Heteroatom-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 136

(31) and (32); Akhlaq et al. 1987a; for similar observations with tetrahydrofufu-
ryl acetate, see Cai and Roberts 1998].

From pulse radiolysis experiments the rate of H-donation by the thiol has been
determined at 7 10 8 dm 3 mol − 1 s − 1 , while from the cis - trans isomerization

data the rate of the H-abstraction by the thiyl radical has been calculated at ca.
10 4 dm 3 mol − 1 s − 1 (Akhlaq et al. 1987a). Similar data have been reported for the
reaction of thiyl radicals with alcohols [ k ( • C(CH 3 ) 2 OH + PenSH) = 1.2

×

10 8 dm 3

×

mol − 1 s − 1 k ; (PenS • + 2-PrOH) = 1.4 10 4 dm 3 mol − 1 s − 1 ; Schöneich et al. 1989, 1990]

and with carbohydrates (2-deoxyribose: 2.7

×

10 4 dm 3 mol − 1 s − 1 ; Pogocki and
Schöneich 2001). Thus, in these systems, the rate of H-donation by the thiol is
four to five orders of magnitude faster than the rate of H-abstraction by the thiol,
that is, these data are not in contradiction with the general conclusion that thiols
do reduce C -centered free-radicals and that the reverse reaction is usually not
observed. On the other hand, when the C

×

H BDE is lower such as in the case of
the pentadienylic position in polyunsaturated fatty acids, the rate of H-abstrac-
tion by thiyl becomes very fast [ k (RS • + RH)

−

10 7 dm 3 mol − 1 s − 1 ; Schöneich and
Asmus 1990; Schöneich et al. 1992], and it is not very likely that the reverse ('re-
pair') reaction is of any importance here.
An H-transfer may also occurs intramolecularly such as in DDT via a five-
membered transition state [reaction (33);Akhlaq and von Sonntag 1986]. In the
given case, an H 2 S forming chain reaction is induced [cf. reaction (34) followed
by the (slow) H-abstraction of the thus-formed radical from DTT] which comes
to a halt when the thiyl radical is complexed as the disulfide radical anion at
higher pH values [cf. reaction (40)].

≈

With GSH carbon-centered besides thiyl radical are formed upon • OH attack,
notably at pH > 7 (Sjöberg et al. 1982; Eriksen and Fransson 1988). It has been
shown subsequently that this is due to an intramolecular H-transfer [reaction
(35); Grierson et al. 1992]. When the a-NH 2 group is no longer protonated as in
neutral solution the C H is only weakly bound and equilibrium (35) is shifted to

the side of the C -centered radical.

−