The crystal structures of L-tryptophan hydrochloride and hydrobromide.

Abstract

Abstract The hydrochloride and hydrobromide of l-tryptophan crystallize in the monoclinic space group P21. The cell dimensions of the hydrochloride are: a=7.45, b=5.30, c=14.67 Å, β=98.80°, and those of the hydrobromide are: a=7.64, b=5.41, c=14.58 Å, β=100.47°. There are two molecules in the unit cell. The two compounds are isomorphous. The crystal structure of the hydrobromide was first determined by the heavy atom method with two-dimensional data, and then that of the hydrochloride was refined by three-dimensional least-squares procedure. The molecular and crystal structure is mainly discussed for the hydrochloride here. The average of six C–C bond lengths in the benzene ring is 1.396 Å, that of C–N bond lengths in the indole ring is 1.384 Å. In the carboxyl group, one C–O bond length, 1.147 Å and two O–C–C angles, 106.8° and 125.7° are significantly different from those reported for amino and carboxyl acids so far investigated. The molecule consists of two planes of the carboxyl group and of the indole ring, the planes making an angle of 70.6° with each other. As to the conformation about the C(α)–C(β) bond, C(γ) lies at the gauche position with both of the carboxyl and the amino groups. The molecules are arranged in double layers parallel to the ab plane; the polar layer is held together by a network of the three N–H··· Cl− hydrogen bonds, 3.17, 3.19 and 3.24 Å and one O–H···C1−, 3.04 Å, and in the non-polar layer, the indole rings are packed mainly by van der Waals forces, the shortest C···C and C···N contacts being 3.74 and 3.42 Å respectively.