Helix Formation by Poly(amino acid)s in Neat Ionic Liquids and Their Mixtures with Buffer

Many polypeptides and proteins dissolve in neat ionic liquids (ILs) or aqueous buffers containing a significant level of IL as added electrolyte. Conformation and activity are difficult to assess when the IL content is high, but numerous experiments suggest that native secondary structures are stabilized while native tertiary structures are not. In the mixed IL/buffer solvents examined, insolubility was always noted across a window of intermediate IL content. To understand structure and solubility in the presence of ILs more systematically, two helix-forming poly(amino acids)s, poly(l-lysine) and poly(l-glutamic acid), were adopted as model systems to probe structure and solubility as IL content and temperature varied; the IL was tributylehtylphosphonium diethylphosphate, which at high concentration, dissolves both poly(amino acid)s as α-helix. The corresponding mixed solvent solubility/conformation maps, determined by circular dichroism and FTIR, show regions of solubility and order (α-helix, β-sheet) positioned differently than for an aqueous buffer containing added simple electrolyte (i.e., KCl).