Phase contrast imaging

A large number of SAD/MAD experiments fail due to radiation damage, which alters the structure and can induce large changes in the intensities of the diffracted beams preventing the user from collecting truly redundant data sets. Tomography could help to overcome this problem, by providing a 3D model of the vitrified crystal surrounded by a solvent and sample holder. The provision of such a 3D model would overcome the need to generate redundancy-dependent empirical models, and would help to separate systematic from random errors. As described in technical overview phase contrast experiments with poly¬chroma¬tic high energy radiation have shown encouraging results. Initial studies (see figure) done at ID15A in collaboration with EMBL indicate that x-ray tomography can be used to locate crystals within a sample holder and water bubble, and to measure the shape. Current programs such as C3D use visible or UV light, and fail in the presence of ice, multiple crystals, or due to lens-effects. X-ray tomography has great potential to overcome these bottlenecks.

3-D phase contrast tomographic reconstruction of heavy-atom soaked protein crystal using high energy white x-ray beam. The protein is the cube which is encapsulated within a cryogenic liquid.