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In-Depth Information
More recently, the formation of an intermediate, peroxynitrate, was observed
in the decomposition of peroxynitrite at a near neutral pH according to the
following proposed reaction sequence (Eq. 5.37) [178]:
ONOOH ONOO
+
−
→
NO O NOO H
−
+
−
+ →
+
2
NO O
−
+
.
(5.37)
2
2
2
2
The formation of
1
Δ
g
O
2
from the decay of ONOO
−
was used as an indicator
for the decomposition of O
2
NOO
−
[179]. A study of the isomerization of
ONOO
−
using
ab initio
also showed the formation of O
2
NOO
−
from the reac-
tion of ONOO
−
and ONOOH [180].
A study on the photon-initiated homolysis of peroxynitrous acid has been
performed in detail [181]. The homolysis of ONOOH at pH 4.0-5.5, studied
using laser flash photolysis at 355 nm, occurred exclusively at the N-O bond
rather than the O-O bond (Eq. 5.38):
ONOOH
•
NO HO
+
•
2
.
(5.38)
The radicals formed in reaction (5.38) recombined with a rate constant of
(1.2 ± 0.2) × 10
10
/M/s [181]. The quantum yields, resulting from excitation of
ONOOH and ONOO
−
at 266 or 355 nm, were similar (15%) within experi-
mental errors. Hence, it was concluded that the bond dissociation energies of
ON-OOH and ON-OO
−
bonds are similar (100 kJ/mol).
5.2.3.3 Reactivity with CO
2
.
The reaction between peroxynitrite and CO
2
was observed almost four decades ago when peroxynitrite was determined to
be unstable in carbonate buffered solution [182]. Now decades later, this reac-
tion was recognized that the bicarbonate buffer inhibits
Escherichia coli
killing
by peroxynitrite [183, 184]. In later years, studies were conducted for the reac-
tion between ONOO
−
and CO
2
, and the reaction mechanism can be presented
by Equations (5.39) and (5.40) [185]:
−
−
4
ONOO CO
+
ONOOC O O
(
)
k
=
( .
2 9 0 3
±
. )
×
10
/M/s
[186]
(5.39)
2
39
k
−
6
=
1 5 10
.
×
/second
[185]
39
ONOOC O O
(
)
−
•
NO CO
+
•−
k
=
1 9 10
.
×
9
/M/s
[185]
(5.40)
2
3
40
k
−
8
=
5 0 10
.
×
/M/s
[187].
40
The formation of the carbonate radical was directly detected using EPR
[37, 188]. Both mixtures of peroxynitrite and bicarbonate at pH 6-9 and per-
oxynitrite and CO
2
-saturated solution in alkaline media yielded carbonated
radicals ranging from a few percent [188] to 35% [37]. In the presence of excess
CO
2
, the oxidation or nitration of several substrates by peroxynitrite did not
exceed more than 33% [143]. Significantly, the rates of decay of ONOO
−
in
these reactions were similar to the rates of formation of oxidized or nitrated
substrates [161, 189-195]. The observations indicate ONOOC(O)O
−
, formed
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