HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 353

by ferrate(V). The second-order self-decay of the HFeO 2− species, which has
a rate constant of ∼10 7 /M/s, is similar to the rate constant of HFeO 2− with the
amino acid anion (Table 6.10). The lower reactivity of the zwitterion species,
RCH NH COO

+ − , than the anionic species, RCH(NH 2 )COO − , is similar to the
reactivity of • OH with amino acids. The results indicate that the Fe V =O system
may abstract a hydrogen atom from the nitrogen within the amino acids. The
pH dependence of the rate suggests the attack of ferrate(V) is also at the
nitrogen site rather than at the sulfur site of Met. In contrast, the reactivity of
• OH with Met undergoes an attack at the sulfur site at the near diffusion-
controlled rate [344].
In addition to amino acids, the reactivity of ferrate(V) with a number of
carboxylic acids has been studied [382, 383]. The rates for the oxidation of
carboxylic acids by Fe(V) depend on the nature of the substituent group at
the α-carbon atom of the acids and decrease in the order α-C-NH 2 > α-C-
OH > α-C-H [382, 383]. It has been suggested Fe(V) oxidizes amino acids and
carboxylic acids by a two-electron step (Fe(V) + amino acids → Fe(III) + NH 3
+ α-keto acids). For example, the oxidation of aspartic acid by Fe(V) yielded
Fe(III), ammonia, and oxaloacetate. The steps of the mechanism are expressed
in reactions (6.142) and (6.143):

( )

3

HFe O

V 2 −

+

−

OOCCH CH NH COO ( )

−

→ Fe

III

+

−

OOCCH C NH COO ( = )

−

4 2 2 2

(6.142)

− − − −

OOCCH C NH COO H O ( = ) + → OOCCH C O COO NH ( = ) + .

(6.143)

2 2 2 3

6.3.2.3 Aminopolycarboxylates. The kinetics of the oxidation of APCs by
ferrate(VI) and ferrate(V) were conducted in detail [384, 385]. The structures
of the studied APCs are given in Figure 6.36. Among the studied compounds,
IDA is a secondary amine (2 o ), and NTA, EDTA, DTPA, and TTHA are ter-
tiary amines (3 o ). The reactions followed a second-order rate law, first order
with respect to the concentration of each reactant. The reactivity of ferrate(V)
was three to five orders of magnitude more reactive than ferrate(VI) (Table
6.9). Both ferrate(VI) and ferrate(V) showed a decrease in the reaction rates
with an increase in pH in the alkaline medium. Significantly, the rate constants
demonstrated the primary amine had the highest rate than the secondary and
tertiary amines, with the order of reactivity primary > secondary > primary
(Table 6.9). The results suggest the oxidation occurred at the N-C bond
because, if the oxidation occurred between the acetate carbons, a distinct sepa-
ration in the reaction rates would not have been observed [386]. Among the
tertiary amines, the rates increased with an increase in the -CH 2 COOH group,
and thus, the rates increased in the order NTA < EDTA < DTPA < TTHA.
A spectral study using premix pulse radiolysis experiments was performed
to determine whether Fe(IV) is formed in the oxidation of APC by Fe(V)
[385]. The spectra obtained for IDA and EDTA are shown in Figure 6.40 [385].