Enantiomer discrimination and ion receptors

Abstract

Highly lipophilic α-, β- and γ- cyclodextrin derivatives were prepared in order to obtain enantiomer selective ionophores for β-aryl ammonium ions and selective ionophores for tetrahedral ammonium ions. The extent of cyclodextrin functionalisation and the homogeneity of the products was investigated by chemical depolymerisation, (^1)H and (^13)C NMR and (+)-FAB-, FD- and ES- mass spectral analysis. For each cyclodextrin, the products of alkylation were found to consist of several constitutional isomers and homologues. These highly lipophilic molecules were incorporated into solvent polymeric membranes and investigated as electrochemical sensors for chiral molecules incorporating an aryl ring. Electrodes using BBPA as the plasticizer were stable and well defined with a limit of detection for ephedrine of -log[c] = 6.5. Interference from serum levels of Na+, K+, Ca2+ and Mg2+ is minimal (-logK(^POT) = 3.9). The electrodes were highly enantioselective in binding ephedrine and closely related homologues. The mechanism of enantiomer discrimination was investigated by several multi-nuclear NMR techniques and the complexation process was investigated by ES-MS. Lipophilic per-O-octylated α-, β- and γ- cyclodextrins exhibit size selectivity and cation discrimination in the binding of +NH4, +NMe4 and +NEt4. Sensors based on per-O-octyl-β-cyclodextrin show excellent sensitivity and good selectivity for +NMe4 over metal cations and may be used for the detection of cationic surfactants. Complexation was studied in situ and competitively by ES-MS as well as by and relaxation time acquisition. The possibility of developing several chiral crown ether and cyclam based ionophores was also investigated.