HALOGENATED SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 104

3.3.3 Reactivity of ClO 2
The reactivity of ClO 2 with a number of inorganic and organic compounds has
been studied [152, 154-162]. The rate laws were first-order with respect to the
concentrations of ClO 2 , and these compounds and their resulting second-order
rate constants varied from 10 −5 to 10 5 /M/s. The rate constants were high for I − ,
NO − , O 3 , H 2 O 2 , and Fe(II). The rate constants for tertiary amines and phenols
were also high at pH ≥ 6. Ammonia, Br − , primary and secondary amines, car-
bohydrates, aromatic hydrocarbons, and compounds containing olefic C=C
double bonds were unreactive with ClO 2 at near neutral pH conditions.
In the oxidation of Fe(II) by ClO 2 , a five-electron transfer has been pro-
posed (Eq. 3.23) [157-159]:

.

(3.23)

ClO + 5 Fe

2 +

+ 4 H

+

→ + Cl

−

5 Fe

3 +

+ 2 H O

2 2

Both inner-sphere and outer-sphere electron transfer pathways were sug-
gested for the reaction [159]. Among the organic compounds, a detailed analy-
sis of the kinetics has been conducted for the phenolic compounds [156]. The
rate constants for the reactivity of ClO 2 with phenoxide anions were approxi-
mately six orders of magnitude higher than their protonated analogues. The
rate constants for phenoxide anions ( k ArO − ) were analyzed using the Hammett
equation (Eq. 3.24):

∑

log k − = 8 2 0 2 . ( ± . ) − 3 2 0 4 . ( ± . ) σ

−

; n = 23 , s = 0 39 . ; r = 0 97 . , (3.24)

o m p , ,

ArO

where σ o m p

− are the constants for ortho-, meta-, and para-substituents. The
negative sign of the slope of Equation (3.24) indicates that the oxidation by
ClO 2 was through a free radical mechanism in which increase in electron
density at the reaction center increased the reaction rate [156]. Furthermore,
the rate-determining step involving primarily an outer-sphere single-electron
transfer step from the phenoxide anion to ClO 2 was supported by the Marcus
correlations [156].

, ,

3.3.3.1 Amino Acids, Peptides, and Proteins. The reactivity of biologically
important molecules including amino acids and some peptides with ClO 2 has
been performed [154, 155, 163-171]. A pseudo-first-order decay of chlorine
dioxide was detected in these reactions [155]. Of the 21 amino acids and three
peptides (l-aspartyl-l-phenylalanine methyl ester (aspartame), l-glycyl-l-
tryptophan and l-tryptophylglycine) reacted, only a few showed reactivity
with ClO 2 at pH 6.0 [155]. Among the amino acids and peptides, Cys, Trp, Tyr,
l-gly-l-Trp, and l-Trp-gly reacted rapidly, while His, Pro, and hydroxypro-
line had slower rates. The second-order rate constants for their reactions are
presented in Table 3.7 [152, 155, 163, 168, 170, 171]. The order of the rate con-
stants for amino acids and peptides was Cys > Tyr > Trp ∼ N -acetyl-Tyr >
gSH > OH-Pro > His > Pro.