W0113

Analysis of Flash Cooling and X-ray Heating of Protein Crystals. S. Kriminski^a, M. Kazmierczak^b and R. E. Thorne^a, ^aLaboratory of Atomic and Solid State Physics, Cornell Univ., Ithaca, NY 14853 USA, ^bDept. of Mechanical Engineering, Univ. of Cincinnati, Cincinnati, OH 45221 USA.

Three problems involving heat transfer from a protein crystal are analyzed: flash cooling in a cold nitrogen or helium gas stream; plunge cooling into liquid nitrogen, propane or ethane; and crystal heating in a cold gas stream due to X-ray absorption. The analysis is used to prioritize experimental parameters affecting the success of low-temperature X-ray data collection procedures and to evaluate methods for reducing crystal heating by X-ray absorption.

For flash cooling in cold gas streams heat transfer is limited by the rate of external convection and not by the thermal conductivity of the sample so that internal temperature gradients and strains during cooling are very small. Helium gas provides only factor-of-three improvements in cooling rates relative to nitrogen because its larger thermal conductivity is offset by its larger kinematic viscosity. Characteristic cooling times vary with crystal size L as L^3/2, and theoretical estimates of these times are consistent with experiments. Plunge cooling into liquid cryogens, which can provide much smaller convective thermal resistances provided that surface boiling is eliminated, can increase cooling rates by more than an order of magnitude. At the same time plunge cooling produces much larger internal temperature gradients and strains that may damage crystals, especially in the case of large crystals when the internal conduction resistance is no longer negligible. Based on this analysis, factors affecting the success of flash cooling experiments can be ordered from most to least important as follows: (1) crystal solvent content and solvent composition; (2) crystal size and shape; (3) amount of residual liquid around the crystal; (4) cooling method (liquid plunge versus gas stream); (5) choice of gas/liquid; and (6) relative speed between cooling fluid and crystal.

Crystal heating by X-ray absorption on present high-flux beam lines is small. For a fixed flux and illuminated area, heating can be reduced by using crystals with areas normal to the beam that are much larger than the beam area.