Hydropersulfides: H-Atom Transfer Agents Par Excellence

Hydropersulfides (RSSH) are formed endogenously via the reaction of the gaseous biotransmitter hydrogen sulfide (H 2 S) and disulfides (RSSR) and/or sulfenic acids (RSOH). RSSH have been investigated for their ability to store H 2 S in vivo and as a line of defense against oxidative stress, from which it is clear that RSSH are much more reactive to two-electron oxidants than thiols. Herein we describe the results of our investigations into the H-atom transfer chemistry of RSSH, contrasting it with the well-known H-atom transfer chemistry of thiols. In fact, RSSH are excellent H-atom donors to alkyl ( k ∼ 5 × 10 8 M –1 s –1 ), alkoxyl ( k ∼ 1 × 10 9 M –1 s –1 ), peroxyl ( k ∼ 2 × 10 6 M –1 s –1 ), and thiyl ( k > 1 × 10 10 M –1 s –1 ) radicals, besting thiols by as little as 1 order and as much as 4 orders of magnitude. The inherently high reactivity of RSSH to H-atom transfer is based largely on thermodynamic factors; the weak RSS–H bond dissociation enthalpy (∼70 kcal/mol) and the associated high stability of the perthiyl radical make the foregoing reactions exothermic by 15–34 kcal/mol. Of particular relevance in the context of oxidative stress is the reactivity of RSSH to peroxyl radicals, where favorable thermodynamics are bolstered by a secondary orbital interaction in the transition state of the formal H-atom transfer that drives the inherent reactivity of RSSH to match that of α-tocopherol (α-TOH), nature’s premier radical-trapping antioxidant. Significantly, the reactivity of RSSH eclipses that of α-TOH in H-bond-accepting media because of their low H-bond acidity (α 2 H ∼ 0.1). This affords RSSH a unique versatility compared to other highly reactive radical-trapping antioxidants (e.g., phenols, diarylamines, hydroxylamines, sulfenic acids), which tend to have high H-bond acidities. Moreover, the perthiyl radicals that result are highly persistent under autoxidation conditions and undergo very rapid dimerization ( k = 5 × 10 9 M –1 s –1 ) in lieu of reacting with O 2 or autoxidizable substrates.

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