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oxidized sulfur through the formation of two-centered, three-electron bonds
with the lone pairs of oxygen [316].
The formation of products related to Met in the oxidation of methionine-
containing peptides by
•
OH is determined by the relative rates of the reactions
of amino acids present in the peptides. When another amino acid had a higher
reactivity than Met, the modification of Met was not observed [261, 309]. For
example, the modification of Met with a −32 mass shift was observed in the
γ-irradiation of peptides HDMNKVLDL and gSNKgAIIgLM(O) but not
in peptides LWMRFA-NH
2
and yggFM(O)R. The influence of other amino
acids was also shown using DFT calculations for the reactions of gly-Met-gly
and gly-Nle-gly tripeptides in which the former had 100 times faster reactiv-
ity than the latter [317]. The importance of protonated counterparts in relation
to the parent molecule in the Met model peptide has also been explored [318].
28
Met and
31
Met residues of ccK8 were shown to be susceptible to oxidation
by
•
OH [319]. The
•
OH-induced oxidation of
27
Met to
27
Met-sulfoxide in the
polypeptide hormone glucagon was also demonstrated [320].
As shown in Figure 4.26, the oxidation of cys by
•
OH radical is complex.
Numerous products were formed, depending on the reaction conditions and
formation of the initial radical species [309, 321, 322]. In the initial step of the
reaction of cys with ROS, the thiyl radical species (RS
•
) was formed through
H
•
HO
2
•
H
2
S
O
2
H
2
O
R
•
Serine
(-16 Da)
ROO
•
RSH
ROH
e
a
-
e
a
-
HS
-
H
•
HO
•
H
•
SO
2
H
2
O
H
2
O
2
RS—SR
H
2
RSH
H
2
O
RSH
HO
2
•
O
2
H
2
O
H
2
O
2
O
2
H
+
RS
-
RS
•
RSOO
•
RSOOH
RSOH
H
2
O
2
RSH
RS
•
RSH
H
2
O
RS
•
HO
2
•
O
2
O
O
2
•-
O
2
HO
2
•
O
O
2
(RS—SR)
•-
RS—SR
R S O
•
R S OH
Sulfinic acid
(+32 Da)
RSH RS
•
R
′
S
-
RS
-
R
′
S
•
RS
•
H
2
O
2
H
2
O
H
2
O HO
2
•
O
O
2
O
O
R'S—SR
R S
•
O
R S OO
•
R
S
OH
O
O
Sulfonic acid
(+48 Da)
R = other part of Cys residue except thiol group
Figure 4.26.
Oxidation of cys by
•
OH radical (adapted from xu and chance [261] with
the permission of the American chemical Society).
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