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O
O
O
OOH
O
OOH
1
O
2
O
-
O
O
-
Fast
O
-
O
-
NH
3
+
O
NH
3
+
NH
3
+
NH
HO
HO
O
O
3
4
OH
O
OH
OH
-CO
2
O
H
2
O
O
-
High pH
N
NH
NH
O
O
O
5
Scheme 4.2
O
O
O
OOH
O
OH
1
O
2
O
Fast
Reduction
HN
HN
O
HN
HN
O
HO
HO
O
6
7
Scheme 4.3
O
-
O
Urea
Asparagine
Aspartic acid
Other materials
O
-
Further peroxidic
species of unknown
structure
1
O
2
O
-
O
NH
3
+
N
NH
NH
3
+
N
NH
O
8
Scheme 4.4
Figure 4.11.
Oxidation of Tyr by
1
O
2
(adapted from Davies [13] with the permission of
the Royal Society of chemistry).
3α-hydrooxypyrroloindole (
2
). The formation of species
2
was supported by the
Raman spectroscopic study in which a new band near 1090 cm
−1
, ascribed to the
stretching vibration of the O-O band in
O
−
, was observed following the reaction
of Trp with
1
O
2
[160]. The formation of kynurenine can occur through NFK
[130]. The decomposition of hydroperoxypyrroloindole and hydroxypyrroloin-
dole can also yield NFK [177, 178].
Figure 4.11 represents Schemes 4.2 and 4.3 for the formation of hydroper-
oxides in the reactions of
1
O
2
with Tyr and peptide-bound Tyr. The formation
of species
3
in Scheme 4.2 has been proven using NMR and MS analysis of
the intermediate [179]. Other species related to species
3
have also been
observed in the reactions of phenols with
1
O
2
[180, 181]. generation of the
cyclized product, 3a-hydroperoxy-6-oxo-2,3,3a,6,7,7a-hexahydro-1
H
-indol-2-
carboxylic acid (species
4
), involved a rapid ring closure at the unprotonated
amine site of Tyr. This ring closure occurs somewhat freely but is slowed down
in peptides, which do not contain Tyr residues at the N-terminus. The delocal-
ization of the nitrogen lone pair in the amide bond of such peptides may result
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