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up to three nucleotides away from the position of the Thy-5-peroxyl radical
(Greenberg et al. 1997). With a G present at the 5
-nucleotide (followed by C- and
T-containing nucleotides), strand breaks and alkali-labile sites occur mainly at
G sites, five times greater than at the following C sites.
Introducing in the 5
′
′
-position a T-containing nucleotide labeled with deute-
rium at either
C
(1
′
),
C
(2
′
) or
C
(4
′
) showed only a marked kinetic isotope effect
(KIE = 3.9) when labeled at
C
(1
). It has been concluded that the observed effects
must be due to an H-abstraction of the Thy-5-peroxyl radical from the neighbor-
ing
C
(1
′
) position. A kinetic isotope effect is only observed when there is a com-
peting reaction. This competing reaction has not yet been identified. However,
some information has been obtained by generating specifically the
C
(1
′
′
) radical
within a short ODN [reactions (11) and (12); Hwang et al. 1999].
The 2-dRL lesion and the 3
-fragment resulting from reaction (13) were identi-
fied by mass spectrometry. The bona fide intermediate (in brackets) expected to
be formed upon treatment with a base such as piperidine was not observed, but
the
′
-elimination product [reaction (14)] and an adduct of the amine [reaction
(15)] were identified.
β
,
δ
For studying mechanistic aspects of DNA strand breakage starting from the
C
(4
′
) radical, ODNs substituted by a phenylselenide group at
C
(4
′
) were photo-
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