Although protein is commonly stated to interfere in the determination of carbohydrate using sulfuric acid solutions of anthrone, such tryptopban-free proteins as insulin, and mixtures of amino acids containing no tryptophan,
give little color during short heating periods with the reagent. The small amounts of nonspecific color can be still further reduced by fhe addition of phosphate ions to repress the catalytic effects of iron in the system. Tryptophan and tryptophan-containing proteins give a characteristic purple color (X,,, = 548 mp) with anthrone in sulfuric acid. When the anthrone reagent was used in an attempt to detect possible carbohydrate in a high-sulfur protein fraction which contains very little tryptophan, from reduced and carboxymethylated wool (l), a bright orange-red color (hmaa = 443 and 542 mp) was produced. This color was found to be specific to the S-carboxymet,hylcysteine present in the protein, and an identical color was given by the free amino acid. Even such closely related amino acids as lanthionine and S-l-and S-2carboxethylcysteines [-CH,CH (NH?) COOH, -CH (CH,) COOH, and -CH,CH,COOH, respectively, replacing -CH,COOH] were found to produce no increase in optical density under the conditions of the test.