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suitable for its nucleation and subsequent growth of crystals (Chayen,
1998).
The application of oil to improve vapour
diffusion experiments
The microbatch method and variations of it have established a new con-
cept in protein crystallization. However, vapor diffusion, which is still the
most popular method of crystallization, does have advantages that are not
fulfilled by microbatch. One such advantage is the possibility to affect the
equilibration rate of the trials (without the risk of the trials drying out) and
thus approach supersaturation more slowly by varying the distance
between the reservoir and the crystallization drop (Luft
et al
., 1996).
However, this cannot be achieved in the popular Linbro and CrysChem
(Hampton Research, USA) plates since a change in the drop-to-reservoir
distance is not sufficient to affect the equilibration rate in such plates
(Mikol
et al
., 1990). A means to slow down the equilibration rate and thus
approach supersaturation more slowly was devised (Chayen, 1997a) by the
introduction of an oil barrier over the reservoir of conventional vapor dif-
fusion trials (hanging or sitting drops) in Linbro and Cryschem plates
(Fig. 3a). It was demonstrated that the type of oil and the thickness of the
oil layer situated above the reservoir dictated the speed of crystallization.
In trials containing an oil barrier, crystals required over a week to grow to
full size, yet their number was reduced and their size and diffraction qual-
ity were far superior (Fig. 3c) to crystals which grew overnight in control
trials which had no barrier (Fig. 3b) (Chayen, 1997a, 1997b).
The contribution of oil to the control
of heterogeneous nucleation
Nucleation is a prerequisite and the first stage of any crystallization. The
ability to control this first stage would be a big step forward in designing
crystallization experiments, and hence studies concerning nucleation are
of high priority in the field of crystal growth.
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