Understanding salt or PEG induced attractive interactions to crystallize biological macromolecules

Phase diagrams of biological macromolecules are governed by an appropriate combination of interaction potentials in solution. Repulsive regimes favor solubility, whereas the presence of attractive potentials may induce a variety of phase transitions, including the desired macromolecular crystallization. The forces at work may be analyzed with a combination of small angle X-ray scattering and of numerical treatments. From the results obtained with a variety of model systems, the respective advantages and drawbacks of using monovalent salts or PEGs as crystallizing agents are discussed.