Solvent-Induced Protein Refolding at Low Temperatures

Protein refolding at low temperatures is shown for a self-assembled system of human serum albumin (HSA) and spin-labeled fatty acids (FAs), in ternary solvent mixtures with usually denaturing cosolvents ethanol or ionic liquids (ILs). When HSA is natively folded, it offers FA binding sites, and the uptake and the distribution of these FA binding pockets have characteristic continuous wave electron paramagnetic resonance (CW EPR) and double electron-electron resonance (DEER) signatures. At room temperature, CW EPR shows that the addition of 35% (v/v) of ethanol or IL leads to HSA being unfolded. A temperature decrease yields bimodal CW EPR spectra with bound FA and free FA signals, indicating at least partial refolding of HSA, which is also confirmed by corresponding DEER data. This finding is based on increased protein stability at lower temperatures and a change in the preferential solvation of the protein by glycerol in the ternary solvent mixtures.