freezing crystals

tivol at tethys.ph.albany.edu tivol at tethys.ph.albany.edu
Tue Nov 8 16:29:34 EST 1994


Augustin de la Calle's response to Phoebe Rice suggested that freezing cry-
stals in liquid nitrogen before inserting them in the cold N2 gas stream
would be a second choice if freezing in propane is not allowed.  We in elec-
tron microscopy have been freezing tissue blocks for some time, and using
liquid nitrogen for this is *not* satisfactory.  The two situations differ
in several ways which may or may not be significant.  The problem with dir-
ect use of liquid nitrogen is that it boils violently and produces a volume
of relatively warm and thermally non-conducting gas which slows the freezing.
In the event that there is not enough total solute disolved in the water,
the water in tissue, and also in a crystal, will expand upon freezing and
distort the structure.  Rapid freezing, high pressure freezing (in the regime
where water does not expand on freezing), and use of cryoprotectants such as
glucose (in a concentration where the solution does not expand on freezing)
are three methods widely used to preserve structures.  If use of liquid nit-
rogen works--the crystal still diffracts--then go ahead, but if not, you
should try one of the other methods.  Cryoprotectants will be the least ex-
pensive and the simplest method, if the particular crystal is stable in them.
Good luck.
				Yours,
				Bill Tivol




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