A Modified Method for the Estimation of Tryptophane. ∗

This paper deals with a modification of May and Rose's 1 method for the determination of tryptophane. In their method the protein is dissolved in HCl which contains Ehrlich's reagent, incubated at 35° for 24 hours, and then allowed to stand for an additional 40 hours at room temperature. The blue color which is thus produced is matched against a similar color obtained by treating casein in an identical manner.

In this procedure such factors as the particular preparation of casein used for the standard and the room temperature may be uncontrolled variables. May and Rose assumed the tryptophane content of casein to be 1.5%. It is based on the data of Hopkins and Cole. 2 Other values have been reported. 3 The uncertainty as to the tryptophane content of casein makes this protein of questionable value as a standard.

In the present work the attempt was made to use tryptophane for the production of the standard color. It was found that the development of the color does not proceed at the same rate when casein and tryptophane are treated with the reagent. Tryptophane reacts more slowly. The color given by tryptophane is not identical with that obtained when casein is used. At 45° both casein and tryptophane react more rapidly than at either 37° or 30°; however, at 45° both substances show only 60-80% of the maximum color intensity of that which was obtained at either of the 2 lower temperatures. Apparently considerable destruction of tryptophane takes place at 45°. At 30° the color is fairly stable and continues so for a period of 48 hours after reaching the maximum intensity. The present data are not entirely in accord with those reported by Holm and Greenbank. 4