Ammonium based ionic liquids act as compatible solvents for glycine peptides

In this article, we have reported the solubilities, apparent transfer free energies (Delta G'(tr)) and UV-visible absorption measurements of glycine peptides (GPs), such as glycine (Gly), diglycine (Gly(2)), and cyclic glycylglycine (c(GG)) in aqueous ionic liquids (ILs), bearing sulfate and phosphate anions with ammonium cation, at T = 298.15 K. Values of solubility were obtained from density (rho) measurements of GPs in water and in aqueous ILs. The ammonium ILs such as diethylammonium hydrogen sulfate (DEAS) [(CH3CH2)(2)NH][HSO4], triethylammonium hydrogen sulfate (TEAS) [(CH3CH2)(3)NH][HSO4], and triethylammonium dihydrogen phosphate (TEAP) [(CH3CH2)(3)NH][H2PO4] have been used in the present study. We observed the positive values of Delta G'(tr) for Gly, Gly(2), and c(GG) from water to ILs. These results reveal that the unfavourable interactions are dominated between ILs and the GPs. This indicates that the ammonium based ILs stabilize the GP structure. Further, we have used the Delta G'(tr) results to evaluate the transfer free energies (Delta g'(tr)) contribution of the peptide backbone unit, also known as glycyl residue, (-CH2C=ONH-) as function of ILs concentration. Our results suggest that all the investigated ammonium ILs are compatible with GPs and act as stabilizers for GPs structure through exclusion of ILs from GPs' surface. Furthermore, UV-vis spectrophotometer measurements are used as evidence for the stability of GPs in aqueous ILs at T = 298.15 K. (C) 2012 Elsevier Ltd. All rights reserved.