The Decarboxylation of Amino Acids, Proteins, and Peptides by N-bromosuccinimide

One of the most widely used methods for the determination of amino acids is the ninhydrin technique perfected by Van Slyke and his coworkers (1). This permits the measurement of amino acid concentration either calorimetrically or by the volume of carbon dioxide released during the reaction. The measurement of carbon dioxide by the Van Slyke manometric apparatus is capable of great precision but is not amenable to the simultaneous analysis of many samples. The calorimetric assay, although not as accurate, makes simultaneous determinations feasible. One disadvantage of both techniques is that they require heating to 100”. The calorimetric procedure suffers from the added disadvantage of sensitivity to atmospheric ammonia. Another method for the decarboxylation of amino acids was recently described (2). This involved heating the amino acids to approximately 150’ in the presence of aromatic aldehydes as catalysts. The yield of the reaction varied from 25 to 70 per cent of theory. In the present paper, we describe a decarboxylation method, based upon work by Barakat et al. (3-6), which lends itself to the determination of amino acids and offers considerable promise in the determination of end group carboxyls in peptides and proteins. The decarboxylating agent is N-bromosuccinimide. This compound has had extensive use as a reagent for the bromination of unsaturated compounds and, with suitable catalysts, for other compounds as well (7). These bromination reactions are carried out in organic solvents, usually carbon tetrachloride. Aside from the observation that N-bromosuccinimide is hydrolyzed to hypobromous acid and succinimide in the presence of water, no serious investigation of the properties of N-bromosuccinimide in an aqueous medium had been undertaken until the work of Barakat. He observed that certain a-substituted carboxylic acids were decarboxylated with a concomitant evolution of bromine when allowed to react with an aqueous N-bromosuccinimide solution. Among the various a-substituted carboxylic acids which were susceptible to this reagent was alanine. It is pertinent to note that the reaction went to completion at room temperature.