An active-site titrant for chymotrypsin, and evidence that azapeptide esters are less susceptible to nucleophilic attack than ordinary esters

The p-nitrophenyl ester of N-methoxycarbonylazaphenylalanine reacts with chymotrypsin in 1:1 stoichiometry, and the enzyme concentration can be quantified from the absorbance of the p-nitrophenolate produced in the reaction. This azapeptide does not decompose at pH 7 in the presence or absence of 1 mM hydroxylamine.