Polymer Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 195

one of the radicals carries a hydrogen in -position. A disproportionation re-

action does not lead to a change in molecular weight. The ratio of recombina-
tion to disproportionation depends considerably on the substituents next to the
radical site (Chap. 6.11). Thus, a change from one kind of radical into another
one (induced by an intramolecular radical transfer process, see above) must also
have an inf luence on the recombination to disproportionation ratio and hence
on the cross-linking yield. As a consequence, product yields ( inter molecular
cross-linking, loop formation, chain scission) strongly depend on the rate of
radical generation and on the polymer concentration (at the same rate of radical
generation but at lower polymer concentrations more radicals are formed per
macromolecule and unit time than at a higher polymer concentration). Such a
behavior is particular to polymers and usually not observed with low-molecular
weight material.
Termination reactions of peroxyl radicals of polymers may eventually lead to
chain scission (see above).

β

9.7
DNA

It has been discussed above that polymer radicals may have lifetimes different
from those of their monomer units. These lifetimes have been shown to depend
on various parameters such as charge and number of radicals per polymer chain.
With respect to the lifetime of DNA radicals, not too many studies are available.
In a very early pulse radiolysis study, it has been stated that very little UV/Vis
absorption decrease of the DNA radicals over the time interval of 300-900 µs was
detected (Scholes et al. 1969). Using Raleigh light-scattering for detection gave
a somewhat complex picture (Lindenau et al. 1976). A relatively fast decrease in
the signal with t 1/2 8 s. These

changes were interpreted as being due to an detachment of DNA segments caused
by DSBs and SSBs, respectively. If this interpretation is correct, these data would
not lead to any information as to the lifetime of DNA radicals. In an experiment
where a

≈ 0.8 ms was followed by a much slower one of t 1/2 ≈

10 − 3 mol dm − 3 calf thymus DNA solution was subjected to a short
pulse of 350 Gy (equivalent to 2

∼ 3 ×

10 − 4 mol dm − 3 DNA radicals) and subsequently
a tritiated nitroxyl radical (TAN) was added after some delay, a biphasic decay
was observed, the slower part exhibiting a half-life of about 10 s (Brustad et al.
1971). It is evident that under these conditions quite a large number of radicals
per DNA strand have been created (one per ten nucleotides, on average), and it is
not unlikely that the rapid part has to be connected with the very fast early part
of radical decay generally shown by polymers having a large number of radicals
per polymer chain. In DNA, this fast decay will certainly slowed down by the
repulsive forces of the phosphate groups.
For DNA in cells and in the absence of O 2 , one has to take into account that
the lifetime of the DNA radicals is not determined by their bimolecular decay
but rather by their reaction with the cellular thiols, mainly GSH (Chap. 12.11).
In the presence of O 2 , the situation becomes more complex, and the lifetime of
the DNA peroxyl radicals is as yet not ascertained. It is expected to be consider-

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