Development of aromatic ethers as solvents for a calix[6]arene derivative and extraction of amino acids and proteins

Calixarene derivatives are excellent host compounds for ionic species in liquid–liquid extraction. However, many studies using calixarene derivatives have been conducted using highly toxic chloroform as a solvent because of their poor solubility in typical hydrocarbons. In the present study, aromatic ethers were developed as solvents for calixarene derivatives to enhance their industrial applicability. The solubility of the calix[6]arene carboxylic acid derivative ^tOct[6]CH₂COOH in 1-octoxybenzene (OB) and 1-butoxybenzene (BB) was 11.3 mM and 24.9 mM, respectively. The cationic guest molecule tryptophan methyl ester was extracted using the ^tOct[6]CH₂COOH host in OB and BB. The cationic lysine-rich protein cytochrome c was also extracted using ^tOct[6]CH₂COOH in BB as shown for ^tOct[6]CH₂COOH in chloroform in previous studies. However, cytochrome c was not extracted using ^tOct[6]CH₂COOH in OB and instead, precipitated. As ^tOct[6]CH₂COOH stabilizes the ammonium group of biomolecules because of its suitable cavity size, the extraction of the tryptophan ester and cytochrome c using ^tOct[6]CH₂COOH was much higher than those using the corresponding calix[4]arene and calix[8]arene derivatives. These results confirm that aromatic ethers are potential solvents for calixarene derivatives for the extraction of biomolecules.