HALOGENATED SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 82

TABLE 3.1. ( Continued )

Compound pH T (°C) k (/M/s) Reference

Pro-gly 7.0 25 2.7 × 10 5 a [27]

c-Ser-Tyr 7.8 25 4.9 × 10 2 a [29]

gSH 5.0, 7.4, and 9.0 25

≥1 × 10 7

[24]

Fe III cytc >1 × 10 5

7.6 25 [29]

a Calculated.
b Reported for modeling the reactivity of proteins.

Figure 3.3. Influence of the pH on the rate of chlorination of dipeptides. Experimental
conditions: [chlorinating agent] = [dipeptide] = 1.2 mM; I (NaCl) = 0.50 M; T = 298.15 K
(adapted from Armesto et al. [27] with the permission of the Royal Society of
Chemistry).

reactivity is Met > Cys >> Cystine ∼ His ∼ α-amino ∼ Arg > Trp > lys >> Tyr
> gln ∼ Asn (Table 3.1).
The second-order rate constants for the chlorination of dipeptides have also
been studied [27, 28, 33-37]. The rate constants for some of the dipeptides
indicate pH dependence, shown in Figure 3.3 [27]. The maximum rate con-
stants were observed at the arithmetic means of the p K a values of HOCl and
the terminal amino groups of the dipeptide [27]. The rate constants for the
studied peptides at neutral pH are summarized in Table 3.1. The rate constants
were on the order of 10 5 /M/s (Table 3.1). The reactivity of HOCl with numer-
ous model proteins showed the rates were highly dependent on the peptide
environment [28]. The reaction of HOCl with cyclic peptides was rapid