A reaction of tryptophan with methyl alcohol which depends upon the formation of formaldehyde.

In 1905 Voisenet described a color reaction given by proteins in the presence of an aldehyde and an oxidizing agent (l), and the following year Rosenheim found that the color produced by tryptophan and formaldehyde is spectroscopically identical with that produced with glyoxylic acid (2). Neither of these substances is readily available in a form suitable for the preparation of reproducible reagents, and formaldehyde has the further disadvantage that an excess inhibits formation of the characteristic violet color. We have found that tryptophan reacts with methyl alcohol and sulfuric acid in the presence of copper sulfate to give a similar color. Although this reaction is apparently caused by the formation of formaldehyde, the new reagent may be useful since it is available in purified form and since an excess of the effective aldehyde is avoided by the conditions of the reaction.