Chloramine-T in radiolabeling techniques. I. Kinetics and mechanism of the reaction between chloramine-T and amino acids.

The determination of the rate and pH dependency of chlorination of amino acids by chloramine-T (CAT) using the model compounds beta-alanine, L-alanine, and L-alanine ethyl ester was investigated. The reactions were studied under pseudo-first-order conditions where the amino acid concentration was in large excess over CAT. The rates of the reactions were determined spectrophotometrically by following the disappearance of CAT. The overall reaction was found to obey second-order kinetics. The effect of pH was studied over the range 6.1 to 12.0. The reaction rate was found to be independent of pH in the range 6.1 to 8.5, and to decrease with increasing pH above 8.5. The pH dependency was rationalized by assuming that un-ionized CAT reacts with the un-ionized amino group of the amino acid. The value of the resulting second-order rate constant (1.9 x 10(7) M-1 min-1) was of the same order of magnitude as rate constants reported for similar N-chlorination reactions. A cyclic transition state involving a water molecule is proposed.