Cooling Rates During Flash Cooling

Flash cooling of macromolecular crystals was investigated by measurement of the cooling rates achieved by different cryoprotectant solutions (glycerol, sucrose and polyethylene glycol) with different sample volumes (0.2-0.8 mm³) and by different cooling agents (cold nitrogen or helium gas, liquid nitrogen and liquid propane). Samples were not cooled instantaneously; it usually took 0.5 to 1 s to reach the glass transition temperature and 1 to 2 s to reach the final temperature. The cooling rate varied from ~50 to 700 K s^-1 depending on the volume of the sample, the cooling agent used and the temperature at the time of measurement. The cooling rate may affect the distribution of unit-cell dimensions, structural states and the average structure throughout the volume of the crystal, and hence its mosaic spread.