Selective Crystallization of l-Histidine Metastable Polymorphs: The Role and Mechanism of Additives

l -Histidine forms A and B usually crystallize simultaneously in pure aqueous solution, posing a significant challenge in selectively obtaining one pure polymorph. In this study, we accomplished the selective crystallization of l -histidine form B using a strategy of additives. A series of additives with selective abilities were discovered and investigated. The results showed that the selective ability of the additives on l -histidine form B varied significantly with the lengths of the carbon chain and the types of substituent groups in the additives. Taking l -valine as a representative, the mechanism of selective crystallization of form B by additives was studied through solution NMR spectroscopy, combined with crystal structure analysis, density functional theory calculations, and molecular dynamics simulations. The results revealed that the additive is prone to bind to the –COO – group of l -histidine through its –NH 3 + group. The difference in molecular stacking between form A and form B originates from their unique dimers (referred to as characteristic dimers in this paper). And the stability of the characteristic dimer of form B is stronger than that of form A in solution in the presence of additives. On the one hand, the additives exhibit a stronger inhibitory effect on the growth of form A with –COO – groups exposed on both sides of the a -axis compared to that of form B with –COO – groups exposed on one side of the a -axis. On the other hand, the addition of additives enhances the relative stability of the characteristic dimer of l -histidine form B compared to that of form A, which makes more of the smallest interacting units of form B in the solution for nucleation and growth. This study offers valuable insights into the crystallization of l -histidine in the pharmaceutical industry and provides guidance for the regulation of polymorphs by using additive strategies.