Crystal structure of DL-Tryptophan at 173K

The crystal structure of pure DL-tryptophan has been determined at 173 K, using large but thin plate formed crystals of C₁₁H₁₂N ₂O₂, which were grown by cooling down a saturated solution of DL-tryptophan in isopropanole/formic acid. The crystals are monoclinic, space group P 2₁/c, with the unit cell constants: a = 18.899(2), b = 5.7445(6), c = 9.309(1) Å, β = 101.776(2), V = 989.4(4), D _calc = 1.371, Z = 4, F(000) = 432, λ.(MoKα) = 0.71073 Å, μ = 0.096 mm^-1. The crystal structure was solved by direct methods and refined by full-matrix least squares to a final R-factor = 4.03% with 3343 unique reflections of which 2104 were observed. The molecule is zwitterionic in the crystal with the protonated amino group and the carboxylate group in unusual conformations with respect to the indole ring system compared to other tryptophan derivatives. A D- and L- molecular pair forms a dimer by N - H ... O hydrogen bonds via a crystallographic inversion center. The dimers are linked by further N - H ... O hydrogen bonds to form a head to head bilayer arrangement of the molecules in the crystal lattice. Except for weak van der Waals contacts there is no relation between indole tails of neighboured bilayers.