Chemistry Reference
In-Depth Information
(Kissner et al. 1997)], and that of reaction (31) is not much slower [
k
= 3.2
×
10
9
dm
3
mol
−1
s
−1
(Goldstein and Czapski 1995)]. The reaction of NO
•
with O
2
•
−
/
HO
2
•
is in competition with its autoxidation [following the rate law -d[NO
•
]/dt =
4
k
[NO
•
]
2
[O
2
] [4
k
= 8
10
6
dm
6
mol
−
2
s
−
1
(Awad and Stanbury 1993)]. In biologi-
cal systems, the NO
•
steady-state concentration is always low [for example, its
intracellular steady-state concentration has been estimated at around 10
−8
mol
dm
−3
(Liochev and Fridovich 1999)].
Peroxynitrous acid is a weak acid [equilibrium (32); for the p
K
a
see above].
Whereas the anion is rather stable, the acid rapidly decomposes quite rapidly
[reaction (37)];
k
= 0.8 s
−1
at pH 7.5 (Kobayashi et al. 1995)] into
•
NO
2
and
•
OH
[for measurements of the volume of activation see Goldstein et al. (1999)] which
either recombine in the cage yielding nitrate plus a proton [reaction (39)] or dif-
fuse out of the cage giving rise to free
•
NO
2
and
•
OH [reaction (41); Mark et al.
(1996); Richeson et al. (1998); Coddington et al. (1999); 28
×
±
4% yield (Gerasimov
and Lymar 1999)].
For some time, there has been a discussion that
•
OH is not the oxidizing spe-
cies formed upon the decay of pernitrous acid (Koppenol et al. 1992; Koppe-
nol 1998b, 1999). This conclusion was based on thermochemical grounds, be-
cause it had been inferred that in the reaction of
•
OH with
•
NO
2
only ONOOH is
formed, but there is now convincing evidence that this reaction leads to almost
equal amounts of pernitrous and nitric acids (Merényi et al. 1999), supporting
also thermochemical calculations concerning the feasibility of
•
OH-production
(Merényi and Lind 1997). The situation got even more complex, when it was real-
ized that CO
2
speeds up the decomposition of peroxynitrite (Keith and Powell
1969; Radi et al. 1993). There is now strong evidence for a peroxynitrite-CO
2
adduct [reaction (33)] (Denicola et al. 1996; Zhang et al. 1997; Lymar and Hurst
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