Extraction behavior of amino acids by calix[6] arene carboxylic acid derivatives

A series of calixarene carboxylic acid derivatives were synthesized for the extraction of amino acids. A calix[6] arene carboxylic acid derivative showed the highest extractability to the target tryptophan ester. The main driving force for the complexation was the interaction between the ammonium cation of the amino acid and the oxygen atoms of the host molecule. Stripping of amino acids was also accomplished by contacting the organic solution with a fresh acidic solution. Based on slope and Job method analyses, it was confirmed that the calix[ 6] arene forms a 1 : 1 complex with the amino acid ester. The structure of the complex between the calix[6] arene and the amino acid was investigated by H-1-NMR and CD spectra. The calix[6] arene includes a guest molecule in the cavity, and the inclusion induces the asymmetrization of the host molecule. This host compound functions as a novel recognition tool for amino acids.