Abstract
Peptides and protein fragments that have been monochlorinated at terminal amino‐N sites are known to react sluggishly with S(IV) dechlorinating agents in wastewater treatment plants. Persistence of these chloramines may imperil downstream aquatic organisms. Reactivity of sidechain amino‐N is explored in this paper, and previous kinetic information on chloramine reduction by S(IV) is summarized. Much like other chloramines, side‐chain chlorinated N‐chloroacetyllysine reacts with SO1/2‐ in borate buffer at 25°C according to the law rate = [SO^2−^~3~ [chloramine](520[H~3~BO~3~] + 5.7 × 10^10^[H~3~O^+^] + 4.1) (time in s, concentration in M). Apparently because of their greater basicity, aliphatic monochloramines such as Nchloroacetyllysine are reduced rapidly at near‐neutral pH, in contrast to terminally chlorinated peptides. At pH > 10, all monochloramines will be reduced at nearly the same, quite slow rate (t~1/2~ ≈ 0.1/[SO^2−^~3~]; time in s, SO^2−^~3~ in M). Microtox™ (Azur Environmental, Carlsbad, CA, USA) bioassays indicate that two terminally chlorinated N‐chloropeptides as well as side‐chain chlorinated N‐chloroacetyllysine are much less toxic to Vibrio fisheri than NH~2~Cl, N‐chloroglycine, and N‐chloropiperidine. An astonishingly simple relationship describes the bioassay results at 5 min exposure as In(EC50) = −3.684 + 0.0221 (MW) (EC50 in mg/L as C1~2~; molecular weight [MW] in Daltons). These exploratory data offer a basis for optimism that S(IV)‐surviving N‐chloropeptides will prove less toxic in natural ecosystems than rapidly reduced chloramines, but further investigations are needed.