REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 175

hydrogen abstraction [51]. Further competitive reactions of RS • with O 2 and
RS − generated the thiyl peroxy radical (RSOO • ) and conjugated disulfide
radical anion (RSSR) •− , respectively. RSOO • could be the precursor species
for the formation of sulfenic, sulfinic, and sulfonic acids, disulfide, and serine,
possibly through thiyl hydroperoxide (RSOOH) as an intermediate [323].
Products of the reactions were confirmed by measuring the mass shifts of +32,
+48, and −16 Da (Fig. 4.26). Formation of cysteine sulfinic and sulfonic acids
was observed in the radiolysis of the cysteine-containing fibronectin peptide
(RcDc) in water [324]. Other studies have shown the radiolysis of cysteine-
containing peptides resulted in the formation of disulfide as one of the primary
products under both aerobic and anaerobic conditions [323, 325]. The exis-
tence of cysteine sulfenic acid and cysteine sulfonamide has also been charac-
terized in the • OH-mediated modification of the cysteine-containing synthetic
peptide HcSAgIgRS [326]. Thiyl radicals, produced during the oxidation of
cys, may react with amino acids in peptides [327-329].
Reactions of seleno-derivatives of amino acids, selenomethionine (SeMet),
selenocystine (Secys), methyl selenocysteine (MeSecys), and selenourea
(SeU), selenocystamine (SeA), and diselenodipropionic acid (SeP) with • OH
have been studied to understand the biological activity of such compounds
[330-332]. Second-order rate constants of the reactions were of the order of
10 9 -10 10 /M/s at pH 7.0. Selenium compounds are usually easier to oxidize and
the oxidation chemistry of these compounds was similar to their sulfur ana-
logues [331]. The reaction of MeSecys with • OH radicals in the pH range from
1 to 7 yielded monomer radicals with an absorption maximum at 350-370 nm.
Diselenides reaction with • OH radicals produced diselenide radical cations
(λ max = 560 nm) by the elimination of either OH − or H 2 O from the adduct.
Secys and SeA, which also contain amino functional groups, produced an
additional transient (λ max = 460 nm), which was likely a triselenide radical
adduct in which a bridge may be formed having the sharing between the three
Se stoms [333, 334].
Acidic Side Chains. c-Terminal decarboxylation of proteins and peptides has
been reported in the generation of the alkoxy radical [307]. Figure 4.27 shows
the mechanism of decarboxylation that produced the α-carbon alkoxy radical
and CO •− . The decarboxylation product had a mass shift of −30, which agrees
with the mechanism proposed in Figure 4.27. This type of decarboxylation
product of glu has been identified in the radiolysis of glu-containing peptides
and proteins [287, 309]. The mass shift of −30 was also observed in the radioly-
sis of Asp-containing peptides such as DSDPR, DRgDS, KQAgDV,
ADSDgK, and DRgDS [287].
Basic Side Chains. The radiolytic oxidation of Lys hydroxylysine produced α-
amino-adipyl-semialdehyde products [335]. However, different products were
obtained in the presence of oxygen. In the radiolysis of the peptide DAHK,
the mass shift products of +14 and +16 were observed [336]. If unstable