Hopkins–Cole reaction

The Hopkins-Cole reaction, also known as the glyoxylic acid reaction, is a chemical test used for detecting the presence of tryptophan in proteins.^[1] A protein solution is mixed with Hopkins Cole reagent, which consists of glyoxylic acid. Concentrated sulfuric acid is slowly added to form two layers. A purple ring appears between the two layers if the test is positive for tryptophan.^[2][3] Nitrites, chlorates, nitrates and excess chlorides prevent the reaction from occurring.^[4]

The reaction was first reported by Frederick Gowland Hopkins and Sydney W. Cole in 1901,^[5] as part of their work on the first isolation of tryptophan itself.