Formation of Reactive Free Radicals in an Aqueous Environment - Free-Radical-Induced DNA Damage and Its Repair - page 20

Table 2 . 5. Kinetic data for the decomposition of some azo radical sources at 37 °C in D 2 O,
pD ~ 7. (Paul et al. 2000)

k/s − 1

Azo compound t ½ /h

1.3 × 10 − 6

2,2'-Azobis(amidinopropane) 2 HCl 148

5.5 × 10 − 6

2,2'-Azobis( N , N '-dimethylisobutyramidine) 37

1.8 × 10 − 7

Azobis[2-methyl- N -(2-hydroxyethyl)propionamide] 1070

3.5 × 10 − 7

3,3'-Azobis(3-cyano-1-butanesulfonate 2 Na +

550

1.4 × 10 − 4

Di(4-carboxybenzyl)hyponitrite (SOST-1) 1. 3

This source of peroxyl radicals has been used to study the peroxyl radical reactions
with nucleobases (Simandan et al. 1998) and thymidine (Martini and Termini 1997;
Chap. 10). The slow H-abstraction reactions of peroxyl radicals prevents their re-
action with DNA in dilute aqueous solution unless they are positively charged and
thus bound to DNA by electrostatic forces (Paul et al. 2000). Otherwise, their com-
peting bimolecular termination reactions are much faster (Chap. 8).

2.4.4
Photolysis of Mercaptopyridine-

N -oxides

The absorption coefficient of H 2 O 2 is rather poor (see above), and in the presence
of strongly UV-absorbing compounds such as DNA or nucleobases, its photoly-
sis cannot be used to generate • OH. Hence, the photolysis of 2- or 4-mercapto-
pyridine- N -oxide (Adam et al. 1998c) or N -hydroxy-4-(4-chlorophenyl)thiazole-
2(3 H )-thione (Adam et al. 2000b) at 350 nm as an • OH source [e.g., reaction (60)]
has found its entrance in the study of DNA free-radical chemistry (Vieira and
Telo 1997; Dias and Vieira 1997). The related N -alkoxpyrimidinethiones gen-
erate alkoxyl radicals whose reactions with DNA have also been investigated
(Adam et al. 1998c).
Alternatively, N -hyroxypyridine-2-thione, a versatile antibacterial and anti-
fungal agent commercially available as Omadine , has been used. As expected, in
the presence of DNA its photolysis causes SSBs (via • OH), but after its photolytic
consumption continuing photolysis at 350 nm caused the photooxidation of G in
DNA and in dGuo (Adam et al. 1999). Apparently this effect is induced by one of
its photolytic products that are largely the N , N

-dioxide, the mono- N -oxide, the
disulfide and the sulfonic acid (Adam et al. 1999).

′