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 H2S solutions can be photolyzed with 193 nm laser pulses and produce a transient species with lambda(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 SO4.(-) and CO3.(-) radicals. At pH > 5, . SH/. S- reacts with SH- (k(f) = 4 x 10(9) L mol(-1) s(-1), k(r) = 5 x 10(5) s(-1)) to form HSSH .(-)/HSS .(2-), with lambda(max) at 380 nm. Both . SH/. S- and HSSH .(-)/HSS .(2-) react rapidly with O-2; the former produces SO2.(-) (k = 5 x 10(9) L mol(-1) s(-1)), and the latter produces O-2.(-) (k = 4 x 10(8) L mol(-1) s(-1)). Both radicals react with olefinic compounds. The monomeric radical oxidizes Fe(CN)(6)(4-), SO32-, ClO2-, 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 x 10(3) L mol(-1), the reduction potential for (. S-,H+/SH-) is calculated to be 0.92 V.