Chemistry Reference
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tives were detected (Paul et al. 1987b). Dinuclosides with intact dCyd and an 8-
oxo-G moieties were also observed. With d(TpA) and d(ApT), the Fo derivatives
are the prominent products, but products with an aldehyde group at the free
5
-end were also detected (Paul et al. 1987a).
With d(TpA) and d(GpC) the same products as observed to be formed by
ionizing radiation were detected upon autoxidation induced by ascorbate plus
phosphate buffer (low amounts of transition metal ions are omnipresent; Ara-
kali et al. 1988).
′
Purines.
The reactions of the
•
OH-adduct radicals of the purines in the presence
of O
2
are still very poorly understood. The dGuo
C
(8)-
•
OH-adduct is reported to
react very fast with O
2
(
k
= 4
10
9
dm
3
mol
−1
s
−1
; Candeias and Steenken 2000).
There is no agreement as to the yield of 8-oxo-G under such conditions. In one
report, its
G
value is given as 0.84
×
10
−7
mol J
−1
(Vieira et al. 1993), but a lower
×
10
−7
mol J
−1
has also been measured (von Sonntag and Schuch-
mann 2001). An even much lower value has been found in the
value of 0.3
×
γ
-radiolysis of air-
10
−7
mol J
−1
; Svoboda and Harms-Ring-
dahl 1999). The latter group reports that in DNA
G
(8-oxo-G) is markedly higher
(0.077
saturated dGuo solutions (
G
= 0.0038
×
10
−7
mol J
−1
). Thus, considerably more work will be required not only
to establish the yield of this crucial product, but also to shed some light on the
mechanism of its formation in the presence of O
2
. When Gua was oxidized with
the Fenton reagent (0.15
×
10
−3
mol dm
−3
H
2
O
2
) the
8-oxo-G yield “oscillated” over a time range of 20 min with very little 8-oxo-G
to start with (White et al. 2003). This cannot be attributed to the Fenton reaction
proper, since under these experimental conditions this reaction is completed af-
ter a few seconds (Chap. 2.5). Similar findings were reported for DNA. There is
no mechanistic explanation yet for these intriguing observations.
10
−3
mol dm
−3
Fe
2+
and 50
×
×
10.3.6
Formation and Properties of Isopyrimidines
Isopyrimidines have been postulated (Haysom et al. 1972, 1975; Al-Yamoor et
al. 1977; Asmus et al. 1978; Al-Sheikhly et al. 1984; Schuchmann et al. 1984a;
Garner and Scholes 1985) or are likely to be involved (Holian and Garrison 1966;
Teoule and Cadet 1974; Cadet 1980) as short-lived intermediates formed upon
oxidation of pyrimidine-6-yl radicals carrying an H-atom at
N
(1) [reactions
(134) and (137)]. Isopyrimidines are also intermediates in the photohydration
reaction (Al-Yamoor et al. 1977; Garner and Scholes 1985). In the Ura system, for
example, the carbocation (immonium ion) is the first intermediate which either
deprotonates at
C
(5) yielding Ura [reaction (136)] or reacts with water giving rise
to the hydrate [reaction (135)]. The latter reaction is reversible in acid solution.
Mechanistically, reaction (-137) accounts for the well-known water elimination
of the photohydrate (for its stabilty in near-to-neutral solutions see Carter and
Greenberg 2001; the other isomer with the OH group at
C
(5) is not acid labile,
and these two hydrates may be distinguished based on their different behavior
in acid solution).
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