Liquid membrane transport of amino acids by a calix[6]arene carboxylic acid derivative

Calix[6]arene hexacarboxylic acid was found to be a useful carrier for transporting amino acids through a liquid membrane. The calix[6]arene, which has a cyclic structure to include a guest molecule of the amino acid ester and bears six ionizable carboxylic acids to contribute electrostatic interaction, exhibited a high transport efficiency compared to its monomer analog and the other calix[n]arene derivatives. The novel carrier successfully transported hydrophobic amino acid esters from the feed phase to the receiving phase. The transport rate could be controlled by changing the pH gradient between the feed and receiving aqueous phases because the complexation proceeds by a proton-exchange mechanism. Furthermore, an optical resolution system was constructed by applying an enantioselective enzymatic reaction for a chiral separation of the amino acids. In the enzyme reaction, the l-form ester was selectively hydrolyzed to the free amino acid. The free amino acid hydrolyzed was not transported, while the unhydrolyzed d-form ester effectively moved to the receiving phase through the liquid membrane containing the calix[6]arene as a mobile carrier.