COOLING RATES DURING FLASH COOLING

Flash cooling of macromolecular crystals was investigated by measureme nt of the cooling rates achieved by different cryoprotectant solutions (glycerol, sucrose and polyethylene glycol) with different sample vol umes (0.2-0.8mm(3)) and by different cooling agents (cold nitrogen or helium gas, liquid nitrogen and liquid propane). Samples were not cool ed instantaneously; it usually took 0.5 to 1 s to reach the glass tran sition temperature and 1 to 2 s to reach the final temperature. The co oling rate varied from similar to 50 to 700 K s(-1) depending on the v olume 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 thro ughout the volume of the crystal, and hence its mosaic spread.