Chemistry Reference
In-Depth Information
oxidation of acetohydroxamic acid by
•
OH,
O
•−
,
•
N
3
,
•
NO
2
, and
CO
•−
has been
studied [219]. The aceto hydroxamic acid has an importance in metal chelation
and enzyme inhibition as well as in scavenging of radicals. The effective reac-
tivity was observed only with
•
OH,
•
N
3
, and
CO
•−
, resulting respective transient
nitroxide radical through a complex mechanism [219].
The addition of ONOO
−
to carbonyl compounds represents an example of
nucleophilic addition [143]. Initially, a fast equilibration between the reactants
and the corresponding tetrahedral adduct anion is formed with a strong ten-
dency of an equilibrium shift toward the reactant side. A fast protonation of
the adduct anion by water and added buffers occurs [220, 221]. The rate
of nucleophilic addition to carbonyls (e.g., amides) is much slower than that
of the reaction of ONOO
−
with CO
2
[77, 161, 221-223]; therefore, the oxidation
of carbonyls by ONOO
−
may not be of biological significance.
Recently, the reactions of ONOO
−
with phenyl and coumarin boronates
have been studied in order to develop boronate-based fluorescent detection
[224]. The reactions were rapid (
k
∼ 10
6
/M/s at pH 7.4). Comparatively, the
reactions of ONOO
−
with aryl boronates were ∼10
6
and 2 × 10
2
times faster
than H
2
O
2
and HOCl [224]. Both H
2
O
2
and HOCl reacted with boronates
stoichiometrically to yield a 100% phenolic product through a two-electron
process. However, in the case of reaction of ONOO
−
with boronates, the
nitrated species as the minor products (10-15%) were also obtained in addi-
tion to phenols as the major products (∼85 to 100%) [224]. The positive effects
of oxygen and inhibitory effects of hydrogen atom donor (acetonitrile and
2-propanol) and general radical scavengers (Tyr, gSH, NAdH, and ascorbic
acid) on the yield of minor product indicates the intermediate of the reaction
was the phenoxy radical [225]. Results of this study may have relevance in
mitigating peroxynitrite-mediated cytotoxicity and therefore warrant further
understanding of the reaction mechanism between peroxynitrite and
boronates.
5.2.3.5 Reactivity with Proteins and Nonprotein Metal Centers.
The
second-order rate constants for the reactions of peroxynitrite with protein and
nonprotein metal centers are given in Table 5.5 [199, 201, 226-243]. The rate
constants vary from 10
3
to 10
7
/M/s. The reactions of peroxynitrite with water-
soluble iron porphyrins at physiological pH have been studied [227, 244-247].
The rate constants vary from 10
5
to 10
7
/M/s. For example, 5,10,15,20-tetrakis
(
N
-methyl-4′-pyridyl)porphinatoiron(III) [Fe(III)TMPyP] rapidly reacts with
ONOO
−
with a rate constant
k
= 5 × 10
7
/M/s [245]. Comparatively, the rate
constant of Fe(III)TMPyP with
O
−
under the same conditions was 1.9 × 10
7
/M/s
[245].
Reactions with metal-centers can be rationalized by Equation (5.42):
n
+
−
n
+
n
+
n
+
•−
•
(
n
+ +
1
)
•
M ONOO
+
→
M -ONOO
→
M -O
+
NO
→ =
M
=
O NO
+
.
(5.42)
2
2
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