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Polymorphism of L-Tryptophan.

A new polymorph of L-tryptophan has been prepared by 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.