Urea and salt induced modulation of protein interactions: implications for crystallization and liquid–liquid phase separation†

Abstract

Urea is a well-known protein denaturant if added at high concentrations. The unfolding of proteins induced by urea is typically attributed to specific mechanisms; however, the influence of urea on protein–protein interactions, which can give rise to protein crystallization and liquid–liquid phase separation (LLPS), remains less understood. In this study, we examine the modulation of protein–protein interactions by urea at non-denaturing concentrations, in combination with sodium chloride. The effects of these additives on the state diagram and protein–protein interactions in lysozyme solutions are analyzed using optical microscopy and small-angle X-ray scattering (SAXS), respectively. Our findings indicate that the addition of urea diminishes net protein attractions, while the introduction of salt enhances them, resulting in respective shifts of the state boundaries. Moreover, the protein–protein interactions can be effectively characterized by a Derjaguin–Landau–Verwey–Overbeek (DLVO) potential. The impact of urea on these interactions can thus be explained entirely by alterations in the dielectric properties of the solution.