Polymer Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 188

either by H-abstraction from a position where the H-atom is more loosely bound,
or add to double bonds. The polymer radical experiences a very high local con-
centration of these functional groups as long as the polymer chains are f flex-
ible, and the rate of this ( intra molecular) reaction will be much higher than an
equivalent inter molecular reaction of low molecular weight models at the same
concentration. In a charged polymer, the rate of this process decreases with in-
creasing charge of the polymer due to a lower f flexibility of segments. On the
other hand, since this process is kinetically of first order while radical recombi-
nation processes are of second order, such a process may only become prominent
when the polymer is highly charged, that is, when the bimolecular termination
becomes slow. Two competing processes are discussed here, H-transfer and

-
fragmentation. In DNA, there is also the possibility that radicals may add intra-
molecularly to a double bond at one of the bases. Evidence for such a reaction is
not yet on firm ground, but this possibility should be kept in mind.

β

9.4.1
Intramolecular H-Transfer

In poly(vinyl alcohol), • OH abstracts preferentially a hydrogen in -position to

the hydroxyl group (forming the tertiary radical, 70%) but also from the methy-
lene group (forming the secondary radical, 30%) [reactions (1) and (2)]. The C

α

H
BDE of the secondary hydrogen is somewhat higher than that of the tertiary hy-
drogen in the

−

group, and thus the secondary radicals undergo the H-ab-
straction reaction (3) ( k 3 = 460 s − 1 ; von Sonntag et al. 1999; see also Chap. 6.5).
Whether the reaction occurs with the first available hydrogen [five-membered

− CHOH −

transition state as depicted in reaction (3)] or over a larger distance, is as yet un-
known. The rate of this H-transfer is only that fast, because it occurs as an intra-
molecular process. In order to compare this first-order rate with known second-
order processes, one may take the concentration of neighboring

groups
in the polymer as 10 mol dm − 3 , that is, the second-order rate constant should not
exceed 50 dm 3 mol − 1 s − 1 by much. For comparison, the somewhat more exother-
mic H-transfer from 2-PrOH to the • CH 2 -CHOH-CH 3 radical [H-abstraction by
a primary alkyl radical as compared to a secondary one as in reaction (3)] has a
rate constant of 430 dm 3 mol − 1 s − 1 (Burchill and Wollner 1972).
In poly(acrylic acid), two radicals are also formed upon • OH-attack. Again,
the secondary radical undergoes intra molecular H-abstraction, leading to the
tertiary radical [reaction (4); Ulanski et al. 1996c].

− CHOH −