Evaluation of Histidine Reactivity and Byproduct Formation during Peptide Chlorination

Abstract

The covalent modifications resulting from chlorine reactions with peptide-bound amino acids contribute to pathogen inactivation and disinfection byproduct (DBP) formation. Previous research suggested that histidine is the third most reactive of the seven chlorine-reactive amino acids, leading to the formation of 2chlorohistidine, 2-oxohistidine, or low-molecular-weight byproducts such as trihalomethanes. This study demonstrates that histidine is less reactive toward formation of chlorine transformation products (transformation time scale of hours to days) than five of the seven chlorine-reactive amino acids, including tyrosine (transformation time scale of minutes). Chlorine targeted tyrosine in preference to histidine within peptides, indicating that chlorine reactions with tyrosine and other more reactive amino acids could contribute more to the structural modifications to proteins over the short time scales relevant to pathogen inactivation. Over the longer time scales relevant to disinfection byproduct formation in treatment plants or distribution systems, this study identified β-cyanoalanine as the dominant transformation product of chlorine reactions with peptide-bound histidine, with molar yields of ∼50% after 1 day. While a chlorinated histidine intermediate was observed at lower yields (maximum ∼5%), the cumulative concentration of the conventional low-molecular-weight DBPs (e.g., trihalomethanes) was ≤7%. These findings support the need to identify the high-yield initial transformation products of chlorine reactions with important precursor structures to facilitate the identification of unknown DBPs.

Characterization of Dissolved Organic Matter and Its Derived Disinfection Byproduct Formation along the Yangtze River

“…Recent studies investigating the chlorination of amino acids indicated that the yields of DBPs such as THMs and HANs were typically low (<5%), while other unidentified transformation products accounted for a greater proportion of TOX. 50 Moreover, the yield of TOCl decreased significantly from 218.5 to 81.4 μg-Cl/mg-C along the Yangtze River. From the perspective of chlorine reaction chemistry, the decreasing trend of the TOCl yield implied that electron transfer rather than halogen substitution played a more important role in chlorination.…”

“…Therefore, the yields of commonly known DBPs are typically low during chlorination of proteins, accompanying a high proportion of UTOX. Recent studies investigating the chlorination of amino acids indicated that the yields of DBPs such as THMs and HANs were typically low (<5%), while other unidentified transformation products accounted for a greater proportion of TOX . Moreover, the yield of TOCl decreased significantly from 218.5 to 81.4 μg-Cl/mg-C along the Yangtze River.…”

“…However, the UTOCl/TOCl ratio showed a slight increase in chlorinated downstream waters. This might be due to the elevated level of proteinaceous organics (Figure ) that can produce more UTOX during chlorination . α-amino groups and side chains of proteins can rapidly react with chlorine to form transforming products, but the cleavage of peptides and carbon–carbon bonds in proteins is difficult, which required a long series of reactions to liberate 1–2-carbon DBPs .…”

“…This might be due to the elevated level of proteinaceous organics (Figure 2) that can produce more UTOX during chlorination. 50 α-amino groups and side chains of proteins can rapidly react with chlorine to form transforming products, but the cleavage of peptides and carbon−carbon bonds in proteins is difficult, which required a long series of reactions to liberate 1−2carbon DBPs. 51 Therefore, the yields of commonly known DBPs are typically low during chlorination of proteins, accompanying a high proportion of UTOX.…”

Hypochlorous Acid: From Innate Immune Factor and Environmental Toxicant to Chemopreventive Agent Targeting Solar UV-Induced Skin Cancer

“…Likewise, imidazole-derivatives (e.g. L-histidine, carnosine, carcinine) and thio-imidazolederivatives (ergothioneine and sea urchin-derived ovothiol) have been identified as potent chlorination stress inhibitors (111)(112)(113).…”