Chemistry Reference
In-Depth Information
The formation of the latter two can be readily explained by a dimerization of the
thiyl radicals [reaction (61)] formed in reaction (60) and a (slow) oxidation by O
2
which occurs in competition (no detailed mechanistic study is available at pres-
ent that accounts for the other major products). The disulfide is not photostable,
but slowly isomerizes to its head-to-tail isomer [reactions (62) and (63)], and
in subsequent reactions is converted into the betaine [reactions (64) and (65)]
which is the photocatalytic agent that causes the oxidation of the guanine moiety
(Adam et al. 1999).
Thus, considerable care has to be taken not to misinterpret photobiological stud-
ies using this
•
OH-source. A further
caveat
has been expressed by Douki et al.
(1999) who observed that product ratios are considerably different when ion-
izing radiation and
N
-hydroxypyridine-2-thione were used as
•
OH sources, and
this has been explained by the H-donating property of the
N
-hydroxypyridine-
2-thione (note the major dimers mentioned above).
2.4.5
Photolytic Generation of Specific Radicals
The generation of specific radicals/lesions within DNA is often achieved upon
photolyzing adequately substituted derivatives such as
tert
-butyl ketone [reac-
tion (66)] or phenylselenide derivatives [reaction (67)]. These and other reac-
tions are discussed in Chapter 10.
R
−
CO
−
C(CH
3
)
3
+ h
ν
→
R
•
+ CO +
•
C(CH
3
)
3
(66)
R
−
Se
−
Ph + h
ν
→
R
•
+
•
Se
−
Ph
(67)
Photolysis is also widely applied to generate the Ura-5-yl radical from 5-halo-
uracils. In aqueous solution, the reaction is a homolytic splitting of the C
X
(X = Br, I) bond (Chap. 10); in DNA, the reaction seems to be much more com-
plex (Chap. 12).
−
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