The I~rei’aration of L-leucine and Its Behavzor in Some Non-aqueous Solvents

Bergmann and Stein (1) have described a procedure for the isolation and purification of naturally occurring n-leucine based upon the precipitation and recrystallization of the slightly soluble salt of this amino acid with naphthalene-fi-sulfonic acid. The product obtained was stated to be 97 per cent pure and to be free from methionine, a common contaminant of less pure preparations of n-leucine (2). We have found that L-leucine so prepared contains significant amounts of those amino acids which are oxidized by bromine in acid solution, and that a naphthalene sulfonate of superior purity can be obtained if the crude leucine is first treated with bromine water. The use of the preliminary bromine oxidation and of more dilute solutions than were used by Bergmann and Stein for the recrystallization of the naphthalene-p-sulfonate gave n-leucine preparations of exceptional purity when judged by qualitative tests and solubility determinations. Characteristic physical constants of n-leucine so prepared are given elsewhere in this paper. In the determination of the optical rotation of a-amino acids in aqueous acid solutions, it has been assumed at times that the concentration of the aqueous acid is not critical provided sufficient acid has been added to assure formation of the amino acid cation. Dunn et al. (3) have pointed out that the specific rotation of solutions of Land n-alanine in hydrochloric acid is dependent upon the hydrochloric acid concentration, and the data presented in Fig. 1 clearly show a similar dependency in the case of either hydrochloric or sulfuric acid solutions of L-leucine. When solutions of n-leucine in 100 per cent sulfuric acid were allowed to stand at 25” for periods exceeding 1 month, no change in specific rotation was observed. Furthermore, the n-leucine recovered from these sulfuric acid solutions was indistinguishable from L-leucine not so treated. As either the lack of formation of the doubly charged cation, [RCHNH&(OH) d++, or the stability of this ion, in respect to the loss of a proton from the a-carbon atom in sulfuric acid solutions, may have been responsible for the optical stability noted, the cryoscopic properties of sulfuric acid solutions of L-leucine were investigated. It was found that the freezing point