REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 172

radicals. Detailed theoretical examination for the reactions of gly with • OH
has been carried out to further understand the simultaneous formation of
three different primary radicals (see Fig. 4.24) [294, 295]. calculations per-
formed by previous studies also showed nitrogen and α-carbon sites are com-
petitive for H-abstraction via an • OH attack to generate different radicals
[304]. The model amide system was also used to understand the reactions of
• OH radicals with peptide systems [305]. The • OH radical attacked the α-
carbon site of formamide; however, addition of a methyl group opened an
alternative attack at the γ-carbon site. The γ-carbon H-abstraction was favor-
able compared to the β-carbon abstraction. Furthermore, H-abstraction of the
nitrogen more likely occurred with the addition of a methyl group on the
nitrogen. geometric structures and stabilization of intermediates supported
the preference of attacks by • OH radicals.
The • OH radical attacked the aliphatic hydrocarbon side chains (e.g., Ala,
Val, Ile, and Leu) indiscriminately, and the reactivity increased with an increase
in the number of c−H bonds and the length of the hydrocarbon side chains
[306, 307]. The hydrogen abstraction by • OH yielded a carbon-centered radical,
which produced a peroxy radical upon reaction with O 2 under aerobic condi-
tions. A number of reactions of the peroxy radical finally form carbonyl,
hydroxide, and hydroperoxide products with mass shifts of +14, +16, and
+32 Da, respectively. The alkoxy radical and O 2 may also be alternatively
formed [307]. Hydroperoxides are generally unstable and decompose to
produce further radicals or carbonyl and alcohol products.
In a recent study, oxidation of peptides containing gly, Ala, Val, and Pro by
• OH/O 2 was studied to quantify the nature and yields of oxidation products
such as alcohols, carbonyls, hydroperoxides, and fragment species [308]. This
study helped to learn the contribution of different pathways to the oxidation
of peptides and proteins. The location of an amino acid in a peptide sequence
influenced the proportions of the formation of different oxidation products.
The hydroperoxides were formed nonrandomly (Pro > Val > Ala > gly) and
their concentrations were inversely related to the yields of both peptide-bound
and released carbonyls. Both side chains and backbone site alcohols were
produced. The overall yields of products were more than that of the initial
• OH generated, which suggest involvement of chain reactions in the oxidation
of peptides.
Sulfur-Containing Side Chains. Met is highly reactive with the • OH radical
(see Table 4.11) and the reaction results in different intermediate radical
species (Fig. 4.25). Methionine sulfoxide (+16 Da mass shift) was produced,
which further oxidized to yield methionine sulfone (+32 Da mass shift). A
formation of aldehyde at the γ-carbon gave a −32 Da mass shift [261, 309]. This
mass shift product has been observed as the minor species in the oxidation of
Met-containing peptides such as HDMNKVLDL [261]. Other mass shift prod-
ucts of −30 Da and (−30 + 16 Da) were due to the c-terminal decarboxylation
from the original molecule and the formation of sulfone, respectively [287].