Three-Electron-Bonded Intermediates in Sulfur Radical Reactions

Three-Electron-Bonded Intermediates in Sulfur Radical Reactions 

Sulfur free-radical chemistry is largely governed by the ability of sulfur to form three-electron bonded intermediates. A case in point is the complexation of a thiyl radical with a thiolate ion (for an analogy with the halide and other pseudohalide systems, see Chap. 5.2). These disulfide radical anions are characterized by strong absorptions in the UV-Vis (Adams et al. 1967). Complexation can occur both intermolecularly as well as intramolecularly. For GSH, for example, the stability constant of the disulfide radical anion is 2900 dm3 mol1 (Mezyk 1996a). The protonated disulfide radical anion is not stable, but such intermediates are known in the cases of the intramolecular complexes [reactions (39) and (40) Akhlaq and von Sonntag 1987]. 

The equilibria of a series of sulfhydryl groups containing amino acids have been investigated (Mezyk 1995, 1996a,b), and details governing these equilibria are now reasonably well understood (Mezyk and Armstrong 1999). 

Metallothioneins are small ubiquitous oligopeptides containing a high proportion of cysteine residues but no disulfide bonds (Tsunoo et al. 1978 Suzuki and Maitani 1983). Mammalian metallothionins are made up of 61 amino acids, 20 ofwhich are cysteine (Hamer 1986 for their protective effects see Chap. 12.10). Despite this, disulfide radical anion formation is mostly bimolecular (Fang et al. 1995). 

The disulfide radical anions are also formed upon the reduction of the disulfide by C02- [reaction (41)] and the thymine radical anion (Willson 1970 Elliot et al. 1984). For example, the reducing hydroxymethyl radical can reduce the disulfide (Anderson et al. 1986), but the rate is only fast when the hydroxymethyl radical is deprotonated (Akhlaq et al. 1989) and thus its redox potential decreased. 

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