Non-ideality of aqueous solutions of polyethylene glycol: consequences for its use as a macromolecular crystallizing agent in vapor-diffusion experiments

Microisopiestic measurements of the concentrations of polyethylene glycol (PEG 8000) paired with the salts sodium chloride, ammonium sulfate and magnesium sulfate heptahydrate have been made in a sitting-drop arrangement with PEG in the droplet and salt in the reservoir. Resulting graphs of the concentrations of PEG and salt that are equivalent with respect to the vapor pressure of water are non-linear, do not intersect their origins, and demonstrate that relatively low (mM) salt concentrations are equivalent to relatively high PEG concentrations. The consequences of each of these observations for macromolecular crystallization by the vapor-diffusion technique when PEG is employed as the crystallizing agent are discussed.