Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 260

Table 10. 22 . γ -Radiolysis if N 2 O-saturated aqueous solutions of MeOH (0.5 mol dm −3 ) in
the presence of 1,3Me 2 Thy (5 × 10 −4 mol dm −3 ). Products and their G values (unit: 10 −7 mol
J −1 ) at a dose rate of 0.19 Gy s −1 . (Schuchmann et al. 1986)

Product G value

5,6-Dihydro-1,3-dimethyl-6-hydroxymethylthymine 1.6

5,6-Dihydro-5,6-di(hydroxymethyl)-1,3-dimethylthymine 0.3

1,3-Dimethyl-6-hydroxymethylthymine 0.08

1,3-Dimethyl-5-(2-hydroxyethyl)uracil 0.12

5,6-Dihydro-1,3-dimethyl-5-(2-hydroxymethyl)-6-hydroxymethyluracil 0.25

5-(2-Hydroxyethyl)-6-hydroxymethyluracil 0.03

5,6-Dihydro-5,6-di(hydroxymethyl)-1,3-dimethyl-5-(2-hydroxyethyl)uracil 0.09

5,6-Dihydro-5-(2-(1,3-dihydroxy)propyl)-1,3-dimethyl-6-hydroxymethyluracil 0.04

5,6-Dihydro-5-(5 ′ ,6 ′ -dihydro-1 ′ ,3 ′ ,5 ′ -trimethyluracil-6 ′ -yl)-6-hydroxymethyl- 0.01

1,3.5-trimethyluracil

0.12 a 5,5 ′ -Bi-(5,6-dihydro-6-hydroxymethyl-1,3-dimethyl)thyminyl

Formaldehyde 0.6

Ethylene glycol n.d.

1,3-Dimethylthymine consumption 2.5

n.d., Not determined
a G value in monomer units

The allylic Thd radical has been generated photolytically [reactions (204-206);
Anderson et al. 2000].
With R = benzyl and in the absence of O 2 , the major product (73%) is the de-

carbonylation product [reaction (209); possible formed to a large extent within
the solvent cage], and the dimer of the allylic radical [reaction (207)] is formed
only in small amounts. Addition of a thiol increases the yield of Thd [reaction
(208)]. If an evaluation of the data reported for the reduction of the allylic • OH-
adduct to 1,3-cylohexadiene by a thiol (Pan et al. 1988), estimated at

′

10 4 dm 3
mol −1 s −1 , is a good guide the rate constant for reaction (208) should be similar.
This would revise an assumed rate constant of 10 6 dm 3 mol −1 s −1 and the con-
clusions as to the repairability of allylic Thy in DNA radicals by cellular thiols
(Anderson et al. 2000).
The allylic radical has also by abstraction of the allylic hydrogen by the
H 2 NC(O)CH 2 • radical in Thd [reaction (210] and its isomer has been produced
from 6-chloromethyluracil by dissociative electron capture and also in low-tem-

∼