REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 165

rich sites of organic molecules [14]. The • OH radical reacts via a hydroxyl
addition to the carbon-carbon double bonds or aromatic rings or through
hydrogen abstraction from saturated carbon sites of the molecules. These reac-
tions produce transient radical species, which undergo further reactions
depending on the radical and structural environment [261]. The factors that
control both types of reactions include the possible number of sites available
for the • OH attack, the electronegativity of the substituents on the target sites,
the strength of the c-H bond, the steric effects, and the nature of the produced
organoradical [278]. For example, among the amino acids, H-abstraction from
cys was the most accessible because the average single bond energies for S-H,
O-H, N-H, and c-H are 363, 459, 386, and 411 kJ/mol, respectively, at 25°c.
The order of c-H reactivity for alkane functional groups was usually ter-
tiary > secondary > primary. Based on several factors involved in the reactiv-
ity of • OH with organic molecules, a kinetic model using a group contribution
method was developed [279]. This model reasonably predicted the rate con-
stants for several organic molecules. Quantum mechanical methods were also
used to estimate aqueous-phase free energy of activation of reactions of • OH
with carboxylate ions [280].
The second-order rate constants for the reactions of • OH with amino acids
are provided in Table 4.11. The variation in rate constants ranged from 10 7 to
10 10 /M/s. The relatively small variation indicates almost all side chains of the
amino acids were oxidized by • OH. Sulfur- and aromatic-containing side chains
had the highest reactivity (Table 4.11). gly had higher reactivity than other
aliphatic amino acids due to the influence of steric effects on the rates [281,
282]. Furthermore, the secondary α-carbon radical produced in the case of gly
appears to be more stable than the tertiary α-carbon radical, formed in other
amino acids. generally, less reactive radical species may have relatively higher
reactivity toward gly and other α-carbon sites of proteins [281]. Peptides
reacted somewhat faster than the parent free amino acids, and the rate con-
stants ranged from 10 8 to 10 9 /M/s (Table 4.11). Proteins were highly reactive
with diffusion-controlled rate constants (Table 4.11).
The rate constants of Table 4.11 suggest the • OH radical can easily cause
damage to both the side chain and backbone of proteins, causing fragmenta-
tion of the proteins. cleavage of the main chains and oxidation of different
residue side chains of proteins by • OH are described below. The focus is on
the progress made in the last few years, particularly the application of mass
analysis of the products formed in the reactions.
4.4.2.1 Main-Chain Cleavage of Protein. Backbone cleavage through
H-abstraction at the α-carbon site is shown in Figure 4.20. Two major pathways
following the initiation of the formation of radicals occur [261]. One pathway
involves the loss of HO • from the peroxy radical and then hydrolysis of the
newly produced imine species. In the other pathway, formation of the alkoxy
radical at the α-carbon takes place, which ultimately results in further frag-
mentation to cleave the backbone of the protein. The scheme in Figure 4.20