Ambient l-lactic acid crystal polymorphism

Abstract

Only one crystalline phase of the metabolite, L-lactic acid (LLA), has been described since its isolation from sour milk as long ago as 1780. Herein, we report the structures of two new crystalline polymorphs of LLA obtained from supercooled melts and characterized by X-ray diffraction and micro-Raman spectroscopy, as well as their transformations from one phase to another. None of the three crystal structures are consistent with the aggregate geometries of LLA in cold matrices, in solution, or those predicted by quantum chemical computations. The latter aggregates feature typical cyclic carboxylic acid dimers not represented in any of the crystal structures. These differences underscore the difficulty of reasoning solid state structure from associations in other media.