HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 316

Most studies on the permanganate oxidation of amino acids and amines
used integration to evaluate k 1 and k 2 . Linear plots of ( v / c ) versus c of the
differential method were applied to determine rate constants for the autocata-
lytic reactions of permanganate with formic acid, dimethylamine, trimethyl-
amine, Gly, L-Ala, and L-Thr.
The kinetics of the permanganate-Gly reaction displayed a curvature in the
plot. The curvature was dependent on the concentration of phosphate in the
reaction mixture. upward and downward curvatures were observed at low and
high phosphate concentrations. A downward curvature increased with an
increase in the initial concentration of permanganate. Effects of other param-
eters of the reaction were also examined. The downward curvature decreased
with an increase in the concentration of the phosphate buffer, pH, and tem-
perature, and decreased under the use of either cationic surfactant (benzyltri-
ethylammonium chloride) or polymeric protective colloidal (gum arabic). The
phosphate concentration dependence demonstrated that phosphate inhibited
the autocatalytic reaction pathway. This was supported by the plotted results
of the bell-shaped rate versus time, which was greatly affected by the concen-
tration of phosphate (Fig. 6.20a). Importantly, the influence of phosphate was
only observed when the differential method was applied (Fig. 6.20B). The
integration method (Eq. 6.72) had almost no concentration dependence for k 1
and k 2 results (Fig. 6.20b). In contrast, the rate constants obtained by the dif-
ferential method (Eq. 6.73) had a strong variation depending on the concentra-
tion of phosphate (Fig. 6.20b). This is an indication of the involvement of
phosphate in the autocatalytic pathway of the mechanism.
The reaction mechanism is expressed in Equations (6.74)-(6.82), in which
the NH 2 CH 2 COO − species was postulated as the reactive form of Gly to react
with permanganate:

NH CH COO

+ −

H NH CH COO

+

+

−

K (6.74)

 3 2 2 2 74

MnO NH CH COO

−

+

−

→ HMnO NH CH COO

−

+

• −

k (6.75)

4 2 2 4 2 75

MnO NH CH COO

−

+

• −

→ MnO

2 −

+ NH CH CO = + k (6.76)

4 2 4 2 2 76

Figure 6.20. (a) Rate versus time plots for the reaction of KMnO 4 (5.12 × 10 −4 M) with
Gly (0.160 M) in KH 2 PO 4 (circles, 0.032 M; triangles, 0.080 M)−K 2 HPO 4 (circles, 0.032 M;
triangles, 0.080 M) buffer, at ionic strength 0.320 M (KCl), pH 6.64, and 25.0°C. (b)
Dependence of the apparent rate constants for the noncatalytic (bottom) and auto-
catalytic (top) reaction pathways, obtained either by the integrated method (empty
circles) or by the differential method with extrapolation at t = 0 (filled circles), on the
total phosphate concentration during the reaction of KMnO 4 (5.12 × 10 −4 M) with Gly
(0.160 M) in KH 2 PO 4 −K 2 HPO 4 buffer ([KH 2 PO 4 ] = [K 2 HPO 4 ] = [phosphate] T /2), at
ionic strength 0.320 M (KCl), pH 6.64, and 25.0°C (adapted from Perez-Benito [234]
with the permission of the American Chemical Society).