Redetermination of l-tryptophan hydro­bromide

Kirsty Stewart

doi:10.1107/S1600536809017322

. 2009 May 14;65(Pt 6):o1291–o1292. doi: 10.1107/S1600536809017322

Redetermination of l-tryptophan hydrobromide

Kirsty Stewart ^a,^*

PMCID: PMC2969532 PMID: 21583152

Abstract

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.

Related literature

For a previous determination of the crystal structure of the title compound, see: Takigawa et al. (1966 ▶). Study of crystal structures of amino acids and their complexes has provided information about aggregation and the effect of other molecules on their interactions and molecular properties, see: Vijayan (1988 ▶); Prasad & Vijayan (1993 ▶). For the structure of histidine hydrochloride monohydrate, see: Takigawa et al. (1966 ▶). Donohue & Caron (1964 ▶). The structures of many amino acids with non-polar side chains feature a double-layered arrangement, see: Harding & Long (1968 ▶); Torii & Iitaka (1970 ▶, 1971 ▶, 1973 ▶).

Experimental

Crystal data

C₁₁H₁₃N₂O₂ ⁺·Br⁻
M _r = 285.13
Monoclinic,
a = 7.6272 (3) Å
b = 5.3840 (2) Å
c = 14.4358 (5) Å
β = 100.688 (3)°
V = 582.52 (4) Å³
Z = 2
Mo Kα radiation
μ = 3.52 mm⁻¹
T = 100 K
0.40 × 0.15 × 0.15 mm

Data collection

Oxford Xcalibur2 CCD diffractometer
Absorption correction: multi-scan (SCALE3 ABSPACK in CrysAlis RED; Oxford Diffraction, 2008 ▶) T _min = 0.334, T _max = 0.621
5749 measured reflections
2731 independent reflections
2507 reflections with I > 2σ(I)
R _int = 0.024

Refinement

R[F ² > 2σ(F ²)] = 0.027
wR(F ²) = 0.063
S = 1.01
2731 reflections
147 parameters
1 restraint
H-atom parameters constrained
Δρ_max = 0.33 e Å⁻³
Δρ_min = −1.14 e Å⁻³
Absolute structure: Flack (1983 ▶), 523 Freidel pairs
Flack parameter: 0.009 (9)

Data collection: CrysAlis CCD (Oxford Diffraction, 2008 ▶); cell refinement: CrysAlis RED (Oxford Diffraction, 2008 ▶); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: ORTEP-3 (Farrugia, 1997 ▶); software used to prepare material for publication: WinGX (Farrugia, 1999 ▶).

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536809017322/at2781sup1.cif

e-65-o1291-sup1.cif^{(16.6KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536809017322/at2781Isup2.hkl

e-65-o1291-Isup2.hkl^{(131.3KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C7—H4⋯Cg1ⁱ	0.95	2.66	3.494 (3)	146
N1—H5⋯Cg2ⁱ	0.88	2.72	3.406 (2)	136
N2—H11⋯Br1ⁱⁱ	0.91	2.56	3.3208 (17)	142
N2—H12⋯Br1ⁱⁱⁱ	0.91	2.42	3.322 (3)	173
N2—H13⋯Br1^iv	0.91	2.52	3.320 (3)	147
C4—H1⋯Br1ⁱⁱⁱ	0.95	2.85	3.750 (2)	159
C10—H9⋯O1^v	1.00	2.49	3.404 (3)	153
C10—H9⋯O1ⁱⁱ	1.00	2.56	3.199 (3)	121
O2—H10⋯Br1	0.84	2.34	3.173 (2)	169

Open in a new tab

Symmetry codes: (i) Inline graphic ; (ii) ; (iii) ; (iv) ; (v) . Cg1 is the centroid of the N1/C1–C3/C8 ring and Cg2 is the centroid of the C3–C8 ring.

Acknowledgments

The author gratefully acknowledges financial support from the University of Kwazulu-Natal. Thanks are also due to Ms C. Janse Van Rensburg for the mass spectrum analysis.

supplementary crystallographic information

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 et al. resulted in more accurate data thus allowing some previously unidentified notable conclusions to be drawn about the crystal structure.

The C—N distance 1.488 (3) Å coincides well with those in e.g. histidine hydrochloride monohydrate (1.495 Å) (Donohue & Caron, 1964). The two C—O bond lengths are 1.316 (3) Å and 1.204 (3) Å for C11—O2 and C11—O1, respectively and the three angles around the C11 atom are O2—C11—O1 = 125.8 (2)°, O2—C11—C10 = 110.0 (2) ° and O1—C11—C10 = 124.1 (2) °.

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···Br^- which 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 of molecule [-x, -1/2 + y, 1 - z]. The metrics of such interactions are reflected in the T-shaped edge-to-face geometry in the non-polar layer.

Experimental

L-tryptophan hydrobromide was obtained in the form of plate-like crystals by dissolving L-tryptophan in concentrated hydrobromic acid followed by slow evaporation at room temperature.

HRMS (ES^-): Found [M—H] 283.0082 C₁₁H₁₂N₂O₂Br Expected 283.0080 (-0.7 ppm).