PMC

Schematic of crystallization-based enantioenrichment mechanisms. (a) For amino acids that preferentially form racemic crystals, in a saturated solution with one enantiomer present in excess, the solid phase would be racemic and the solution phase would be enriched to a greater degree than the overall enantiomeric excess (counting both the solution and solid phase). The enantioenriched solution phase can be physically separated from the racemic solid during aqueous alteration. (b) For amino acids that preferentially form enantiopure crystals, in a saturated solution where one enantiomer is present in excess, that enantiomer will tend to form more crystals that are larger, while the less abundant enantiomer will form fewer crystals that are smaller. In this case the solution phase will be racemic, so racemization would affect both enantiomers equally. However, producing more of the more abundant enantiomer will cause more of it to precipitate, increase the enantiomeric excess.

Ternary phase diagram for T* = 1. The apex of the diagram is shown on the Left and Right with an illustration of the full diagram in the Center. Open circles mark the eutectic points (labeled E and E′), the solid red lines designate the coexistence region of a solution phase in equilibrium with an enantiopure solid phase, the solid blue lines mark the coexistence region of a solution phase with a racemic solid phase, and the region labeled with green text consists of 3-phase equilibrium between a solution at the eutectic point, a near enantiopure solid, and a near racemic solid. Lines of constant enantiomeric excess (see for definition) are shown on the Left as a guide. Tie lines are shown as dashed lines on Right. These connect the compositions of the coexisting solution and solid phases.

Depiction of equilibrium between chiral crystalline solids and their aqueous solution phases for nonracemic mixtures of enantiomers. Rectangles represent solid phase enantiomeric molecules; colored letters represent solution phase molecules in equilibrium with the solid phases. The solution phase composition is known as the eutectic. (A) conglomerates show ee^eut= 0; (B) racemic compounds show nonzero ee^eut.

Graphical representation of the spectroscopic data collected for the present study. (A) MIR spectra of the 33 enantiomeric mixtures in the solid state (KBr pellet); (B) NIR spectra of the 33 enantiomeric mixtures in the solid state (integrating sphere); (C) average MIR spectra for Pure l-DOPA (red) and racemic DOPA (blue); (D) average NIR spectra for Pure L-DOPA (red) and racemic DOPA (blue).