Origins of hydration differences in homochiral and racemic crystals of aspartic acid

The propensity for crystalline hydrates of organic molecules to form is related to the strength of the interactions between molecules, including the chiral composition of the molecular solids. Specifically, homochiral versus racemic crystalline samples can exhibit distinct differences in their ability to form energetically stable hydrates. The focus of the current study is a comparison of the crystal structures and intermolecular forces found in solid-state l-aspartic acid, dl-aspartic acid, and l-aspartic acid monohydrate. The absence of experimental evidence for the dl-aspartic acid monohydrate is considered here in terms of the enhanced thermodynamic stability of the dl-aspartic acid anhydrate crystal as compared to the l-aspartic acid anhydrate as revealed through solid-state density functional theory calculations and terahertz spectroscopic measurements. The results indicate that anhydrous dl-aspartic acid is the more stable solid, not due to intermolecular forces alone but also due to the improved conformations of the molecules within the racemic solid. Hemihydrated and monohydrated forms of dl-aspartic acid have been computationally evaluated, and in each case, the hydrates produce destabilized aspartic acid conformations that prevent dl-aspartic acid hydrate formation from occurring.