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

Abstract

To better understand the thermodynamics and molecular self-assembly mechanism of diastereomeric salt/cocrystal-induced 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 °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:0.5CH4O:0.25H2O salt were prepared via equimolar slurry conversion at 10 °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.