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

Published

July 1, 1999

Author(s)

T N. Das, Robert E. Huie, Pedatsur Neta, S Padmaja

Abstract

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 with 193-nm laser pulses 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 / 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 10⁹ L mol s , k _r = 5 x 10⁵ s ) to form HSSH.^-/HSS2.^-, with λ_max at 380 nm. Both SH/ S and HSSH /HSS react rapidly with O₂; the former produces SO₂ (k = 5 x 10⁹ L mol s ) and the latter produces O₂ (k = 4 x 10⁸ L mol s ). Both radicals react with olefinic compounds. The monomeric radical oxidizes Fe(CN)₆^4-, SO₃ , 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 and with the 4-methoxyaniline radical cation and found to be (0.69 0.03) V vs NHE. From the equilibrium constant K = [HSS ]/[SH ][ S ] = 8 x 10 L mol , the reduction potential for ( S ,H /SH ) is calculated to be 0.92 V.

Citation

Journal of Physical Chemistry A

Volume

103

Pub Type

Journals

Keywords

disulfide radical, free radicals, kinetics, pulse radiolysis, sulfide ion, sulfide radical

Citation

Das, T. , Huie, R. , Neta, P. and Padmaja, S. (1999), 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, Journal of Physical Chemistry A (Accessed July 18, 2024)