Crystal Structures of L -(R)-Cysteine-Mandelic Acid Diastereomers

The crystal structures of the title compounds, L -(R)-cysteine- D -(R)-mandelic acid, (I), and L -(R)-cysteine- L -(S)-mandelic acid, (II), have been determined. The structures show a common fashion in molecular recognition in hydrophilic layers, except for the arrangement of phenyl rings against mercapto groups. Crystallographic and physicochemical experiments have clarified the entropic and enthalpic advantage of (II) than (I). It plays a key role in the optical separation of mandelic acid with chiral L -cysteine.