Solid-State NMR and the Crystallization of Aspartic and Glutamic Acids

We used high-resolution solid-state ¹³C NMR, with cross-polarization and magic-angle spinning, to study chirality in the crystallization of aspartic acid. We show that, contrary to a recent report, dl-aspartic acid crystallizes over most of its temperature range as racemic crystals rather than a conglomerate of enantiomeric crystals, regardless of whether the solution was prepared by solution of the racemate or by mixing solutions of the pure enantiomers. Over virtually the entire solution temperature range at 1 bar pressure, racemic crystals of aspartic acid are thermodynamically stable, whereas the conglomerate is metastable. In contrast, in agreement with the literature, glutamic acid crystallizes as a conglomerate under thermodynamic conditions. Solid-state NMR results are confirmed by powder X-ray diffraction.