Kinetics of the reactions between sulfide radical cation complexes, [S therefore S](+) and [S therefore N](+), and superoxide or carbon dioxide radical anions

The reaction of superoxide radical anions with sulfide radical cation compl exes represents an important sulfoxide-forming process. Here, absolute rate constants for the reaction of sulfur-sulfur [>S...S <](+) and sulfur-nitro gen ([>S.. NH2R](+)) three-electron bonded sulfide radical cation complexes with superoxide and, for comparison, carbon dioxide radical anion have bee n measured by pulse radiolysis. For two different sulfur-sulfur bonded spec ies, the intermolecular complex from dimethyl sulfide and the intramolecula r complex from 1,5-dithia-3-hydroxycyclooctane, the rate constants for the reaction with superoxide are on the order of 1.6 x 10(10) M-1 s(-1) and wit h the carbon dioxide radical anion on the order of (6.5 +/- 0.2) x 10(9) M- 1 s(-1). The fact that the stronger reducing carbon dioxide radical anion s hows the lower rate constant can be rationalized by the higher internal reo rganization energy of (CO2-)-C-. as compared to O-2(.-). The rare constant for the reaction of superoxide with the sulfur-nitrogen bonded radical cati on of Met-Gly, k = 5.3 x 109 M-1 s(-1), is 3-fold lower as compared to that of the reaction with the sulfur-sulfur bonded radical cation complexes.