Enantiopure and quasiracemic crystals of 7-substituted tryptophan derivatives: modulation of the molecular arrangement for functionalized crystals

Abstract

Amino acids are widely distributed in natural systems, and their crystals hold great promise as functional materials. However, the variety of natural amino acids is inherently limited. In this study, the crystal structures and properties of two 7-substituted tryptophan derivatives have been investigated. The crystal of L-7-cyanotryptophan exhibited a different molecular arrangement from that of natural L-tryptophan, primarily due to the involvement of the cyano and indole amino groups in forming intermolecular hydrogen bonds. Meanwhile, L-7-azatryptophan formed two distinct crystals, α and β. In particular, the α-crystal featured one-dimensional pores capable of including solvent molecules. Furthermore, co-crystallization of L-7-cyanotryptophan with D-tryptophan afforded a quasiracemic crystal, which exhibited fluorescence at a longer wavelength compared with the racemic tryptophan crystal.