Role of amide nitrogen in water chlorination: Proton NMR evidence

A large fraction of the dissolved amino-N in natural waters and wastewaters is contained in amide groups, for example, in proteins. Whether this N pool reacts with and modifies the chemical behavior of chlorine during water disinfection is unclear. To investigate this issue, water-suppressed, proton NMR spectra have been obtained for aqueous, acetylated glycine, alanine, and alanylalanylalanine before and after treatment with NaOCl at near-neutral pH. N-Chlorination of N-acetylglycine (aceturic acid) induced cis-trans rotation about the amide bond at ambient temperature. N-Bound Cl+ induced a measurable downfield chemical shift in the acetyl methyl resonance in N-acetylglycine, partial derivative delta = 0.15 ppm (cis) and partial derivative delta = 0.3 ppm (trans), and in N-acetylalanine methyl resonances, acetyl methyl partial derivative delta = 0.3 ppm (trans) and side chain methyl partial derivative delta = 0.2 ppm. Chlorination of N-acetylalanylalanylalanine produced partial derivative delta values similar to N-acetylalanine. The spectral effects were reversible, the original spectra being regenerated upon dechlorination with sulfite. Negligible substrate decomposition was observed. Rate constants for chlorination of N-acetylalanine near neutral pH are k(f) = 1.58 x 10(-3) M-1 s(-1) and k(b) = 7.57 x 10(-7) s(-1) where K-eq = 2.1 x 10(3). Because of both sluggish formation kinetics and an unfavorable equilibrium constant, N-chloramides are predicted to be unimportant under typical disinfection conditions.