Thermodynamic and Molecular Recognition Mechanism of Diastereomeric Salt/Cocrystal-Induced Chiral Separation

To better understand the thermodynamics and molecular self-assembly mechanism of diastereomeric salt/cocrystalinduced chiral separation, a series of 1:1 cocrystals and salts consisting of chiral valines (VAL) and tartaric acid derivatives were synthesized via different methods. Powders of these as-screened cocrystals/salts were characterized by PXRD, TGA, DSC, FT-IR, and Raman spectroscopy. The crystal structures of the five cocrystals/salts were determined and analyzed. It was found that both DBTA and DTTA form diastereomeric salt pairs with VAL enantiomers. Interestingly, L-DMTA cocrystallizes with D-VAL and L-VAL via hydrogen bonding and proton transfer, respectively. Considering this particularity, the differential isothermal (10 degrees C) ternary phase diagrams (TPDs) of D-DMTA and L-VAL (D-VAL) cocrystals (salt) were constructed in the mixed solvent of MeOH/ H2O. Moreover, a pure L-VAL:D-DMTA cocrystal and D-VAL:D-DMTA:0.5CH(4)O:0.25H(2)O salt were prepared via equimolar slurry conversion at 10 degrees C. In situ Raman spectroscopy was applied to monitor the molecular assembly process during the incubation of the cocrystal/salt. Molecular dynamics simulation was employed to rationalize the molecular recognition mechanism, demonstrating the excellent chirality preference of L-DMTA toward L-VAL instead of D-VAL. Calculations of density functional theory approved the synergistic instead of antagonistic effects of binding energy and solvation free energy toward chiral separation.

Automated summary

A series of 1:1 cocrystals and salts consisting of chiral valines and tartaric acid derivatives were synthesized to study the thermodynamics and molecular self-assembly mechanism of diastereomeric salt/cocrystal-induced chiral separation. The study found that the diastereomeric salts between different chiral molecules and the affinity and solvation free energy play important roles in molecular preferences.