A simple and rapid method for mounting protein crystals at room temperature

Cryocooling of protein crystals for X-ray data collection has now become a routine method in the majority of biostructural laboratories. The improvement of facilities at synchrotron sources and their increased use has made it essential to have properly frozen crystals for optimal data collection. Although in general crystals can be cooled without significant damage, there are often cases in which crystals with slight disorder or twinning problems suffer considerably during the freezing process. In other cases, poor or mosaic diffraction may be blamed on the cryoprotectant or cooling protocol. Many crystals may be wasted in searching for the best freezing conditions when the intrinsic quality of the crystals is poor. In principle, the collection of room-temperature diffraction data would provide a reference that would allow the detection of crystal damage caused by addition of cryoprotectant or by cryocooling. In practice, however, many investigators are reluctant to do this, one reason being that capillary mounting of crystals is a tedious method, especially for those who are new to crystallography. Here a simplified method for mounting crystals at room temperature is reported, which requires little expertise and no expensive equipment.