Polymorphism of l-Tryptophan.

A new polymorph of l-tryptophan was prepared through crystallization from the gas phase, with structure determination carried out directly from powder XRD data augmented by periodic DFT-D calculations. The new polymorph (denoted β) and the previously reported polymorph (denoted α) are both based on alternating hydrophilic and hydrophobic layers, but with substantially different hydrogen-bonding arrangements. The β polymorph exhibits the energetically favourable l2-l2 hydrogen-bonding arrangement, which is unprecedented for amino acids with aromatic side chains. The specific molecular conformations adopted in the β polymorph facilitate this hydrogen-bonding scheme while avoiding steric conflict of the side chains.