Heteroatom-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 138

7. 4 . 3
Addition of Thiyl Radicals to C − C Double Bonds

Thiyl radicals add to C C double bonds, but this reaction is strongly reversible

(Ito and Matsuda 1979, 1981, 1982; Ito 1992, 1995). As a consequence, cis - trans
isomerization may occur [reactions (37) and (38)].

−

Typical examples are unsaturated fatty acids (Ferreri et al. 1999; Sprinz et al.
2000, 2001; Adhikari et al. 2001). The equilibrium constants for the oleic and
linoleic systems are in the order of 10 dm 3 mol − 1 and the reverse reaction in the
order of 10 6 s − 1 (Sprinz et al. 2000 and Sprinz, pers. comm.). In polyunsaturated
fatty acids, such isomerizations could, in principle, also occur by an H-abstrac-
tion/H-donation mechanism as discussed above. However, the rate of H-dona-
tion of RSH to the pentadienylic radicals must be very low (see above), and isom-
erization has been considered to occur only by the addition/elimination path-
way (Sprinz et al. 2000). With the nucleobases, any thiyl addition can only be
detected when the short-lived adduct is trapped by a fast reaction (Chap. 10.10).

7. 4 . 4
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 pseudo-
halide systems, see Chap. 5.2). These disulfide radical anions are characterized
by strong absorptions in the UV-Vis (Adams et al. 1967). Complexation can oc-
cur both intermolecularly as well as intramolecularly. For GSH, for example, the
stability constant of the disulfide radical anion is 2900 dm 3 mol − 1 (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 protonated intramolecular complexes absorb at slightly shorter wave-
lengths, while the thiyl radicals do not absorb in the wavelength region of inter-
est. The five-membered complex is the most stable one (von Sonntag 1990), and
its stability decreases both to smaller and larger ring sizes (Table 7.4). It can be
seen from this table that in the case of the 4-membered ring system the absorp-
tion maximum is shifted significantly to longer wavelength than in the other