Enantioselective transport of D,L-phenylalanine and D,L-phenylglycine through a bulk liquid membrane containing cinchona alkaloid derivatives as chiral selectors

Different cinchona derivatives were tested as chiral mobile carriers for enantioselective transport of D,L-phenylglycine and D,L-phenylalanine through a bulk organic liquid membrane (BLM). The effects of several parameters such as carrier nature, membrane solvent, buffer, solute and carrier concentrations on BLM enrichment were evaluated. Only D,L-phenylglycine is subjected to enantioselective transport at a certain degree; the maximum enantioselectivity ratio (the initial transport rates of the L-enantiomer relative to the antipode) was reached by employing O-allyl-N-(9-anthracenylmethyl) cinchonidinium bromide as carrier. In all cases the highest selectivity was observed during the initial stages of the process, indicating the rate of amino acid release from the source phase (SP) to the membrane organic phase (MP) as the driving factor. Chiral enrichment appeared to depend on thermodynamic factors more than on kinetic ones, since the complex formation was observed at (SP)/MP) interface, while complex decomposition was evidenced at (RP)/(MP) interface, where RP is receiving phase. The last phenomenon was promoted by H+ ions present in the RP.