HALOGENATED SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 109

content as well as in the transition temperature and endothermic transition
enthalpy of heat-induced unfolding [173]. The decrease in the concentrations
of ClO 2 with time in reactions with BSA and g6PD was exponential. The
denaturing of proteins by ClO 2 was also confirmed by the absorbance and
fluorescence spectra of proteins with and without ClO 2 [173]. Residues of Trp
and Tyr were destroyed in the oxidation process. Product analysis showed the
Trp residue was converted into NFK, DOPA, and 2,4,5-trihydroxyphenylalanine
(TOPA) in the ClO 2 -treated proteins [173]. Experiments with 18 O labeling of
peptides in the reaction with ClO 2 provided evidence of the incorporation of
oxygen from ClO 2 into the oxidation products.
3.4 CONCLUSIONS
Significant information on the HOCl reactivity with amino acids and proteins
is known to assess targets for the reactions of proteins with HOCl. However,
similar knowledge on other oxidants, HOBr and HOSCN, is lacking. The
chemistry of HOBr appears to be similar to HOCl, but much higher reactivity
of HOBr than HOCl has been observed, particularly for the aromatic amino
acids. These results have important implications for the biological environ-
ment. Chlorination results in chloramines, which can either transform into
products or react with side chains of proteins. The products include oxyacids
and carbonyls. Side chains of thiols are highly reactive with HOCl, which first
form sulfenyl chloride before converting into sulfoxides and sulfonamides.
Halamines such as NH 2 Cl, NH 2 Br, gly-Cl, taurine-Cl, and taurine-Br, formed
from the reactions of HOCl and HOBr, may also play a role in the reactions
of biological systems. Reduction of halamines may produce damaging halogen-
and nitrogen-centered radicals in the process. However, there are limited rate
studies of the reactions of these oxidants with proteins that have been con-
ducted. HOSCN has not shown much reactivity with free amino acids, but the
side chains in proteins have significant reactive rates with HOSCN. The reac-
tions of HOSCN with protein thiols form a disulfide and a dimer. This suggests
the need to examine reactions of HOSCN with different proteins in order to
understand the role of HOSCN in protein oxidation in vivo and in cellular
signaling.
ClO 2 is generated from the chlorite ion by using chemical, electrochemical,
and biocatalysis methods. The use of Cld to catalyze the chlorite ion may be
imperative not only in generating ClO 2 but also in gaining insight on the
interaction of the chlorite ion with heme proteins and metalloporphrins and,
thus, warrants further studies. ClO 2 showed reactivity with Cys, Trp, and Tyr,
and the formation of oxidized products occurred through the incorporation of
oxygen from ClO 2 .
ClO 2 as a disinfectant has shown equal or better effectiveness than com-
monly used chlorine. The concern of halogenated disinfection by-products
(e.g., trihalomethane and halo acids) can be minimized using ClO 2 . Aside from