Polymer Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 191

9.4.2
Intermolecular H-Transfer

In charged polymers, a strong inf luence of charge on the rate of H-abstraction is
also observed with external thiols as hydrogen donors. A case in point is poly(U)
(Fahey et al. 1991). As mentioned above, base-centered radicals can abstract an H
atom from the sugar moiety, and the resulting radical undergoes chain scission
(see below). This reaction can be prevented by thiols which donate an H-atom to
the base radical. Since poly(U) is a charged polymer, the rate of H-transfer to the
polymer strongly depends on the charge of the thiol (Chap. 11.2).

9.4.3
Homolytic β -Fragmentation and Hydrolysis

Homolytic -fragmentation is a common process in free-radical chemistry. It

is the reverse of the better known polymerization. Although

β

-fragmentation is
often rather slow and has to compete with the bimolecular termination reaction
of free radicals, it can become of considerable importance in long-lived radicals
such as the poly(acrylic acid) radicals (charged at high pH), where this reac-
tion leads to chain scission [reaction (11); von Sonntag et al. 1995; Ulanski et al.
1996c].

β

In poly(methacrylic acid), where this type of reaction is 50 times faster (von

Sonntag et al. 1995; Ulanski et al. 1999a), even at room temperature subsequent
depolymerization occurs as a very efficient process [reactions (12) and (13);
Ulanski et al. 1999a,b].

∼

Because of a ready addition of the methacrylic acid released in reaction (13) to
all radicals in this system, the situation of equilibrium polymerization is ap-
proached even at room temperature (Ulanski et al. 2000).
In carbohydrates, -fragmentation of radicals adjacent to the glycosidic link-

age [e.g., reactions (14) and (15)] will also lead to chain scission.

β