Abstract
As a test of the labelling characteristics of photogenerated reagents, an aryl azide was photolysed in the aromatic-binding locus of a protein of known tertiary structure. The acyl-enzyme derived from the reaction of alpha-chymotrypsin with the p-nitrophenyl ester of p-azido[(14)C]cinnamate was isolated and photolysed. About 60% of the acyl group is covalently bound to the protein after photolysis and deacylation, and labelled enzyme is inactive. The covalently attached label is localized in the C chain of chymotrypsin, and there are firm indications that the major labelled tryptic fragment of the C chain is that which constitutes the aromatic-binding locus of the enzyme. The high degree of labelling of that portion of the protein molecule predicted on the basis of the known chemistry and structure of alpha-chymotrypsin, provides gratifying confirmation of the utility of the photo-labelling method.