Reduction Potential of the Sulfhydryl Radical: Pulse Radiolysis and Laser Flash Photolysis Studies of the Formation and Reactions of ·SH and HSSH·- in Aqueous Solutions

Formation and reactions of the ·SH/·S - and HSSH· - /HSS· 2- radicals in aqueous solutions have been studied by excimer laser flash photolysis and by pulse radiolysis. Acidic H 2 S solutions can be photolyzed with 193 nm laser pulses and produce a transient species with λ max at 240 nm, ascribed to the ·SH/·S - radical. Solutions of SH - can be photolyzed also with 248 nm laser pulses to produce the ·SH/·S - radical. The same radical is formed by oxidation of SH - ions with SO 4 · - and CO 3 · - radicals. At pH > 5, ·SH/·S - reacts with SH - ( k f = 4 × 10 9 L mol -1 s -1, k r = 5 × 10 5 s -1 ) to form HSSH· - /HSS· 2-, with λ max at 380 nm. Both ·SH/·S - and HSSH· - /HSS· 2- react rapidly with O 2; the former produces SO 2 · - ( k = 5 × 10 9 L mol -1 s -1 ), and the latter produces O 2 · - ( k = 4 × 10 8 L mol -1 s -1 ). Both radicals react with olefinic compounds. The monomeric radical oxidizes Fe(CN) 6 4-, SO 3 2-, ClO 2 -, and chlorpromazine. The dimeric radical is a weaker oxidant toward ferrocyanide but reduces N -methylpyridinium compounds. The reduction potential for the dimeric radical at pH 7 was determined from one-electron transfer equilibria with Mo(CN) 8 3- and with the 4-methoxyaniline radical cation and found to be 0.69 V vs NHE. From the equilibrium constant K = [HSS· 2- ]/[SH - ][·S - ] = 8 × 10 3 L mol -1, the reduction potential for (·S -,H + /SH - ) is calculated to be 0.92 V.