HALOGENATED SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 83

(2.0 × 10 1 /M/s per amide group). However, the incorporation of a charged side
chain decreased the reactivity roughly by an order of magnitude. Similar
charge dependence was observed for the rates of reaction of N -acetyl blocked
amino acids, di- and tripeptides. The rates were generally much slower
( k = 10 −3 -10 −1 /M/s) [28]. The chlorination rates of the amide bonds in the tri-
peptide models were greater than those of the blocked amino acids. generally,
chlorination primarily occurs at the side chain (and N -terminal α-amino
group) rather than at the backbone sites of the protein. The second-order rate
constants for the oxidations of glutathione (gSH) and Fe III cytc by HOCl/
OCl − showed rapid reactivity in the acidic to basic pH range (Table 3.1).
3.1.1.2 Products of HOCl Oxidation. Products for the reactions of amines,
α-amino groups, and lys with HOCl have been studied in detail [33]. The
reactions yielded unstable monochloramines. In the presence of excess HOCl,
dichloroamine species were formed [38, 39]. Recently, a detailed product anal-
ysis for the chlorination reaction of gly in water was studied by applying
isotopically enriched ( 13 C and 15 N) samples of gly and 1 H, 13 C, and 15 N NMR
spectroscopy [40]. The results showed the C1 carboxylic acid carbon of gly
was quantitatively converted to CO 2 , while the C2 methylene carbon of gly
was transformed to CO 2 and formaldehyde monohydrate. CO 2 was the pre-
dominant product at pH 6-9. However, the formaldehyde monohydrate was
the likely product under acidic reaction conditions (pH < 6). The proposed
mechanism for the chlorination of gly is provided in Figure 3.4 [40]. Initially,
N -chloroglycine (I) was obtained when one equivalent of aqueous chlorine
reacted with gly. N -Chloroglycine was converted to N , N -dichloroglycine (II)
when more than 1 equiv of aqueous chlorine was used. The formation of N -
chloromethanimine (III) as a transitory product occurred through the decar-
boxylation and elimination of HCl of labile N , N -dichloroglycine (II). Under
acidic pH, N -chloromethanimine (III) may be hydrated to form N -
chloroaminomethanol (IV), which is quantitatively transformed to formalde-
hyde monohydrate (VI). One other possibility to consider is the elimination
of the second mole of HCl from N -chloromethanimine (III) to likely form
cyanide (V), which, upon chlorination, yielded CNCl (VII). The hypochlorite-
assisted catalytic hydrolysis of CNCl (VII) formed CO 2 , also previously
reported [41]. Further hydrolysis of CNCl (VII) and/or decomposition of
chloroaminomethanol (IV) in the presence of excess chlorine yielded N 2 and
nitrate.
The products of the chlorination of other amino acids have been studied
mostly for His and Trp. Product analyses for the reactions of gln and Asn with
HOCl have not been carried out, possibly due to the slow rates of reactions
(see Table 3.1). The reaction of l-Arg with HOCl was rapid, and the products,
identified by MS, were monochlorinated and dichlorinated adducts of l-Arg
[30]. The proposed structure of the chlorinated adducts (Fig. 3.5) showed the
formation of chloramine at the guanidine group [30]. An important role of
chloramines in endothelial cell dysfunction was suggested because such species