Revealing the effects of polymeric additive coupling supersaturation on the formation mechanism of L-tryptophan spherulite

Abstract

L-tryptophan, an essential amino acid for protein synthesis and neurotransmitter regulation, tends to crystallize in flake crystals in the crystallization process, resulting in difficulty in processability and poor product quality. Faced with this challenge, in this work, the approach of spherulite growth induced by the polymeric additive was applied to form spherulites of L-tryptophan with effectively improved powder properties. In detail, L-tryptophan spherulites were fabricated through cooling crystallization under the effect of hydroxypropyl methylcellulose (HPMC). Then, the formation mechanism of L-tryptophan spherulites was analyzed based on the results of experiments and molecular simulations. Besides, the effect of supersaturation coupling additive concentration on the morphology of L-tryptophan spherulites was investigated. It turns out that the formation of L-tryptophan spherulites follows the classical non-crystallographic branching for spherulitic growth, in which HPMC preferentially adsorbs on the (001) face of L-tryptophan to introduce stress triggering the branching. Furthermore, the tap density of the prepared L-tryptophan spherulites was improved and nearly 4 times that of the L-tryptophan flake crystals, and the angle of repose of the L-tryptophan flake crystals was 46°, while the angle of repose of L-tryptophan spherulites was 29°, exhibiting the improved flowability for the post-processing. To sum up, this study provides a feasible method for the manufacture of L-tryptophan crystals with good product quality, as well as making efforts to reveal the mechanism of spherulitic growth, which may provide new insight into the structure design of organic crystals.