Heteroatom-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 140

RSSR + • CH 2 O − [RSSR] • − + CH 2 O → (41)

Disulfide radical anions are rather strong reductants (E = -1.7 V; Wardman
1989), and it has hence been proposed that in cellular systems these intermedi-
ates may contribute to the repair of DNA radicals (Prütz 1989).
Similar three-electron-bonded intermediates must also be formed in the
well-known thiyl radical-induced scrambling of disulfides (Owen and Ellis
1973). Mechanistic details have been studied using the pulse radiolysis tech-
nique which allowed the identification the three-bonded intermediate and the
determination of some equilibrium constants, but also gave evidence that this
system may be more complex than described by reactions (42) and (43) (Boni-
facic and Asmus 1984).

The rate constant of the similar reaction (44) varies strongly with the substituent
R and decreases in the case of R = phenyl by two orders of magnitude on going

from R = methyl to R = tert -butyl, R = 2-propyl lying in between (Pryor and
Smith 1970).

′

RSSR + • R' → RSSR' + R • (44)

Addition to the disulfide function is also observed with • OH (Bonifacic et al.
1975b). But here, in addition to the substitution reaction (45), generation of a
disulfide radical cation and a hydroxide ion is also observed [reaction (46)]. The
high solvation energy of the latter certainly provides additional driving force.
There is EPR evidence that RSO • is also formed [reaction (47); Gilbert et al. 1975].
In dithiodipropionic acid and in cystine, the thiol is indeed a major product (30-
40%; Elliot et al. 1981).

Cationic species are also formed when sulfides (Meissner et al. 1967; Adams
1970; Bonifacic et al. 1975a; Janata et al. 1980; Hiller et al. 1981; Davies et al. 1984;
Ramakrishna Rao et al. 1984) or thioureas (Wang et al. 1999; Schuchmann et al.
2000) react with • OH. Especially stable are the dimeric radical cations [reactions
(48)-(50)]. In the case of thiourea, the high stability of the dimeric radical cation
may contribute to the driving force which leads, in acid solution, to its forma-