The Hydroxyl Radical - Free-Radical-Induced DNA Damage and Its Repair - page 50

Table 3. 2 . Rate constants (unit: of dm 3 mol − 1 s − 1 ) of • OH with some alcohols (Buxton et
al. 1988) and the position of H-abstraction (in percent). (Asmus et al. 1973)

Alcohol Rate constant -CH 3 (%) -CH 2 - (%) -CH- (%) -OH (%)

MeOH

9.7 × 10 8

93 - - 7.0

1.9 × 10 9 EtOH 13.3 84.3 - 2.5

1.9 × 10 9

2-PrOH 13.3 - 85.5 1.2

t BuOH

6 × 10 8

95.7 - - 4.3

and von Sonntag 1977). In alcohols and carbohydrates, the oxygen-bond hydro-
gens are quite tightly bond (BDE = 435 kJ mol −1 ), and hence alkoxyl radicals are
formed only in low yields. In water, their detection is complicated due to their
ready conversion into -hydroxyalkyl radicals by a water-assisted 1,2-shift [e.g.,

reaction (28): Berdnikov et al. 1972; Gilbert et al. 1976; Schuchmann and von
Sonntag 1981], and hence their yields given in Table 3.2 might have been slightly
underestimated.
The activation energy for H-abstraction from MeOH as measured over a very
wide range, from 22-390 °C, has been found to be 13.3 kJ mol −1 (Feng et al. 2003).
Because of the high reactivity of • OH in its H-abstraction reactions the H/D-iso-
tope effects are rather small, e.g., k (CH 3 OH)/ k (CD 3 OH) = 2.5, k (CH 3 CH 2 OH)/
k (CD 3 CD 2 OH) = 1.6 and k ((CH 3 ) 2 CHOH)/ k ((CH 3 ) 2 CDOH) = 1.5 (Anbar et al.
1966b). More recently, a value of 1.96 has been reported for the EtOH system
(Bonifacic et al. 2003). These values are of interest in comparison with H/D-
isotope effects observed for the reaction of • OH with the sugar moiety of DNA
(Balasubramanian et al. 1998; Chap. 12).
The rates of • OH addition to C-C double bonds and of H-abstraction are both
close to diffusion-controlled. When both reactions can be given by a substrate
molecule, addition will be the generally preferred route. This even holds for mol-
ecules that have very weakly bound hydrogens. The 1,4- and 1,3-cyclohexadi-
enes provide a good example (Pan et al. 1988). Both contain two C-C double
bonds and four weakly-bound pentadienylic hydrogens. In 1,4-cyclohexadiene
the two double bonds are separated, while in 1,3-cyclohexadiene the double
bonds are conjugated. In the case of 1,4-cyclohexadiene, H-abstraction occurs to
an extent of 50%, in the other isomer it is only 25%. Here, • OH addition mainly
(50%) yields the allylic radical. Even more pronounced is the situation in the
case of toluene. Although there are three weakly bound benzylic hydrogens, H-
abstraction occurs with a yield of only 4% (Christensen et al. 1973). Thus, also
with thymine, • OH addition to the C (5)- C (6) double bond is much preferred
over an abstraction of a (weakly bound) allylic hydrogen at the C (5)-CH 3 group
(Chap. 10). This is in marked contrast to, for example, the behavior of peroxyl
radicals (Chap. 8).

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