields (RSSSR).- radical anions which decay into either RSS. + RS- (preferred pathway in case of PenSSSPen reduction; k{PenSSSPen + e(aq)-} = 2.0 x 10(10) M-1 s-1, k{PenSSSPen + CO2.-} = 2.3 x 10(8) M-1 s-1), or RS. + RSS- (preferred pathway in case of CySSSCy reduction). Trisulfide radical anions could directly be identified in the case of the cysteine compound with (CySSSCy).- absorbing at 425 nm and exhibiting a half-life of ca 4-mu-s. Radical cations (PenSSSPen).+ are indicated in the oxidation of penicillamine trisulfide.