REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 176

Figure 4.27. Decarboxylation of c-terminal carboxyl (adapted from xu and chance
[261] with the permission of the American chemical Society).
ε-hydroxylysine formed due to the attack of • OH on the ε position of Lys, it
would convert into 2-amino-adipyl-semialdehyde with a loss of ammonia [337].
However, this product has not been identified in the radilolysis of Lys [261].
The deamination of Ser-containing peptides (Ala-Ser, Ser-Ala, Ala-Thr, and
Ser-Ser-Ser) has been observed [338]. The presence of a hydroxyl group facili-
tated the degradation of the main chain of the N-terminal amino residue of
the peptide, resulting in amides. However, the hydroxyl group at the c-terminal
residue inhibited deamination. The deamination process decreased in oxygen-
ated solutions of di- and tripeptides [338]. The study on the radiolysis oxidation
of Thr and its derivative showed the presence of an α,β-aminoalcohol group
in compounds was responsible for the c-c bond cleavage [338, 339]. Initially,
nitrogen- and carbon-centered radicals were formed, which ultimately frag-
mented to cause degradation.
Products obtained in the oxidation of Arg by • OH are shown in Figure 4.28.
A hydrogen abstraction from the δ-carbon of the side chain and O 2 addition
produced a peroxy radical [340-342]. This initial attack on the δ-carbon has
been confirmed by EPR and NMR studies [343, 344]. A similar approach has
been used to identify radicals in reactions of • OH with aliphatic amines and
polyamines as well as compounds containing one or two guanidine groups
[343]. The loss of a guanidine group from the radical leads to the formation
of γ-glutamyl semialdehyde.
A number of products were formed in the reaction of His with • OH in which
some oxidation products have not been fully identified [14, 306]. Mass shift
products of +16, −22, −23, +5, and −10 Da have been identified in the oxidation
of the peptide DAHK and proteins (Fig. 4.29) [261]. Mass shifts of +16 Da and
−22 Da correspond to 2-oxohistidine and aspartic acid, respectively [345]. In