Chemistry Reference
In-Depth Information
5.2.2 Nitrogen Dioxide Radical
Nitrogen dioxide (
•
NO
2
) produces and participates in a variety of biological
reactions [65, 115, 125, 126]. The kinetics and mechanism of the reactions of
•
NO
2
with nitroxides, thiols, peptides, and proteins have been studied [65, 125,
127]. Reactions of piperidine, pyrrolidine, and oxozolidine nitroxides (RNO
•
)
by
•
NO
2
radicals have been performed [65]. The kinetics of the reactions dem-
onstrated nitroxides are the most efficient scavengers of
•
NO
2
at physiological
pH (
k
= (3-9) × 10
8
/M/s). Reactions were dependent on the ring size and
nature of the side chain of nitroxides, similar to the reactions of nitroxides
with
CO
•−
[65]. The reactions produced the respective oxoammonium cation,
which could be efficiently scavenged by ABTS
2−
, forming ABTS
•−
[65].
The reactivity of
•
NO
2
with gSH and Cys has been performed using pulse
radiolysis [127]. Formation of the thiyl radical was observed (Eqs. 5.26, 5.27):
GSH NO
+
•
→
GS NO H
•
+
−
+
+
(5.26)
2
2
CysSH NO
+
•
→
GysS NO H
•
+
−
+
+
.
(5.27)
2
2
The rate constants of reactions (5.26) and (5.27) increased linearly with
an increase in pH between ∼5.7 and 8.0. The slopes of the plot of log
k
versus pH were determined as 0.63 ± 0.02 and 0.57 ± 0.01 for the reactivity of
•
NO
2
with gSH and cysteine, respectively. At pH 7.4, the rate constants for
reactions (5.25) and (5.26) were estimated as ∼2 × 10
7
/M/s and 5 × 10
7
/M/s,
respectively.
Nitrosylation of peptide and proteins by
•
NO
2
has been carried out [128-
130]. The reactivity of gly-Tyr with
•
NO
2
in aqueous solution was strongly pH
dependent with rate constants varying from 3.2 × 10
5
/M/s at pH 7.5 to
2.0 × 10
7
/M/s at pH 11.3. The reaction generated phenoxyl radicals, which
could further react with
•
NO
2
to yield nitrotyrosine (
k
∼ 3.0 × 10
9
/M/s), the
predominant final product in the neutral solution. The reaction of
•
NO
2
with
cysteine-thiolate was faster (
k
∼ 2.4 × 10
8
/M/s at pH 9.2), which involved the
transient formation of cystinyl radical anions. The reactivity of
•
NO
2
with gly-
Trp was relatively slow (
k
∼ 10
6
/M/s at pH 9.0). Reactivity was not observed for
•
NO
2
with Met-gly and (Cys-gly)
2
in the pulse radiolysis experiments. At
pH 7.0-9.0, the selective nitration of Tyr in the interaction of
•
NO
2
with histone,
lysozyme, and ribonuclease A was found [129]. Nitration of tyrosine in hydro-
philic and hydrophobic environments has recently been performed, which
suggested the involvement of free radical mechanisms [128]. The involvement
of radical was supported by the restrictions imposed on obtaining a large yield
of protein 3-nitrotyrosine formation
in vivo
due to the presence of strong
reducing systems such as gSH. Elimination of important soluble reductants
allowed nitration of Tyr [128].
Recently, the reactivity of
•
NO
2
with various oxidation states of myoglobin
was studied [125]. The interconversion of various oxidation states of myoglo-
bin was observed (Eqs. 5.28-5.32):
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