Optimum solubility (OS) screening: an efficient method to optimize buffer conditions for homogeneity and crystallization of proteins.

One of the most critical steps in the preparation of protein samples for structural studies by X-ray crystallography is to obtain biochemically pure and conformationally homogenous protein samples. Very often, the purified sample does not meet these qualifications and therefore does not crystallize. A screening method, Optimum Solubility Screen, has been developed that consists of two steps. The first step selects a better buffer than that used during purification. 24 different buffers ranging from pH 3 to pH 10 are screened using a vapor-diffusion method and very small amounts of protein. The solubility of the protein is first determined by visual examination using a light microscope and those drops that remain clear after 24 h are further evaluated using dynamic light scattering. If the results from the first step are still not satisfactory, a second step explores a variety of chemical additives in order to improve the monodispersity of the protein sample. In 64% of the cases, crystallization was successful from proteins that had initially shown high levels of aggregation. This screen can be configured to perform in an automated high-throughput mode and can be expanded for additional buffers and additives.