A ninhydrin reaction differentiating N-terminal glycine peptides from other peptides.

The products of the reaction of ninhydrin with a-amino acids are, in general, the corresponding aldehyde of one less carbon atom than the original compound, COZ, and, at pH 5 to 7, an intensely colored compound (thought to be the anion of diketohydrindylidene-diketohydrindamine), which contains nitrogen from the amino acid. If the pH is kept at 2.5 or lower, this colored compound is not produced and the nitrogen from the amino acid appears in a form that is readily convertible to free NH, on subsequent alkalization (1). The reaction with peptides must be different because it produces neither aldehydes (2-4) nor CO2 (5). However, the reaction does produce a color at pH 5, an observation which indicates that NH, may be a product under the proper experimental conditions. This paper is concerned with the question of the formation of NH3 by peptides and proteins on treatment with ninhydrin, and with the analytical possibilities of such a reaction.