Colorimetric enantiodiscrimination of mandelic acid by indicator displacement assay

The colorimetric enantiodiscrimination between mandelic acid and L-proline-Cu (II) is exploited to develop enantioselective indicator displacement assays. The sensitivity of the assay could be tuned by using a colorimetric indicator. The chromophoric ligand, pyrocatechol violet, effectively competes with the mandelic acid guest for open coordination sites on L-proline-Cu (II). The ΔA could be increased to 0.12 by changing the ratio of (+)-and (-)-mandelic acid concentrations that were found to be optimal from the displacement experiments. The resultant enantiomer excess versus ΔA relationship is linear. From the calibration curves, the absorbance values of the unknowns may be calculated for the enantiomeric excess value and the colorimetric enantiodiscrimination of mandelic acid can thus be obtained.