Formation of metastable disinfection byproducts during free and combined aspartic acid chlorination: Effect of peptide bonds and impact on toxicity

Abstract

The formation and occurrence of haloacetonitriles (HANs) in drinking water is of increasing concern because recent data have shown that they are the major contributors to DBP-associated toxicity of disinfected waters. Earlier research on HAN formation had established free amino acids as important HAN precursors due to their high reactivity with chlorine. However, free amino acids are unlikely to be the primary precursors for HANs in natural waters, mainly because the actual concentrations of these compounds are too low to sufficiently account for observed HAN formation. On the other hand, combined amino acids (i.e., peptides and proteins) are of much higher abundance even though it is unclear if they can contribute to HAN formation given that nearly all the amino nitrogen is tied up in peptide linkages. In order to clarify the reactivity of combined amino acids with chlorine to form HANs, dichloroacetonitrile (DCAN) formation kinetics was compared between free aspartic acid and two aspartyl-containing tetrapeptides (i.e., Asp-Asp-Asp-Asp and Arg-Gly-Asp-Ser). Results indicated that aspartyl residue could also lead to DCAN formation upon chlorination, whereas the rate of DCAN formation was much slower compared to that from free aspartic acid chlorination. Moreover, DCAN formation from the two model peptides was catalyzed by high pH. This is because chlorine-induced peptide backbone degradation is the key to DCAN formation from the chlorination of combined amino acids and this slow stepwise process is base-catalyzed. Perhaps most importantly, regardless of the precursors, DCAN was continuously formed but simultaneously degraded especially at alkaline pHs, leaving the corresponding N-chloro-2,2-dichloroacetamide (N–Cl-DCAM) and dichloroacetic acid (DCAA) as major end products. It was observed that over increasing chlorine exposure, there exists an important transition from initial organic precursors through metastable chlorination intermediates (e.g., DCAN and N–Cl-DCAM) and finally to stable end products (e.g., DCAA). By weighting DBP concentrations by their respective cytotoxic potencies, it is estimated that the aggregate cytotoxicity of chlorinated water would reach its maximum at relatively short chlorine contact times. In general, shorter water age and lower pH both resulted in higher levels of metastable intermediates (i.e., DCAN) and thus higher levels of aggregate calculated cytotoxicity. The resulting toxicity profile is different from the prevailing notion that supports current DBP regulations. Therefore, there is a risk that by placing regulatory limits and control strategies exclusively on regulated end products (e.g., HAAs), the overall toxicity of drinking water might be inadvertently elevated.