Characteristic extraction and transport properties of crown ethers and cryptand [2.2.2] for amino acid methylesters

The liquid-liquid extraction and the transport through liquid membrane of complexes of a series of amino acid methylesters and crown ethers such as 18-crown-6 (18C6), benzo 18-crown-6 (B 18C6), cryptand [2.2.2] (222), and diaza crown ether [2.2] [22] were investigated. The experimental results strongly suggest that amino acid methylesters are extracted and transported by 18C6 and B18C6 more efficiently than the native amino acids. It was observed that crown ethers 18C6 and B18C6 exhibit different transport selectivities for the series of amino acid methylesters under study. The extraction constants (log K-ex) of the amino acids involved were also determined. The extractability and the transport proved to be essentially controlled by factors such as the structure of the macrocycle, the nature of the amino acid, the nature of the anion, and the thermodynamic properties. The bottom line is the feasibility of optimal separation of amino acid derivatives.