Chemistry Reference
In-Depth Information
TABLE 5.3. Second-Order Rate Constants for Oxidation of Compounds by H
2
O
2
and
HCO
-
at 25°C [72-74]
k
(H
2
O
2
)
k
(
HCO
−
)
Compound
(/M/s)
(/M/s)
k
(
HCO / H O
4
−
)
k
(
)
2
2
N
-Methylmorpholine
a
3.5 × 10
−5
1.6 × 10
−2
457
N
,
N
-dimethylbenzylamine
b
1.0 × 10
−4
4.2 × 10
−2
420
Cysteine
c
1.7 × 10
1
9.9 × 10
2
58
glutathione
c
1.8 × 10
1
1.9 × 10
3
105
N-Acetylcysteine
c
1.7 × 10
1
1.5 × 10
3
88
Methionine
a
7.5 × 10
−3
4.8 × 10
−1
640
a
Solvent: d
2
O.
b
Solvent: acetone-d
6
/d
2
O.
c
Values are for thiolate anion.
the carbonate leaving group [22]. The oxidation of methyl disulfides by
HCO
−
resulted in the formation of thiosulfone and sulfonate products [73]. The mecha-
nism of the oxidation of aryl sulfide by
HCO
−
has also been studied at the
aqueous/cationic micellar interface [24]. The cetyltrimethylammonium carbon-
ate (CTAHCO
3
) significantly enhances the overall oxidation rate of sulfides
compared to the addition to cetyltrimethylammonium chloride (CTACl) and
bromide (CTABr) [24]. The oxidation of aliphatic amines by
HCO
−
was over
400-fold greater than H
2
O
2
alone (Table 5.3) [72]. The enthalpy of activation
was significantly lower for the oxidation by
HCO
−
than that of H
2
O
2
, resulting
in an increased rate. The secondary aliphatic amines were oxidized to corre-
sponding nitrones, while tertiary amines gave corresponding
N
-oxides in high
yields.
The scheme for the activation of H
2
O
2
by
HCO /CO
−
in the oxidation of
Cys and Met is presented in Figure 5.9 [73, 74]. In the scheme,
k
1
is the second-
order rate constant for the oxidations by
HCO
−
(see Table 5.3). The left side
of the scheme is the proposed perhydration of CO
2
to form percarbonic acid
in equilibrium with its conjugate base,
HCO
−
. The right side of the scheme in
Figure 5.9 is the equilibrium of
CO /HCO
2
3
2
−
, which may be catalyzed by car-
bonic anhydrase and various model complexes. The scheme demonstrated the
reaction of
HCO
−
with substrate S via O atom transfer formed back to
HCO
−
(top portion). This supports the role of
HCO
−
as a catalyst in the oxidation of
Met and Cys. The possible involvement of
HCO
−
in protein oxidation has also
been explored [73, 74]. For example,
in vitro
and
in vivo
studies have demon-
strated the enhancement of the oxidation of the human neutrophil elastase
inhibitor αI-PI in the presence of
HCO
−
[74]. The loss of inhibitory activity
was essentially complete at the longest reaction time when the highest con-
centration of
HCO
−
was used. Evidence was provided for the oxidation of two
methionines (Met351 and Met358) in the reactive center loop of αI-PI. A
3
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