HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 332

TABLE 6.5. Pseudo-First-Order Rate Constants for Decomposition of [Fe IV (O)
(N 4 Py)] 2+ by Amino Acid Substrates Ac-AA-NHtBu ([Fe] = 1 mM) a,b
Entry k obs (/s) c Ac-AA-NHtBu k rel

1 Cys 1.7(1) 340,000

2 Tyr 3.4(1) × 10 −1 68,000

3 Trp 1.7(1) × 10 −2 3,400

4 Met 3.2(1) × 10 −3 640

5 Gly

5.8(2) × 10 −5

12

3.8(2) × 10 −5

6 His 7.6

7 Ser 3.3(7) × 10 −5 6.6

8 Ala 2.5(2) × 10 −5 5.0

9 Asp 1.7(3) × 10 −5 3.4

10 Gln 1.6(2) × 10 −5 3.2

11 Thr 1.4(6) × 10 −5 2.8

12 Phe

1.4(4) × 10 −5

2.8

1.4(1) × 10 −5 13 Asn 2.8

14 Arg·HCl d 1.4(1) × 10 −5 2.8

15 Lys·HCl d 1.4(2) × 10 −5 2.8

16 Glu 1.3(1) × 10 −5 2.6

17 Val 1.3(2) × 10 −5 2.6

18 Leu 7.4(3) × 10 −6 1.5

19 Pro 5.0(8) × 10 −6 1.0

20 Ile

5.0(5) × 10 −6

1.0

a Reaction conditions: 1 : 1 H 2 O/CH 3 CN, 298 ± 2 K, 10 mM substrate (10 equiv).
b Background rate for decomposition of [Fe IV (O)(N4Py)] 2+ in the absence of a substrate was
5.0(2) × 10 6 /s.
c Number reported is the average of at least three runs; error as standard deviation is given in
parentheses.
d HCl salts of the basic amino acids were used to avoid strongly basic conditions.
Adapted from Abouelatta et al. [334] with the permission of the American Chemical Society.

The kinetic isotope effect (KIE) and the analysis of products were per-
formed to understand the mechanisms of the reactions of [Fe IV (O)(N4Py)] 2+
with amino acids [334, 335]. Gly reacted 12 times faster than the control reac-
tion (Table 6.5). In D 2 O/CD 3 CN, the rate was similar to the rate in H 2 O/
CH 3 CN, which suggests the deprotonation of an amide proton of Ac-Gly-
NhtBu did not participate in the rate-determining step of the decomposition
of [Fe IV (O)(N4Py)] 2+ . However, when deuterated Gly at the α position (2,2- d 2 )
was used, the decomposition of the ferryl species was much slower
( k obs = 1.2 × 10 5 /s), resulting in a KIE of 4.8. The generation of a glycyl radical
was proposed (Fig. 6.30). A hydrogen atom transfer (HAT) may also occur
to yield a glycyl radical. In the case of other aliphatic amino acids, Ala,
Val, Leu, Ile, and Pro, rapid decomposition of the ferryl species did not occur
(Table 6.5). This observation suggests the abstraction of the α-H atom with