Polynucleotides and Single-Stranded DNA Fragments - Free-Radical-Induced DNA Damage and Its Repair - page 328

Although strand breakage must have sugar radicals as the precursor, strand
breakage is a much more important process than can be accounted for by the
primary • OH-attack at the sugar moiety ( G (strand breakage) = 2.4

10 −7 mol ×

J −1 ; Table 11.2).
It is hence obvious that a radical transfer must occur from the base to the
sugar moiety [reactions (4)−(7)]. In agreement with this, strand breakage and
the decay of the absorption of the base radicals follow the same kinetics (Jones
and O'Neill 1991). This radical transfer is also evident from the high yields of
unaltered Ura ( G (Ura) 10 −7 mol J −1 ; Deeble and von Sonntag 1984; Deeble

et al. 1986; Hildenbrand et al. 1993). There must be more than one precursor.
This is evident from the kinetics of base release: only 20% are released during
(or immediately after) irradiation, while 80% are liberated at a much later stage,
50% in a fast and 30% in a slow process. The fast and the slow processes are only
observable at elevated temperatures (Table 11.3).
In basic solution, the observed rate of strand breakage after • OH-attack is low
(0.4 s -1 ) and independent of pH, but increases by about three orders of magni-
tude in acid solution (Bothe and Schulte-Frohlinde 1982) (in neutral solution, a
similar value, 3.7 s -1 , has been obtained by time-resolved light-scattering; Jones
and O'Neill 1991). Thus, there is a spontaneous and a H + -catalyzed reaction.
The major part (70%) of the • OH-induced strand breakage is prevented by
thiols such as DTT (Lemaire et al. 1987). This has been interpreted as being due
to a reduction of the C (5)- • OH-adduct [reaction (9); k = 1.5

≈ 3.0 ×

10 6 dm 3 mol −1 s −1 ].
The (oxidizing) C (6)- • OH-adduct may not react with DTT (for the paradoxical
behavior of thiols in their reactions with reducing and oxidizing radicals see
Chaps. 6.5 and 7.4) and continue to contribute to strand breakage via a radical
transfer to the sugar moiety. The rate of reaction of thiols with the negatively
charged polyanion poly(U) most strongly depends on the charge of the thiol
(Table 11.4).
TNM nearly completely suppresses strand breakage ( G (strand breaks) = 0.2

×

10 -7 mol J -1 remaining; Lemaire et al. 1987) and Ura release (Deeble and von
Sonntag 1984). TNM oxidizes rapidly the C (5)- • OH-adduct, but may (on a much
longer time scale) also form an adduct with the C (6)- • OH-adduct. This would
prevent the radical transfer reactions from the base • OH-adducts to the sugar
moiety leaving • OH-reactions with the sugar moiety as the only source of strand
breakage. This low yield of directly induced strand breaks (Washino et al. 1983)

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