Herzig and Meyer's Reaction Applied to Proteins and Amino-Acids

This reaction [Herzig and Meyer, 1894] has been known for a consider- able time as a means of detecting the presence of, and estimating, methyl groups attached to nitrogen in organic compounds. It consists in heating the substance to a high temperature in the presence of excess of hydriodic acid, so that the methyl groups leave the nitrogen to form methyl iodide, and the hydrogen of the hydriodic acid takes the place of the methyl group. The methyl iodide is then carried over into alcoholic silver nitrate, where silver iodide is formed. The precipitate of silver iodide can be weighed and from its amount the percentage of methyl attached to nitrogen can be calculated.