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To reduce radiation damage of the radicals formed by ionizing X-ray photons, data collection is usually performed at ~100 K, prolonging significantly the life of the crystal (see Nave, C. & Garman, E.F. Towards an understanding of radiation damage in cryocooled macromolecular crystals. Journal of synchrotron radiation 12, 257–260 (2005): "up to 70 times more absorbed dose can be delivered to a cryocooled macromolecular crystal compared with one at room temperature before significant degradation in crystal order occurs").

Cryogenic temperatures (~100 K, or 90-120 K) close to the boiling point of liquid nitrogen (77 K) reduces thermal vibrations, and flexible parts of the protein can become distinct in electron density.

Symptoms of radiation damage: 

    During data collection
        decrease in diffraction intensity
        decrease in resolution
        increase in unit cell volume
        color changes
    During model rebuilding-refinement
        breakage of disulfide bonds
        demethylation of Met
        decarboxylation of Asp, Glu and the C-ter
        loss of OH from Tyr

Ref. Biomolecular Crystallography By Bernhard Rupp       

Cryoprotectant database for protein crystals:Common cryoprotectans:

A large number of compounds have been used successfully for the cryoprotection of macromolecular crystals (Table 1). [...] Finding the right cry-oprotectant for a particular system is to some extent a matter of trial and error. In many instances, the cryoprotection of crystals grown from aqueous/organic mixtures or from buffers of low salt concentration can be achieved easily by the addition of organic compounds [...] in a large proportion of these cases the following guidelines will lead to success. If a polyethylene glycol (PEG) with a molecular weight of less than 4000 or MPD is used as the precipitating agent, increasing the concentration of the precipitant can yield a good cryobuffer. For crystals induced by PEGs with molecular weights of 4000 and higher, the addition of small PEGs (i.e. PEG 400 or PEG 600) is recommendedFor crystals grown by salting out at relatively low salt concentrations, MPD and ethylene glycol are promising candidatesIn other cases, glycerol at concentrations up to 50% seems to be an almost universal cryoprotective agent for a wide range of commonly used crystallization buffers. 

A review paper published in 2004,