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CO
2
-
CO
2
-
CO
2
-
or
R =
NH
3
+
NH
3
+
NH
2
+ ClO
2
•
R
H
+ ClO
2
-
CO
2
-
H
NH
3
+
+ ClO
2
•
H
+
+
H
+
H
+
H
R
R
OClO
ClO
2
•
N
N
Trp-OClO Adduct
-H
+
Ring Closure
OCl
CO
2
-
CO
2
-
O
O
H
2
O
Product A
NH
NH
O
H
H
H
+
H
+
CO
2
-
O
NH
3
+
NH
H
O
N-Formylkynurenine (NFK)
Figure 3.18.
The proposed mechanism for the reaction of ClO
2
· with Trp (adapted from
Stewart et al. [171] with the permission of the American Chemical Society).
generated short-lived adducts, which decayed rapidly to yield dopaquinone
(from Tyr) and
N
-acetyldopaquinone (from NAT). DOPA also consumed
2 mol of ClO
2
to yield dopaquinone and 2 mol of
ClO
−
. The cyclization of
dopaquinone occurred at pH > 4 to form cyclodopa, which subsequently oxi-
dized to dopachrome.
There are few studies on the oxidation of peptides and proteins by ClO
2
[167, 173, 174]. Recent work focused on BSA and glucose-6-phosphate dehy-
drogenase (g6PD) of baker's yeast (
Saccharomyces cerevisiae
) as model pro-
teins [173]. The denaturation of proteins by ClO
2
decreased the α-helical
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