Chemistry Reference
In-Depth Information
TABLE 6.7. Observed Rate Constants for the Reactions of Methyl-Substituted
Amino Acids (10 mM) with Ferrate(V) at pH 8.8 and 23°C in 0.025 M Phosphate
Compound
Structure
k
(/M/s)
Gly
H
2
CNH
2
COOH
3.0 ± 1.0 × 10
5
N-Methylglycine
H
2
CNH(CH
3
)COOH
1.2 ± 0.3 × 10
5
N-Dimethylglycine H
2
CN(CH
3
)
2
COOH
<5.0 × 10
3
(estimated upper limit)
(CH
3
)
2
CNH
2
COOH
1.2 ± 0.3 × 10
5
α-Aminobutyric
7.0 ± 0.2 × 10
4
β-Aminobutyric
HOOCCH
2
CH(CH
3
)NH
2
Adapted from Rush and Bielski [382] with the permission of Harwood Academic Publishers
GmbH).
substitution at pH 8.8 (Table 6.7). Dimethylglycine showed no reactivity with
ferrate(V). Other substituted molecules were only slightly less reactive than
Gly. The comparative reactivity suggests that the attack of ferrate(V) was most
likely at the nitrogen atom of the amino acids. A recent study showed the
simultaneous formation of acetate, carbon dioxide, and ammonia for the oxi-
dation of Gly by ferrate(VI). This indicates the possibility of an attack on
nitrogen or the α-carbon atoms of Gly by the
FeO
2−
molecule to explain the
observed products [378].
In the presence of excess ferrate(VI) and a
60
Co gamma ray source under
steady-state conditions, a chain mechanism has been proposed for the oxida-
tion of amino acids (AAs) (Eqs. 6.126-6.131) [377]:
Initiation and propagation steps
FeO
2
−
+
AA
•
→
FeO
3
−
+
products
(6.126)
4
4
FeO
3
−
+
RCH NH COO
(
+
)
−
→
FeO
4
−
+
AA
•
(6.127)
4
3
4
FeO
4
−
+
RCH NH COO
(
+
)
−
→
Fe
3
+
+
AA
•
(6.128)
4
3
Termination steps
FeO
3
−
+
AA
•
→
FeO
4
−
+
products
(6.129)
4
4
FeO
4
−
+
AA
•
→
Fe
3
+
+
products
(6.130)
4
Fe
3
+
+
AA
•
→
Fe
2
+
+
products
.
(6.131)
Radical-radical reactions as well as reactions between similar ferrate
species are other possible termination steps. The rates of the individual initia-
tion step of the mechanism have been determined independently [379]. The
second-order rate constants for the reactivity of ferrate(VI) with radicals are
1.4 × 10
9
/M/s, 1.2 × 10
9
/M/s, and 3.3 × 10
8
/M/s at pH 12.4 and 24°C for Gly, Ala,
and Asp, respectively. Ferrate(V) and ferrate(IV) have shown three to five
orders of magnitude higher reactivity than ferrate(VI). The rate constants for
ferrate(V) with radicals are thus expected to be much greater than ferrate(VI).
Search WWH ::
Custom Search