HALOGENATED SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 89

TABLE 3.2. Second-Order Rate Constants (with 95% Confidence Limits) at 22°C
and pH 7.2-7.5 for Reactions of Hypobromous Acid with Amino Acids, Free
α-Amino Groups, and Amides
Substrate Substrate

k (/M/s) k (/M/s)

gly 2.6 × 10 6 N -Acetyl-Cys 1.2 × 10 7

Cyclo(gly) 2 9.0 × 10 2 ( N -Acetyl-Cys) 2 3.4 × 10 5

Ala 1.6 × 10 6 N -Acetyl-Met-OMe 3.6 × 10 6

Cyclo(Ala) 2

2.5 × 10 2 2.6 × 10 5 N -Acetyl-Tyr

7.0 × 10 −2 3.7 × 10 6

N -Acetyl-Ala N -Acetyl-Trp

N -Acetyl-Ala-OMe 2.1 × 10 0 Cyclo(Ser) 2 5.5 × 10 2

Val 1.7 × 10 6 Cyclo(Asp) 2 5.0 × 10 1

N -α-Acetyl-lys 3.6 × 10 5 Propionamide 3.3 × 10 0

N -Acetyl-Arg-OMe 2.2 × 10 3 2-Methyl propionamide 1.5 × 10 0

Trimethylacetamide 1.5 × 10 0

Data are taken from Pattison and Davies [64].

and ∼450 times higher than with HOCl, respectively. Rates were also faster
for HOBr than HOCl for the double bonds of unsaturated fatty acid chain
model compounds such as pentenoic acid and sorbate [65].

3.1.2.2 Products of HOBr Oxidation. The reactions of HOBr are usually
analogous to those of HOCl. HOBr is reactive with thioether, thiol, disulfide,
and amine functions [1]. The oxidation of amino acids and dipeptides by HOBr
in alkaline bromine resulted in nitriles with one less carbon atom [66]. The
oxidative deamination of nitriles at pH 9.4 of di- and higher peptides yielded
N -(α-ketoacetyl) peptides. Several studies showed the formation of bro-
moamines (R-NHBr) and bromamides (R-C(O)-NBr-R′) as major products
in the bromination of amino acids, peptides, and proteins [64, 67-69]. Bro-
moamines are usually less stable ( t 1/2 < 15 minutes) than the bromamides
( t 1/2 > 15 minutes) [68, 70]. However, the amides of bromamides of gly and
Ala had half-lives of 12 and 35 minutes, respectively. generally, N -bromo
species were less stable than corresponding chloro-species [71, 72].
The decay products of N -bromo species of the soybean trypsin inhibitor
(STI), lysozyme, and bovine serum albumin (BSA) were followed using dif-
ferent analytical techniques such as UV-vis spectroscopy, fluorescence, elec-
tron paramagnetic resonance (EPR) spectroscopy with spin trapping, and
high-performance liquid chromatography (HPlC) [67, 68]. Radical intermedi-
ates in the oxidation of BSA by HOBr were observed. Initially, nitrogen-
centered radicals on side-chain and backbone amide groups formed, which
readily converted into C-centered radicals through rearrangement reactions.
These radicals play an imperative role in the fragmentation of proteins. The
final stable products include protein carbonyls, brominated Tyr residues,