Peroxyl Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 155

that this reaction occurs via a five-membered transition state (Bothe et al. 1977),
and largely due to steric reasons only the 1,2- but not the 1,4-hydroxycyclohexa-
dienylperoxyl radical eliminates HO 2 • (Pan et al. 1993).

The rate of HO 2 • elimination from -hydroxyalkylperoxyl radicals strongly de-

pends on the flanking substituents that also govern the strength of the resulting
C

α

O double bond (for a compilation see von Sonntag and Schuchmann 1997).
The O 2 • − -elimination reactions may be divided into three groups. Those per-
oxyl radicals that have an -OH or -NH function in the

−

-position make up the
first group. Such peroxyl radicals play a major role in nucleobase peroxyl radical
chemistry [cf. reactions (12) and (13)]. Upon deprotonation at the heteroatom
by OH − [reactions (10) and (12)], the peroxyl radical anion is formed (cf. the en-
hancement of the acidity of the functions

α

to the peroxyl group discussed above;
for the thermodynamics of the various equilibria that are involved in these reac-
tions see Goldstein et al. 2002). As before, the driving force for the elimination
reaction is the formation of a double bond [in addition to the energy gain by the
formation of the stabilized O 2 • − radical [cf. reactions (11) and (13)].

α

The peroxyl radical anion formed in reaction (10) has an immeasurably short
(<< 10 − 6 s − 1 ) lifetime, i.e., k 11 is much larger than k − 10

[H 2 O], and even at high
[OH − ] the rate of acetone formation is essentially given by k 10 ×

×

[OH − ] (Bothe et

al. 1977). The situation is similar for other -hydroxyalkylperoxyl radical an-

ions (Rabani et al. 1974; Ilan et al. 1976; Bothe et al. 1983) with the exception

α