STUDIES IN THE DETERMINATION OF TRYPTOPHAN. MODIFIED FISCHL PROCEDURE.

Abstract A modified Fischl procedure for determining tryptophan is described. The method is more convenient than the original Fischl procedure, since aqueous solutions of tryptophan or protein hydrolyzates may be employed, and it is more sensitive. It is also more satisfactory than Fischl's method for the estimation of tryptophan in the intact protein. Differences in reactivity of tryptophan residues in the protein molecule are reflected by differences in the rate of formation of chromogen under cold conditions and the somewhat low values obtained for the number of reactive tryptophan residues in the molecule when compared with the commonly accepted values. However, analyses obtained after heating the protein for 90 min in acid solution and after correcting for the loss of tryptophan that occurs under these conditions, furnish results that are either similar to, or higher than, those reported by other workers.