Biology Reference
In-Depth Information
Protection of crystallization samples
and crystals by the oil
It is generally believed that external disturbances such as vibration can
cause excess nucleation and lead to the formation of smaller crystals or to
crystal imperfections. Trials under oil are preserved from physical shock
since the nuclei and the forming crystals are buoyed and cushioned by the
viscous oil, making trials resistant to vibration and allowing unmounted
crystals to be easily transportable.
The presence of the oil can offer the additional benefit of protecting
crystals which have formed in the oil from dissolution. Using vapor dif-
fusion one often encounters problems concerning changes in drop vol-
ume, particularly when precipitants such as polyethylene glycol and
volatile solvents are used. The absorbance of a volatile precipitating agent
(Yonath
et al
., 1982) or a slight change in temperature can cause enlarge-
ment of the drops, thus diluting the protein and causing dissolution of
crystals; this can occur during the short space of time when crystals are
being observed under the microscope. Provided the crystals are incubated
under a sufficiently thick layer of paraffin oil, the volume of the drops
remains constant and no dissolution occurs (Conti
et al
., 1996) unless the
solubility of the protein is temperature dependent [e.g. Zagari
et al
.,
(1994)].
Limitation of Crystallizing Under Oil
Application of organic molecules as precipitants
and/or additives
Experimental data has indicated that most macromolecules which were
tested could be crystallized under oil [e.g. Chayen (1998), and references
therein]. The oils described above do not interfere with the common
precipitants such as salts, polyethylene glycol (PEG), Jeffamine and
2-methyl-2,4-pentanediol (MPD). Moreover, samples containing detergents
have also been crystallized under oil (Snijder 2003; Stock
et al
., 1999).
However, not every case is suitable for crystallization under oil. A limitation
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