Temperature-switchable deep eutectic solvents for selective separation of aromatic amino acids in water

Amino acids are vital compounds in animal and human nutrition. However, it is still a great challenge to develop a cost-effective process for selective separation of amino acids. In the present study, a class of deep eutectic solvents (DESs) with short-chain alkanolamines as hydrogen bond acceptors (HBAs) and phenolic compounds as hydrogen bond donors (HBDs) have been explored for selective separation of aromatic amino acids for the first time. It is found that these DESs exhibit upper critical solution temperature (UCST)-type phase behavior in water, and their phase transition temperature is related with the pKa values of HBAs and HBDs as well as the hydrophobicity of HBDs. The UCST-type phase separation process is investigated by dynamic light scattering (DLS), FT-IR spectroscopy and temperature-variable 1H NMR in details, and the stability of the DESs in water is also examined. Interestingly, these UCST-type DESs/water systems are applicable to selective separation of aromatic amino acids. The separation factor between L-tryptophan and L-tyrosine as well as between L-tryptophan and L-phenylalanine can reach up to 476.1 and 87.9, respectively, which is about 20 and 19 times that reported in ionic liquids-based extraction solvents.