Fast parallel enantiomeric analysis of unmodified amino acids by sensing with fluorescent β-cyclodextrins

Modified cyclodextrins bearing a metal binding site and a dansyl fluorophore 6-deoxy-6-N-(Nα-[(5-dimethylamino-1-naphthalenesulfonyl)aminoethyl]-aminocylamino-β-cyclodextrin, containing L-Phe (S-1), L-PhGly (S-2) or L-Pro (S-3) moieties, were used as enantioselective fluorescence sensors for the discrimination of enantiomers of the amino acids valine and proline, according to a ligand exchange mechanism. The best conditions to perform enantiomeric analyses were studied and a fast protocol using fluorescence quenching by the copper(II)/amino acid complexes in a fluorescence microplate reader was developed, allowing the detection of samples with high enantiomeric excess. Calibration of the fluorescence response as a function of enantiomeric composition was obtained using the Stern–Volmer model; the calibration curves showed good linearity, allowing fast evaluation of enantiomeric excess within 6% error. Two calibration curves and triplicate analyses of six valine samples were performed in two minutes.