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O
O
O
O
N
N
N
N
HO
•
O
2
or
H
S
H
S
O
S
O
•
O
O
•
H
O
O
•
O
HO
2
•
O
O
O
O
S
N
N
N
O
N
O
2
H
2
O HO
•
O
•
O
Sulfoxide
+16 Da
•
O
O
•
S
O
S
O
S
O
-32 Da
O
N
O
N
•
S
O
O
S O
Sulfone
+32 Da
Figure 4.25.
Oxidation of Met by
•
OH radical (adapted from xu and chance [261]
with the permission of the American chemical Society).
In the radiolytic oxidation of Met, the addition of the
•
OH radical to the
sulfur atom and hydrogen abstraction was involved [310]. At pH > 3.0, the
adduct released OH
−
to yield a sulfur-centered radical cation >S
•+
. This step
was catalyzed by an electron-rich heteroatom, such as N or S, on neighboring
side chains [311]. The sulfur-centered cation may also be stabilized by the
peptide bond through the formation of an intramolecular three-electron
bonded cyclic transient [312, 313]. Theoretical calculations also showed the
cyclic S-N bonded species in the one-electron oxidation of the Met peptide,
in which the intramolecular three-electron interaction was between the S atom
and the π orbital of the amide group [314]. The S-N bond radicals may convert
into carbon-centered radicals located on the α-carbon moiety of the peptide
backbone [312]. The dimeric radical cation has also been postulated as the
intermediate of the reaction [315], which may deprotonate from −cH
3
or
−cH
2
- next to sulfur atom, followed by conversion into α-(alkylthio)alkyl
radicals on the side chain of Met [312]. The factor analysis of the transient
spectra obtained from the
•
OH radical-induced oxidation of cyclo-(D-Met-L-
Met) showed the formation of a radical cation via the stabilization of the
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