Flash-cooling of macromolecular crystals in a capillary to overcome increased mosaicity

This paper describes the usefulness of flash-cooling in a capillary (FCC) for X-ray diffraction data collection from macromolecular crystals. FCC may be applied when conventional cooling using a cryoloop fails. This technique cools crystals in a capillary instead of in a cryoloop and thus cools crystals more slowly than conventional cooling. Measurements of cooling rates have shown that the time taken to reach the final temperature is two to eight times longer using FCC than using a cryoloop, depending on the volume of cryoprotectant solution around the crystal. Using this cooling technique, the crystal structures of isocitrate dehydrogenase and protein L-­isoaspartate O-­methyltransferase have been solved at 1.95 and at 2.50 Å resolution, respectively. Both crystals could not be cooled by the conventional method using a cryoloop. Moreover, diffraction data from crystals of the hypothetical proteins PH-A and PH-B were also collected successfully using the FCC method. These results show that some crystals, especially larger ones, need to be cooled slowly.