Redetermination of l-tryptophan hydrobromide

The redetermined crystal structure of the title compound, C11H13N2O2 +·Br−, is reported. Data collection at 100 K about three crystallographic axes resulted in a crystal structure with significantly higher precision in comparison to the two-dimensional data collected at 176 K [Takigawa et al. [(1966) Bull. Chem. Soc. Jpn, 39, 2369–2378]. The carboxyl group and indole ring system are planar, with maximum deviations of 0.002 (2) and 0.007 (2) Å, respectively, and make an angle of 70.17 (1)° with each other. The molecules are arranged in double layers of carboxyl and amino groups parallel to the ab plane, stabilized by an extensive network of N—H⋯Br and O—H⋯Br hydrogen bonds. The polar layer is held together by a network of three N—H⋯Br hydrogen bonds and one O—H⋯Br hydrogen bond. In the non-polar layer, the indole rings are linked mainly by electrostatic N—H⋯C interactions between the polarized bond N—H (H is δ+) of the pyrrole unit and two of the ring C atoms (δ−) of the benzene rings of adjacent molecules. The distances of these electrostatic interactions are 2.57 and 2.68 Å, respectively. C—H⋯O and C—H⋯π interactions are also present.

The redetermined crystal structure of the title compound, C 11 H 13 N 2 O 2 + ÁBr À , is reported. Data collection at 100 K about three crystallographic axes resulted in a crystal structure with significantly higher precision in comparison to the twodimensional data collected at 176 K [Takigawa et al. [(1966) Bull. Chem. Soc. Jpn, 39, 2369-2378. The carboxyl group and indole ring system are planar, with maximum deviations of 0.002 (2) and 0.007 (2) Å , respectively, and make an angle of 70.17 (1) with each other. The molecules are arranged in double layers of carboxyl and amino groups parallel to the ab plane, stabilized by an extensive network of N-HÁ Á ÁBr and O-HÁ Á ÁBr hydrogen bonds. The polar layer is held together by a network of three N-HÁ Á ÁBr hydrogen bonds and one O-HÁ Á ÁBr hydrogen bond. In the non-polar layer, the indole rings are linked mainly by electrostatic N-HÁ Á ÁC interactions between the polarized bond N-H (H is + ) of the pyrrole unit and two of the ring C atoms ( À ) of the benzene rings of adjacent molecules. The distances of these electrostatic interactions are 2.57 and 2.68 Å , respectively. C-HÁ Á ÁO and C-HÁ Á Á interactions are also present.

Comment
The crystal structures of amino acids and their complexes have provided interesting information about aggregation, and the effect of other molecules on their interaction and molecular properties (Vijayan, 1988;Prasad & Vijayan 1993).
The crystal structure of L-tryptophan hydrobromide was determined some 40 years ago (Takigawa et al., 1966). The final refinement was carried out to only R = 0.101 with no data available for H atoms. The reported structure possesses almost identical crystal parameters to the structure reported here in terms of space group and unit-cell dimensions and angles.
However, the collection of data at 100 K about three crystallographic axes in comparison to the data reported by Takigawa The planarity of the carboxyl group with the α-carbon has been established in many investigations and the deviations of the amino nitrogen range from 0.00 to 0.82 Å for the amino acids so far investigated. For the present molecule, the amino nitrogen is 0.094 Å out of the plane, so the amide group is essentially planar in this case.
The mean plane through the atoms of the indole ring with the methylene carbon attached to it forms a dihedral angle of 70.17 (1) ° with the mean plane of the carboxyl group.
The structures of many amino acids with non-polar side chains have the arrangement of a double layered system (Torii & Iitaka 1970;Torii & Iitaka, 1971;Torii & Iitaka, 1973;Harding & Long, 1968) which is characteristic for a structure containing polar and non-polar groups together. The polar layer is held together by a network of hydrogen bonds between the halide ions and the amino nitrogen and the halide ions and the carboxyl group.
The amino nitrogen forms three N-H···Br hydrogen bonds, in the lengths of 2.56 (1) Å, 2.41 (1) Å and 2.52 (1) Å. The three acceptor halogen ions are approximately at the three vertices of a regular tetrahedron centred around the nitrogen atom, with the fourth vortex positioned in the direction of the α-carbon.
The fourth hydrogen bond completing the network is a O-H···Brwhich is 2.34 (1) Å in length.
In the non-polar layer, the indole rings are packed in a manner similar to that found for typical aromatic molecules. A weak electrostatic interaction with a separation of 2.677 (1) Å exists between N1-H5 from the pyrrole moiety and the slightly positively charged C6 from the benzene moiety of a neighbouring symmetry related molecule [-x, -1/2 + y, 1 -z].
A similar interaction with a separation of 2.573 (1) Å exists between the same N1-H5 and C7 from the benzene moiety    Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.