The structural effect of heterocyclic amino compounds in liquid-liquid extraction using a calix[6]arene carboxylic acid derivative

The extraction behavior of heterocyclic compounds was investigated with a cyclic ligand calix[6]arene carboxylic acid derivative. Pyridine and pyrimidine bearing an amino group were quantitatively extracted with this host ligand. The presence of the amino group and the magnitude of pK_a in the guest molecules significantly affect the extraction efficiency. The host ligand provides low extractability for guest molecules having a carbonyl group, which decreases the magnitude of pK_a. The calix[6]arene carboxylic acid derivative exhibits a high extractability compared to other calix[6]arene derivatives and commercially available extractants. The important factors for extractants in obtaining high extractability were found to be the introduction of carboxylic acid groups and the inclusion effect of the macrocyclic structure. Among all nucleotide bases tested, the calix[6]arene carboxylic acid derivative showed the highest specificity for adenine.