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Cl
H
H
2
N
NH
H
2
N
NH
N
NH
C
C
C
NH
NH
NH
+ HOCl
(CH
2
)
3
(CH
2
)
3
(CH
2
)
3
CH
CH
CH
H
2
N
COOH
N
COOH
N
COOH
Cl
H
Cl
H
N
a
-chloro-
L-arginine
N
a
,
N
w
-dichloro-
L-arginine
L-arginine
Figure 3.5.
Proposed structures of chlorinated l-Arg products. (Adapted from [30]
with the permission of American Society of Biochemistry and Molecular Biology).
chloramine was formed, which went through either a thermal or metal-
catalyzed process to yield radicals. The other Trp side chain then reacted with
radicals intermolecularly to produce kynurenine. Recently, the formation of
chloramine in the enzymatic chlorination of Trp has also been proposed [43].
The direct oxidation of the side chain of Trp to yield 2-hydroxyindole deriva-
tives of Trp has also been suggested [44-46]. The chlorination products of Tyr
were 3-chloro-Tyr and 3.5-dichlorotyrosine [47-50]. The direct attack of HOCl
and a chlorine transfer from the initially produced chloramine are the two
proposed mechanisms for chlorination of the phenolic side chain of Tyr [33].
The chlorination of thiols, including sulfur-containing amino acids, has been
studied extensively [26, 51-54]. Methionine sulfoxide was the reaction product
in the oxidation of Met by HOCl. Evidence for the formation of an intermedi-
ate, a chlorosulfonium cation, was provided (Eq. 3.4) [26]. The hydrolysis of
the resulting cation forms sulfoxide. Oxidation of Met and Tyr residues in
myoglobin by HOCl has been observed [55]:
−
CH S CH CH NH COO HOCl
CH S Cl CH CH NH COO OH
(
)
(
)
+
3
2 2
2
(3.4)
+
−
−
→
(
)(
)
(
)
+
.
3
2 2
2
The mechanism for the oxidation of Cys and cystine by HOCl is summa-
rized in Figure 3.6 [33]. Similar to Met, the initial formation of sulfenyl chloride
was proposed, which was ultimately converted to cysteic acid (RSO
3
H) in
aqueous solution through the intermediates, sulfenic (RSOH) and sulfinic
(RSO
2
H) acids [45, 51, 56]. Sulfenyl chlorides played a major role in the pro-
duction of thiyl radicals in the reaction system, which were detected in the
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