Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 215

Table 10.4. G (base release) (unit: 10 −7 mol J −1 ) from some pyrimidine nucleosides and
2 10 −7 mol J −1 ) at differ-

ent dose rates: pulsed electron-beam irradiation ( ∼ 6 Gy per 2 µs pulse, high dose rate) and
γ -irradiation (0.013 Gy s −1 , low dose rate; Aravindakumar et al. 2003)

-deoxynucleosides induced by the SO 4 • − radical [ G (SO 4 • − ) = 3.3 ′ ×

Nucleoside/2 ′ -deoxynucleoside Electron beam γ -Radiolysis

Cyd 2. 8 7. 5

Urd 0.72 4.5

5MeUrd n.d. <0.05

dCyd 0.14 1.64

dUrd 0.06 0.6

1.0 a Thd 0.2

n.d., Not determined
a 0.7

10 −7 mol J −1 at 1.3 Gy s −1 , 0.85 10 −7 mol J −1 at 0.1 Gy s −1 × ×

substantial part (22% of primary SO 4 • − ) of Ura release must be due to this radi-
cal transfer from the base to the sugar moiety.
Mechanistic details will be discussed below, since the Urd seems to be anal-
ogous to the Cyd system, and the latter has been investigated in more detail
(Schulte-Frohlinde and Hildenbrand 1989; Hildenbrand 1990; Catterall et al.
1992; Naumov et al. 2001; Aravindakumar et al. 2003). Interestingly, 5MeUrd
does not undergo base release (Table 10.4), indicating that in this system the
pathways dominating the reactions of Urd and Cyd are not followed. Whether
the oxidation at methyl is the competing reaction is not yet known. A detailed
mechanistic study would be required to shed light into this interesting phenom-
enon.
dCyd and Cyd differ dramatically in their reactions with SO 4 • − . dCyd gives
rise to a base-centered radical [reactions (36)−(41); Hildenbrand et al. 1989]
which is formed in the tautomerization reaction (39) (Naumov et al. 2001).