Salt-promoted protein folding, preferential binding, or electrostatic screening?
The extended coil/molten globule conformational equilibrium exhibited by ferricytochrome c in 10 to 20 mM HCl was examined using free boundary capillary electrophoresis. Addition of the osmolyte glucitol, also called sorbitol, to shift the conformational equilibrium toward the molten globule markedly diminished the mobility of the protein. This diminution can be entirely assigned to the relative viscosity of the added glucitol. The insensitivity of the viscosity corrected protein mobility to added glucitol suggests that both the extended coil and molten globule conformations of cytochrome c are free draining in an electrophoresis measurement. Addition of a neutral salt to shift the conformational equilibrium toward the molten globule conformation also markedly diminished the mobility of the protein. This diminution can be entirely assigned to the electrostatic screening afforded by the added salt. The onset of the conformational transition observed by optical measurements and the onset of electrostatic screening observed by mobility measurements appear to be in common for some but not all neutral salts. The exception suggests that preferential binding of the anion of a neutral salt to the molten globule conformation and not electrostatic screening is principally responsible for the shift in the conformational equilibrium of cytochrome c in acidic solutions.