Growing protein crystals that are of sufficient size and quality for conventional X-ray structure analysis can be extremely challenging. Many biologically significant proteins, including membrane proteins, tend to form crystals which diffract poorly due to a lack of good crystallinity. The presence of disorder within the crystal can make structure determination with conventional protein crystallography methods challenging as only relatively small amounts of disorder are tolerated. Methods of studying disorder and the associated crystal strain in biological and protein crystals have thus far been limited to relatively low spatial resolution techniques, such as reciprocal space mapping (RSM), which preclude the study of smaller, micron-sized crystals.

The three-dimensional strain distribution in nanocrystals of ‘radiation hard’ materials (e.g. Au, Pb and diamond) has been previously studied using a technique known as Bragg coherent diffractive imaging (BCDI). BCDI is able to determine atomic displacements within a crystal using the continuous diffraction pattern around a Bragg peak [1].

Advances in detector technology and sensitivity have recently allowed us to extend these ideas to disordered micron-sized protein crystals. Our current work is focused on incorporating this additional information into strategies for structure determination for disordered crystals in cases where conventional crystallographic approaches fail. The key to this technique is in the high-resolution characterisation of the disorder and strains within the protein crystal which can be used to refine the recovery of the protein structure.

Here we present the underlying theory behind our approach to protein structure determination and discuss the potential implications for improving the quality of structures obtained from imperfect protein crystals.