Chemistry Reference
In-Depth Information
10
4
s
−1
, Chatgilialoglu
et al. 2004), possibly due to conformational differences. Thus, the
C
(5
10
4
, von Sonntag 1987b;
k
8BrdAdo system (
k
≈
2.3
×
≈
1
×
) radi-
cal has a much longer lifetime, and as a consequence high yields of the 5
′
-alde-
hyde are formed in the presence of an oxidant such as TNM or O
2
(von Sonntag
1987b). H-abstraction of the
C
(2
′
H leads to the release of Ade [reactions (244)
and (245); Chatgilialoglu et al. 2004]. The oxidizing radical formed upon Ade
release is monitored by TMPD [reaction (246)], and from a computer analysis of
the data a rate constant of
k
245
= 1.1
′
)
−
10
9
dm
3
mol
−1
s
−1
has been arrived at (in the original paper, it has been suggested that TMPD is
oxidized by the precursor radical cation, but this will deprotonate too rapidly to
undergo this reaction).
10
5
s
−1
and
k
246
= 4.6
×
×
The Ado
C
(2
) radical has been generated separately in a photolytic experiment
using an adequately substituted Ado derivative, and from a competition of Ade
release and reduction by GSH
k
GSH
/
k
245
= 4.3 dm
3
mol
−1
has been arrived at.
Based on the assumption that
k
GSH
should be around 10
6
dm
3
mol
−1
s
−1
a reason-
able (within a factor of 2) agreement with the above
k
245
has been obtained. Con-
sidering, however, that
′
-hydroxyalkyl radicals react in aqueous solution at least
50 times faster with thiols (Chap. 7.4), the rate of Ade release may even be much
faster then estimated from the above-mentioned computer analysis of the data.
A detailed study on the reaction of 8BrGuo with reducing radicals, notably
e
aq
−
but also with other reducing radicals, has shown that in these reactions it
behaves differently from 5BrUra and 8BrdAdo (Ioele et al. 2000). In this case,
a comparatively long-lived (
k
= 5
α
10
4
s
−1
) intermediate absorbing at around
600 nm is formed, and in H
2
O vs D
2
O, there is a strong kinetic isotope effect
of
k
H
/
k
D
= 8.0 in its decay. In the presence of
t
BuOH, equal amounts of Br
−
and
Guo are formed, and when the reaction is carried out in D
2
O, deuteration occurs
at
C
(8). Based on spectral similarities, it is suggested that on the way to these
products the G
•
+
or G
•
are intermediates, but it is difficult to see wherefrom the
reduction equivalent should come. It is not expected that these intermediates
are capable of abstracting an H atom from
t
BuOH with a rate constant of about
10
4
dm
3
mol
−1
s
−1
as would be required for a complete reduction of the inter-
×
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