Enantiomer discrimination using lipophilic cyclodextrins studied by electrode response, pulsed-gradient spin-echo (PGSE) NMR and relaxation rate measurements

The diastereoisomeric complexes formed between 2,6-di-O-alkyl-a- and β-cyclodextrins and the arylammonium ions propranolol, ephedrine and amphetamine have been studied by electrode response and NMR methods. Enantioselectivity in binding propranolol is 3.3 : 1 with 2,6-di-O-dodecyl-β-cyclodextrin in favour of the (+)-enantiomer as revealed by measurement of the association constant using pulsed-gradient spin-echo (PGSE) NMR methods. In all of the cases of enantiodifferentiation studied here, the (+)-enantiomer is more strongly bound by the cyclodextrin. Relaxation rate measurements of the host and guest proton NMR resonances highlight the importance of hydrogen-bonding in enantiomer discrimination.