REACTIVE INORGANIC OXY-SPECIES OF C, N, S, AND P - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 212

free energy change does not fully controlled the reaction rates of the reactions
with organic compounds. The QSARs for the effect of substituents on the
rates for the same categories of substrates with different oxidants were devel-
oped [51]. The sensitivity to substituent groups increased in the order
•

3 1 2 2 3 . Furthermore, this order was correlated appro-
ximately with the inverse order of reduction potentials for one-electron trans-
fer. This suggests that the magnitude of second-order rate constants for
oxidation is defined by the reaction mechanism.
The rate constants of the reactions of CO •− with carbohydrates were deter-
mined in the range of 10 5 -10 7 /M/s and the rates increased with increasing pH
from 8.0 to 13.0 [52]. This indicates that carbohydrates were activated toward
oxidation due to deprotonation. At low pH, the mechanism involved hydrogen
abstraction and/or electron/proton transfer [52]. However, deprotonated car-
bohydrates reacted via electron transfer at high pH [52]. Table 5.2 represents
the rate constants of the reactions of CO •− with compounds of biological inter-
est. The rate constants vary from 4.0 × 10 5 /M/s to 1.4 × 10 9 /M/s [1, 27, 43, 44,
49, 53-61]. The activation energies for the reactions of CO •− with organic
compounds were generally low and also independent of the driving force, and
hence, oxidation occurred via inner-sphere mechanisms [40].
Butyl amine had the lowest reactivity, while the reducing substrate, ascor-
bate, had the highest reactivity. Primary amines have been reported to react
primarily by hydrogen abstraction [40, 42, 62]. The reactivity of the anionic
form of gly at pH 11 was higher than the neutral species at pH 7.5 (Table 5.2).
Aromatic amines appear to react by electron transfer, suggested by their
higher rate constants with CO •− (Table 5.2). Cys reacted by electron transfer
from the −S − group [40]. Reactions of Cys and Met most likely involve an
addition to the sulfur [40]. Significantly, the sulfur-containing amino acids
reacted similarly with CO •− ; however, Met showed a negative temperature
dependence, while the rates for the reaction of cystine with CO •− increased
with an increase in temperature. The observed results suggest the formation
of the perthiyl radical through an intermediate complex in the cystine reaction
with CO •− (Eqs. 5.11, 5.12):

•−

OH CO < ≈ O ClO O < ≈

CO

•−

+ RSSR ( RSSR CO )

• −

(5.11)

3 3

( RSSR CO OH )

• −

+

−

→ RSS ROH CO

•

+ +

2 −

. (5.12)

3 3

Comparatively, the reaction of CO •− with Met initially resulted in an inter-
mediate complex (Eq. 5.13). The decomposition of the complex requires
charge separation (Eq. 5.14), a different pathway in comparison to the reaction
of CO •− with cystine. As the temperature increases, decomposition of the
complex back to reactants is able to compete with the path of forming prod-
ucts. Thus, this causes negative temperature dependence for the overall reac-
tion of CO •− with Met:

CO

•−

+ RSR RSR CO

• −

(5.13)

3 3