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O
O
O
e - aq
H +
HC
CH 3
CH 3
CH 3
Reaction 24
-
H
NH
NH
NH
H 3 C
H 3 C
H 3 C
H
O
O
O
HC +
SO 4 •-
H 2 O/-H +
CH 3
CH 3
CH 3
Reaction 25
+
H
NH
NH
NH
H 3 C
H 3 C
H 3 C
HO
FIG. 1.9 Reaction of a solvated electron with phenylalanine followed by protonation
forms a cyclohexadienyl radical ( reaction 24 ). Strong oxidants like SO 4 or direct
photoionization are able to induce ring radical cations at all aromatic amino acids
( reaction 25 ) (according to Clare et al., Biochim. Biophys. Acta 1504: 196-219, 2001).
reactions between aromatic side chain-derived radicals have been defined. The
reduction potentials of peptide radicals suggest that the ultimate source for
oxidizing equivalents is likely to be Tyr residues (or Trp in the absence of these
side chains). Thus, peptide radicals are able to oxidize Tyr residues via the
formation of the ring radical cation, and subsequent deprotonation to give the
phenoxyl radical. These reactions are in equilibrium, so Tyr phenoxyl radicals
can be repaired by high concentrations of thiols such as Cys, yielding thiyl
radicals. This process is enhanced by excess of thiol anions, as the thiyl radicals
generated are removed via the formation of the disulfide radical anion.
Reaction of Trp with N 3 results in the generation of the neutral indolyl
radical. If such species are generated on peptides or proteins that also contain
Tyr residues, rapid oxidation of the latter residues to give phenoxyl radicals is
observed via electron transfer (91). This type of transfer process has been
investigated in a 62-amino-acid peptide (erabutoxin B) that contains single
Trp (Trp-25) and Tyr (Tyr-29) residues. Slow transfer is observed in this case;
this is attributed to the rigid nature of this peptide that contains four disulfide
bonds (92). This study suggests that rapid electron transfer requires either
direct contact of the reactive residues or contact via suitable intermediate
species, and that the peptide backbone does not provide a transfer pathway.
Disulfide bonds (cystine residues) can act as a major source for electrons
arising from electron transfer by reducing species. Thus, initial addition of
solvated electrons to both the backbone carbonyl groups of peptide bonds and
at some side chain sites (e.g., aromatic residues) can result in the ultimate
reduction of cystine groups. The yield of initial electrons that end up at disul-
fide sites depends on the protein; with lysozyme, it is nearly 65%, whereas with
RNase A, it is nearly 20%. The latter observation is of particular interest as
the disulfide groups in this protein are internalized and inaccessible to species
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