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Re: [ccp4bb]: indexing

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On Wed, 19 Apr 2000, Quan Hao wrote:

> We have collected some data on station F1 at CHESS. The crystals appeared to
> be hexagonal thin plates and diffracted to about 2.2A. We collected 75
> degrees of data for each of the three crystals. The spot shapes looked OK
> but we could not index the spots using DPS, DENZO or MOSFILM. The spots
> seemed to align along straight lines on all images and one could see the
> hexagonal features. Has anyone come across this before? Your help is greatly

Think of twinning! Think of pseudo-crystalline aggregates!

We once came across something possibly related to your problem.
Our xtals looked quite hexagonal and we indexed in some P6xx
space group. Later it turned out that the true space group
was P3xx and we had a meroedric twin. Those "crystals" were
aggregates of mini-crystals in two different orientations
(relative rotation by 60/180 deg.) whose diffraction intensities
(not amplitudes !) added up in the same spots. Since there is
no problem with the cell dimensions, this twinning can be very
precise and can simulate real mono-crystals out to quite a decent

Try detwinning. If is doesn't work you might have to grow
crystals in a different space group.

 Dr. Manfred Buehner             Phone:  +49-931-888-4100
 Physiologische Chemie           FAX:    +49-931-888-4150
 Theodor-Boveri-Institut fuer Biowissenschaften
 Biozentrum der Universitaet Wuerzburg
 Am Hubland,  D-97074 WUERZBURG,  Germany
 Email:  buehner@biozentrum.uni-wuerzburg.de