HALOGENATED SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 80

1.0

HOSCN H OBr

HOCl

0.8

0.6

0.4

0.2

OBr -

OCl -

- OSCN

0.0

4.0 5.0 6.0 7.0 8.0 9.0 10.0

pH

Figure 3.2. The species of aqueous Cl 2 , Br 2 , and HOSCN as a function of pH at 25°C.

of the compound, which contribute to an overall second-order reaction. The
second-order rate constants ( k ) of HOCl and OCl − for a particular compound
varied significantly with pH in the chlorination reactions [19]. HOCl was gener-
ally the major reactive species. The variation of HOCl species as a function of
pH is presented in Figure 3.2. The reactivity of Cl 2 with inorganic molecules
generally derived from an initial electrophilic attack on HOCl and k for organic
compounds varied from <10 −1 to 10 9 /M/s. The possible pathways in the reaction
mechanism include oxidation, addition, and electrophilic substitutions [19]. In
a recent study, the roles of chlorine monoxide (Cl 2 O) and Cl 2 have been dem-
onstrated in the chlorination of molecules that are of environmental interest
[20]. The proposed reactive species, Cl 2 O and Cl 2 , were generally present at low
concentrations but were shown to play a greater role than HOCl in the chlori-
nation process.
Table 3.1 reports the second-order rate constants for the chlorination of
amino acids and peptides [21-32]. Met and Cys reacted most rapidly with
chlorine ( k > 10 7 /M/s). Aromatic acids, with the exception of Tyr, showed
reasonable reactivity ( k ∼ 10 4 -10 5 /M/s). Tyr had reactivity of the order of
10 1 /M/s. Ser and lys reacted similarly. Arg, Asn, and gln showed the lowest
reactivity ( k ∼ 10 −2 -10 1 /M/s). The rate constants for the chlorination of
gly, Ala, β-Ala, Val, and iso -leu were on the order of 10 4 -10 5 /M/s, similar to
the observed rate constants for basic aliphatic amines [19]. The order of