REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 167

shows the role of oxygen in achieving significant peptide fragmentation. In the
absence of oxygen, less yield of fragmentation is expected.
Formation of the α-carbon radicals may also result from the • OH attack at
the β-carbon (c-3) position (Fig. 4.21). The initial H-abstraction generates
carbon-centered radicals, followed by the reaction with O 2 to yield peroxy
radicals [283] (Fig. 4.21). The alkoxy radical may form from several different
pathways using different precursors such as decomposition of intermediate
hydroperoxides and termination reactions of the peroxy species with other
radicals [284, 285]. The β-scission of the alkoxy radicals at a rate of >10 7 /second
may lead to the cleavage of the backbone [283]. The nature of the substituents,
R and R′, determines the rate of the reactions [286]. The side chains of Asp,
Val, and Leu usually go through such β-scission reactions to yield carbonyl
products. The oxidation of gly side chain by • OH via the abstraction of H from
the γ-carbon site is shown in Figure 4.22. The γ-carbon peroxy radical, which
forms in the presence of O 2 , ultimately causes production of the modified chain
products of different mass shifts [12] (Fig. 4.22). Decarboxylation with a mass
shift of −30 was the major product, while mass shifts of +14 and +16 were minor
products [287]. The dehydropeptide readily converted to keto form, which on
hydrolysis formed a new amide and a keto acid-containing protein fragments
[288].
The oxidation of Pro to the 2-pyrrolidone derivative by • OH is presented
in Figure 4.23. Acid hydrolysis of the derivative to the γ -amino butyric acid
leads to backbone cleavage of the protein [289, 290]. Oxidation of the c-5 site
of Pro produced a pyroglutamyl residue with a mass shift of +14 Da, which on
acid hydrolysis yielded glutamyl, causing backbone cleavage of the peptide.
The products of mass shift +16 were also formed (Fig. 4.23). glutamyl semial-
dehyde was the equilibrium product of 5-hydroxyproline.

R

•

R R ′

R R ′

O R ′

•

O O

α O

HO • H 2 O O 2

N N

N

R1 H R2 H

R1 R2

H

R1 R2

O

O

O

O

R ′

R

R •

O

O R ′

N

O

•

Fragments

H N

R1 R2

H

R1 R2

O

(

R1, R2 = N- and C-terminal parts of protein
R, R' = Substitutents at β -carbon of side chain

)
(Backbone cleavage)

O

(Side-chain cleavage)

Figure 4.21. Backbone cleavage by radical transfer from the β-carbon at side chains
(adapted from xu and chance [261] with the permission of the American chemical
Society).