Jeroen brings up a good point. Back in the old days, around 5 B. C.
(Before Cryo), we would use a chilled air generator to blow a stream
of cold air along the capillary axis to keep the crystals just above
their freezing point--it made a huge difference in crystal lifetime.
I recall a colleague devising an apparatus from a 50 ml conical tube.
The bottom was cut off and cold air was blown in from the other end.
Windows were cut in either side to allow the beam to pass & covered
in mylar. This way the entire capillary was contained within the
cold tube, so no temperature gradients formed along the length of the
capillary (temp gradient => distillation => dead crystal). Later, we
purchased a very clever goniometer head from Nonius that had a
plastic cylinder attached to goniometer head, with a swivel, so the
hose supplying cold air didn't get tangled during data collection...
I've often thought duplicating this apparatus when we encounter cryo
problems, but I'm always stymied when trying to find a cheap and
simple source of cold air. Any bright ideas?
On Jul 10, 2007, at 5:00 AM, mesters wrote:
Mary,
freezing habitually increases mosaicity. In your case, the high
water content adds to the problem.
Try not to freeze the crystal but collect at sub-zero temperature
(in short glass capillaries or use oil plugs instead).
You have to optimize the "close to freezing" data-collection
temperature.
I collected complete synchrotron datasets (of GCPII in buffer with
PEG1500 and PEG400) at 260-263 Kelvin which resulted in mosaicity
values of as small as 0.07 degrees! At 277 K, the crystals only
last for a few images and freezing did not work (for the buffer
mentioned before).
- J. -
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Patrick J. Loll, Ph. D. (215) 762-7706
Associate Professor FAX: (215) 762-4452
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Drexel University College of Medicine
Room 10-102 New College Building
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