Chemistry Reference
In-Depth Information
Table17.3
Therateconstantsk
o
(M
−1
s
−1
)forthereactionofnitroxideswithperoxylandHO
2
•
radicals
15,19 - 21
t-BuOO
•
CH
3
OO
•
CH
2
(OH)OO
•
HOO
•
Nitroxide
10
7
10
7
10
8
10
8
TEMPO
(2
.
8
±
0
.
2)
×
(5
.
1
±
0
.
1)
×
(1
.
0
±
0
.
1)
×
(1
.
1
±
0
.
1)
×
10
6
10
6
10
7
10
7
TEMPOL
(3
.
3
±
0
.
2)
×
(5
.
4
±
0
.
2)
×
(4
.
4
±
0
.
1)
×
(2
.
7
±
0
.
3)
×
4-amino-TEMPO
(1
.
0
±
0
.
2)
×
10
6
(1
.
5
±
0
.
2)
×
10
6
ND
<
1
×
10
7
10
5
10
5
10
6
TEMPONE
(2
.
8
±
0
.
2)
×
(5
.
4
±
0
.
2)
×
ND
<
6
×
10
7
TEMPENE
ND
ND
ND
(7
.
9
±
0
.
2)
×
10
5
10
6
10
6
10
6
3-CP
(8
.
1
±
0
.
6)
×
(1
.
1
±
0
.
1)
×
(9
.
0
±
0
.
1)
×
(1
.
1
±
0
.
1)
×
10
5
10
6
10
6
3-AP
(9
.
6
±
0
.
6)
×
(1
.
5
±
0
.
6)
×
ND
(1
.
1
±
0
.
2)
×
10
4
10
4
10
5
CHDO
(5
.
0
±
0
.
5)
×
(6
.
7
±
0
.
5)
×
ND
(1
.
6
±
0
.
2)
×
10
4
PTIO
ND
ND
ND
<
2
×
ND - not determined.
In contrast, the reactivity of nitroxides towards HO
2
•
and RO
2
•
is highly dependent on the ring size
and, to a lesser extent, on the side chain. Furthermore, in the case of piperidine nitroxides, the rate of
formation of R
2
N
+
=
O is both H
+
and general acid catalyzed (Equations 17.4 and 17.5).
20,21
H
+
−→
R
2
N
+
=
O
R
2
NOX
+
+
HX
(17.4)
R
2
N
+
=
O
A
−
R
2
NOX
+
HA
−→
+
HX
+
(17.5)
The uncatalyzed rate constants (
k
o
) derived for HO
2
•
and alkyl peroxyl radicals are given in Table 17.3,
demonstrating that the most efficient scavenger is TEMPO, which has the lowest oxidation potential among
the studied nitroxides (Table 17.1).
In the case of thiyl radicals (RS
•
), the reaction also proceeds via the formation of an adduct with
the general structure R
2
NOSR
. However, the decomposition of the adduct does not yield the respective
oxoammonium cation but instead the respective amine via a complex mechanism (Scheme 17.2).
28
The rate constant for the reaction of nitroxides with thiyl radicals is independent of the structure of the
nitroxide and thiyl radicals and is similar to that determined for
•
NO
2
and CO
3
•
−
radicals,
k
=
(
5
−
7
)
×
10
8
M
−
1
s
−
1
(Table 17.4).
28
At physiological pH the unstable adduct R
2
NOSR
decomposes mainly via heterolysis of the N-O bond
to yield the respective amine and sulfinic acid (R
S(O)OH). In acidic solutions the protonated form of the
adduct decomposes via homolysis of the N-O bond to yield the reducing sulfenyl radical (R
SO
•
)and
the aminium cation radical (R
2
NH
•
+
), which can oxidize thiols
45
forming the respective amine and thiyl
radicals (Scheme 17.2).
17.3 Nitroxides as SOD mimics
The ability of nitroxide to catalyze superoxide dismutation has been previously demonstrated by the
persistence of its concentration under a flux of superoxide.
10,11
It has been demonstrated that piperidine
and pyrrolidine nitroxides catalyze the dismutation of superoxide by utilizing the R
2
NO
•
/R
2
N
+
=
O redox
couple (Equations 17.6 and 17.7).
19,20,46
H
+
−→
R
2
NO
•
HO
2
•
R
2
N
+
=
O
+
+
H
2
O
2
(17.6)
R
2
N
+
=
O
O
•
2
R
2
NO
•
+
−→
+
O
2
(17.7)
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