Polymer Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 194

and (24)]. Thus, under certain conditions, small changes in pH can have a dra-
matic effect on product yields in such systems.
It is noted that the 1,2-H shift [reaction (21)] always competes with the

-
fragmentation reaction of oxyl-radicals [reaction (25)]. In the main chain of
polymers,

β

-fragmentation causes a chain break. The ratio of the importance
of reaction (21) vs. reaction (25) is determined by the stabilization of the radical
that is released in the

β

-fragmentation process (25) (Gröllmann and Schnabel
1980; Schuchmann and von Sonntag 1982).
In DNA, the peroxyl radicals at C (3

β

) could, in principle, give rise to
oxyl radicals that may lead to strand breakage through

′ ) − C (5 ′

β -fragmentation. In con-

trast to the C (4 )-mechanism in the absence of O 2 , details are, however, not yet

established. For developing mechanistic concepts, we still fully rely on model
systems such as those described above.

′

9.5
Oxidation and Reduction of Polymer Radicals

The reduction and oxidation of radicals are discussed in Chapter. 6.3−6.5. That
in the case of radicals derived from charged polymers the special effect of re-
pulsion can play a dramatic role was mentioned above, when the reduction of
poly(U)-derived base radicals by thiols was discussed. Beyond the common oxi-
dation and reduction of radicals by transition metal ions, an unexpected effect
of very low concentrations of iron ions was observed in the case of poly(acrylic
acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly
reproducible, but when the glass ware had been washed with EDTA to eliminate
traces of transition metal ions, notably iron, from its surface, results became
reproducible. In fact, the addition of 1

10 − 6 mol dm − 3 Fe 2+ reduces in a pulse
radiolysis experiment the amplitude of conductivity increase (a measure of the
yield of chain scission; Chap. 13.3) more than tenfold and also causes a signifi-
cant increase in the rate of the chain-breaking process. In further experiments,
this dramatic effect of low iron concentrations was confirmed by measuring the
chain scission yields by a different method. At present, the underlying reactions
are not yet understood. These data are, however, of some potential relevance to
DNA free-radical chemistry, since the presence of adventitious transition metal
ions is difficult to avoid.

×

9.6
Products of the Termination of Polymer Radicals

Polymer radicals may recombine upon bimolecular termination. When this
cross-linking takes place between two polymer molecules the molecular yield
increases. When there are more than one radical on the polymer chain, cross-
linking between these two sites will form only a loop (Fig. 9.1), that is, no increase
in the molecular weight ensues despite the fact that cross-linking has occurred.
In competition to recombination, radicals may disproportionate, when at least