Thermodynamic characterization of the biocompatible ionic liquid effects on protein model compounds and their functional groups

The stability of proteins under co-solvent conditions is dependant on the nature of the co-solvent; the co-solvent can alter a protein's properties and structural effects through bimolecular interactions between its functional groups and co-solvent particles. Ionic liquids (ILs) represent a rather diverse class of co-solvents that are combinations of different ions, which are liquids at or close to room temperature. To quantify the bimolecular interactions of protein functional groups with biocompatible ILs, we report the systematic and quantitative apparent transfer free energies (ΔG′_tr) of a homologous series of cyclic dipeptides (CDs) from water to aqueous solutions of ILs through solubility measurements, as a function of IL concentration at 25 °C under atmospheric pressure. The materials investigated in the present work included the CDs of cyclo(Gly-Gly), cyclo(Ala-Gly), cyclo(Ala-Ala), cyclo(Leu-Ala), and cyclo(Val-Val). The ILs used such as diethylammonium acetate ([Et₂NH][CH₃COO], DEAA), triethylammonium acetate ([Et₃NH][CH₃COO], TEAA), diethylammonium dihydogen phosphate ([Et₃NH][H₂PO₄], DEAP), triethylammonium dihydogen phosphate ([Et₃NH][H₂PO ₄], TEAP), diethylammonium sulfate ([Et₃NH][HSO ₄], DEAS) and triethylammonium sulfate ([Et₃NH][HSO ₄], TEAS). We observed positive values of ΔG′ _tr for CDs from water to ILs, indicating that interactions between ILs and CDs are unfavourable, which leads to stabilization of the native structure of CDs. The experimental results were further used for estimating the transfer free energies (Δg′_tr) of the peptide bond (-CONH-), the peptide backbone unit (-CH₂CONH-), and various functional groups from water to IL solutions. Our results explicitly elucidate that a series of all ammonium ILs act as stabilizers for tested model compounds through the exclusion of ILs from CDs surface.