Electrostatic Potentials Around The Proteins Preferably Crystallized By Ammonium Sulfate

Ammonium sulfate (AS) is one of the most popular precipitants in protein crystallization. However, AS is not always effective for all kinds of proteins. Some proteins are easily crystallized by utilizing AS, while others are not. Polyethylene glycols (PEGs) is also frequently used in protein crystallization. To clarify the reason a protein has a preference for a precipitant in crystal growth, we investigated electrostatic potentials for 100 kinds of proteins that had been crystallized by high concentrations of AS. Most of the proteins have a common shape for the isosurface of their electrostatic potentials. The positive and negative areas of electrostatic potential are almost equally separated. The contact between the positive and negative areas is limited to a narrow region on the protein surface. The separation of the electrostatic potential is neat even at the contact region. In contrast, the separation between positive and negative areas is not clear for the proteins preferably crystallized by PEGs. The positive and negative areas of electrostatic potential are fragmentary, and the isosurface at the contact region is complicated. These findings suggest that not only the local interaction between AS ions and a protein molecule but also the surroundings of the protein are responsible for crystal growth.