Towards atomic resolution with crystals grown in gel: the case of thaumatin seen at room temperature

One reason for introducing a gel in the crystallization medium of proteins is its ability to reduce convection in solution. This can lead to better nucleation and growth conditions, and to crystals having enhanced diffraction properties. We report here the X-ray characterization at room temperature of high-quality crystals of the intensely sweet thaumatin prepared in a sodium tartrate solution gelified with 0.15% (m/v) agarose. Using a synchrotron radiation, these crystals diffracted to a previously unachieved resolution. A diffraction dataset was collected from four crystals at a resolution of 1.2 A with a R(sym) of 3.6% and a completeness of 99%. Refinement was carried out to a final crystallographic R-factor of 12.0%. The quality of the electron density map allowed for the observation of fine structural details in the protein and its solvation shell. Crystallization in gel might be used more generally to improve the quality of macromolecular crystals. Advantages provided by the gelified medium in the frame of structural studies are emphasized.