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k, = 7.05 (f1.4) X k2 = 6.8 (f1.4) X cm3/molecule s cm3/molecule s The value of k l can be compared with previous absolute measurements at 298 K and pressures below 10 Torr: 6.1 (*O.l);' 6.0 (f0.7);2 5.93 (10.45);3 5.5 (f0.6)4 X lo-" cm3/molecule s. Our rate constant is 20% larger than the average of these four measurements, well within the stated error limits, and is only 15% larger than the measurement1 with the smallest stated error limits that was carried out at the lowest pressure. Therefore, our value of kl agrees with all previous measurements to within experimental uncertainty, verifying that there is no pressure dependence to this reaction from to 700 Torr. Only one other absolute mea-surement has been made of kz. At 298 K and 100 Torr of argon, Wine and Semmes'O obtain k2 = 8.04 (f0.57) X cm3/ molecule s, which again agrees with the value from our experiments to well within the error limits. Acknowledgment.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):l 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 f 1.0) X lo6 M-' s-l and k(PenSS' + 4 ) = (5.1 * 1.0) X lo6 M-I PI. Probably the most interesting finding about perthiyl radicals is that inorganic Sod2is 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, RSSO;, addition of oxygen to the latter to yield a sulfonyl peroxyl, RSS0200', followed by a bimolecular radical-radical reaction of these peroxyl radicals (in analogy to the fate of peroxyl radicals in general) leading to RSS03'. Cleavage of SO3 from RSSOj and hydrolysis would then result in SO,". Reduction of trisulfides yields (RSSSR)'-radical anions which decay into either RSS' + RS-(preferred pathway in case of PenSSSPen reduction; k(PenSSSPen + ea;) = 2.0 X 1O' O M-' s-l, k(PenSSSPen + CO;-) = 2.3 X 10s M-I s-l), 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 ps. Radical cations (PenSSSPen)'+...

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