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-amino group of
Lys side chains) undergo intramolecular abstraction, and this generates carbon-
centered radicals at either C3 or the
Side-chain aminyl radicals (e.g., those formed from the
-carbon (69).
-carbon position accounts for more than
90% of the radicals formed with a series of alanine-derived peptides on reac-
tion with HO . This is due to the greater stability of the
Hydrogen abstraction from the
-carbon radical over
the primary alkyl radical formed on hydrogen atom abstraction from the
methyl side chain (74). However, the yield of such backbone-derived radicals
decreases markedly when there are side chains present, which can form stabi-
lized radicals, or when steric factors play a role.
-Carbon radicals decay
mainly by dimerization in the absence of O 2 (75). In the former case, significant
yields of cross-links involving side chain-derived radicals have been identified;
in the presence of O 2 , peroxyl and alkoxyl species are also generated (76).
-Carbon-centered radicals are also generated on addition of the solvated
electron to backbone carbonyl groups (Fig. 1.3, reaction 6). The resulting mid-
-amido radicals, formed on protonation of the initial adduct,
decay primarily via reaction with other radicals in the absence of O 2 . Thus, the
reaction with an
-carbon radical results in the repair of both species (Fig. 1.3,
reaction 7). The initial adduct species also undergo electron transfer reactions
with acceptors such as disulfide (Fig. 1.3, reaction 8) or His residues. The main
chain cleavage via reaction 9 (Fig. 1.3) is believed to be a minor process.
e - aq + ~C(O)NH~
Reaction 6
H +
~ C(O - )NH~ →~ C(OH)NH~
~ C(OH)NH~ + ~NH C(R 1 )C(O)~ →
Reaction 7
~ C(O - )NH~ + RSSR →
~C(O)NH~ + (RSSR) -
Reaction 8
e - aq + ~CH(R)C(O)NHCH(R 2 )~ →
~CH(R) C(O - )NHCH(R 2 )~ →
~CH(R)C(O)NH 2 + CH(R 2 )~
Reaction 9
FIG. 1.3
-carbon-centered radical is gener-
ated at the backbone carbon ( reaction 6 ). The formed midchain
After addition of a solvated electron, an
radical decays in the absence of O 2 via the reaction with other radicals ( reaction 7 ), in
this case resulting in the repair of both species. Electron transfer reactions are possible
between disulfides or histidine residues ( reaction 8 ). A cleavage of the main chain is a
minor process in this scenario ( reaction 9 ) (according to Clare et al., Biochim. Biophys.
Acta 1504: 196-219, 2001).
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