Protein crystalline, cryoprotection

III. X-Ray Studies at Subzero Temperatures Physical-Chemical Basis of Cryoprotection of Proteins in Solution and in the Crystalline... 

All these results are encouraging for investigators planning to use X-ray diffraction in mixed solvents at subzero temperatures and the rest of the present article will be devoted to a discussion of methods and preliminary results in this field. The methodology for cryoprotection of protein crystals, its physical-chemical basis, and the specific problems raised by the crystalline state, as well as the devices used to collect data at subzero temperatures, will be described. Limitations and perspectives of the procedure will be discussed critically. First attempts to determine the structure of productive enzyme-substrate intermediates through stop-action pictures will be described, as well as investigations showing that X-ray diffraction at selected normal and subzero temperatures can reveal protein structural dynamics. 

Work in solution is an absolute prerequisite for further studies of enzyme-substrate intermediates in the crystalline state. According to the Arrhenius relationship, k = A exp(—E IRT), which relates the rate constant k to the temperature, reactions normally occurring in the second to minute ranges might be sufficiently decreased in rate at subzero temperatures to permit intermediates to be stabilized, and occasionally purified by column chromatography if reactions are carried out in fluid solvent mixtures. Therefore, the first problem is to find a suitable cryoprotective solvent for the protein in question. 

---