Chemistry Reference
In-Depth Information
10.3.8
Reactions of O
•
−
At very high pH,
•
OH deprotonates (p
K
a
(
•
OH) = 11.9), and the reactions that
are observed are due to that of O
•
−
. The reactions of O
•
−
are typically one or-
der of magnitude lower than those of
•
OH. In some of the studies reported be-
low, experiments have been carried out at pH 13. This pH is not high enough
to exclude major contributions of
•
OH. This has to be taken into account when
consulting these papers. In contrast to
•
OH, O
•
−
only very reluctantly adds to
C
C double bonds, but it is a good H-abstractor and has also a considerable oxi-
dation power (Chap. 3.1). At pH 13, all nucleobases are deprotonated, and their
reduction potentials are quite low (ranging between 0.63 V (Gua) and 0.88 (Ura);
Jovanovic and Simic 1986). For Ura, Cyt, Ade and Gua, it has been suggested
that the reaction of O
•
−
with their anion and dianions occurs by ET, a reaction
that is also given by Br
2
•
−
under these conditions (Ioele et al. 1998). In the case
of Cyt, an H-abstraction from the amino group has also been considered as an
alternative. With Thy, 1MeUra and 1MeCyt H-abstraction at methyl dominates
(Ioele et al. 1998), and this is also reported for other methyl-substituted pyrimi-
dines (Luke et al. 2003). The one-electron oxidized nucleobase (di)anions of Ura,
Thy (formed via Br
2
•
−
) and Ade decay unimolecularly with rate constants near
10
4
s
−1
. Two alternatives are envisaged, nucleophilic substitution at
C
(6) by OH
−
or protonation at carbon by water [reaction (154)]. Since the rate of reaction does
not depend on the OH
−
concentration, the second alternative may be favored. It
is analogous to the also slow protonation at carbon of the radical anion that is
discussed in the next paragraph.
−
O
O
H
H
H
2
O
(154)
N
N
H
+ OH
O
N
H
O
N
H
In nucleosides,
G
(base release) increases dramatically at high pH (Fujita 1984).
This has been suggested to be due to an OH
−
-induced transformation of a base
•
OH-adduct radical into a sugar-centered radical that releases the base. How-
ever, it has been subsequently shown that this effect is due to the involvement
of the basic form of
•
OH, the O
•
−
radical [Scholes et al. 1992; reactions (160) and
(162)]. The O
•
−
radical undergoes H-abstraction from the sugar moiety rather
than addition to the base (for rate constants see Chatgilialoglu et al. 2005). Some
of these sugar-derived radicals will release the base (see below). Reactions (155)-
(162) show the various p
K
a
values that are involved in the Urd system given as an
example. The deprotonation of ribonucleosides at the 2
′
-position at high pH has
recently been substantiated (Velikyan et al. 2001).
Search WWH ::
Custom Search