Crystallization and vitrification in aqueous systems

Abstract

SUMMARY Some key ideas and experimental findings concerning the probability that crystallization of a liquid or its binary solutions will occur at moderate cooling rates are discussed. The use of cryoprotectants and of pressure to favourably influence these probabilities is rationalized, and some of the newer findings on small sample supercooling phenomena are reviewed.

Flash-cooling and annealing of protein crystals

“…Better diffraction results are obtained when crystals are¯ash-cooled to temperatures below water's bulk glass transition at T g 9 140 K by immersion in a liquid cryogen (nitrogen or propane) or by insertion into a cold stream of gas (nitrogen or helium). Characteristic cooling times, limited by the crystal's thermal conductivity and heat capacity and by heat transfer to the surrounding medium, are estimated to be $0.1±1 s (Teng & Moffat, 1998;Walker et al, 1998) and are long compared with important microscopic timescales in bulk water (Angell & Choi, 1986).…”

Macromolecular Cryocrystallography

“…At temperatures below the temperature corresponding to "r,ose, a regime is entered that is dominated by the quenching of particle fluctuations, resulting in an increase in To,t as icecrystal formation is greatly retarded. For pure water, Tnose is of the order of 10 -6 tO 10 -10 S (Angell & Choi, 1986). If cooling is performed on a timescale that is fast compared to Those, only limited crystallization will occur.…”

“…This problem has plagued the field of cryoelectron microscopy for decades and numerous techniques have been developed to suppress the formation of ice lattices in biological specimens. Excellent reviews describing the various methods of cryoprotection and the underlying physicochemical principles are available (Angell & Choi, 1986;Steinbrecht & Zierold, 1987;Echlin, 1992). Here, we concentrate on the principles of Kanno, Speedy & Angell (1975) and Franks (1985)].…”

“…These kinetics can be understood in terms of a simple model based on the temperature dependence of two characteristic times (Uhlmann, 1972;Angell & Choi, 1986). For illustration, see Fig.…”

Cryocrystallography of Biological Macromolecules

“…Addition of organic solutes to the stabilizing solutions used for protein crystals lowers the freezing point of the respective solutions and slows down the crystallization kinetics of ice [15]. The first condition for a cryoprotective buffer is that the buffer should, at a given cooling rate, not show any detectable crystalline ice.…”