Extraction of Tryptophan with Ionic Liquids Studied with Molecular Dynamics Simulations

Abstract

Extraction of amino acids from aqueous solutions with ionic liquids (ILs) in biphasic systems is analyzed with molecular dynamics (MD) simulations. Extraction of tryptophan (TRP) with the imidazolium-based ILs [C(4)mim][PF(6)], [C(8)mim][PF(6)], and [C(8)mim][BF(4)] are considered as model cases. Solvation free energies of TRP are calculated with MD simulations and thermodynamic integration in combination with an empirical force field, whose parametrization is based on the liquid-phase charge distribution of the ILs. Calculated solvation free energies reproduce successfully all observed experimental trends according to the previously reported partition of TRP between water and IL phases. Water is present in ILs as a cosolvent, due to direct contact with the aqueous phase during extraction, and is found to play a major role in the extraction of TRP. Water improves solvation of cationic TRP by 7.8 and 5.1 kcal/mol in [C(4)mim][PF(6)] and [C(8)mim][PF(6)], respectively, which is in the case of [C(4)mim][PF(6)] sufficient to extract TRP. Extraction in [C(8)mim][PF(6)] is not feasible, since the hydrophobic octyl groups of the cations limit the water concentration in the IL. The solvation of cationic TRP is 2.4 kcal/mol less favorable in [C(8)mim][PF(6)] than in [C(4)mim][PF(6)]. Water improves the solvation of TRP in ILs mostly through dipole-dipole interactions with the polar backbone of TRP. Extraction is most efficient with [C(8)mim][BF(4)], where hydrophilic BF(4)(-) anions substantially increase the water concentration in the IL. Additionally, stronger direct electrostatic interactions of TRP with BF(4)(-) anions improve its solvation in the IL further. The solvation of cationic TRP in [C(8)mim][BF(4)] is 3.4 kcal/mol more favorable than in [C(8)mim][PF(6)]. Overall, the extractive power of the ILs correlates with the water saturation concentration of the IL phase, which in turn is determined by the hydrophilicity of the constituting ions. The results of this work identify relations between the extraction performance of ILs and the basic chemical properties of the ions, which provide guidelines that could contribute to the design of improved novel ILs for amino acid extraction.