Looking for crystals suitable for three-beam diffraction

[Edgar Weckert <Edgar.Weckert@physik.uni-karlsruhe.de>]

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Dear All,

even if my concern is not specific to this newsgroups I would like to 
use this possibility to contact you.

During the last years we developed a three-beam diffraction technique
also for the determination of triplet phases from small macromolecular
compounds. For that purpose mainly crystals of known structures have
been used. For those of you interested: the method and some results
are described in Weckert et al. (1999) Acta Cryst. D55, 1320-1328
(http://journals.iucr.org/d/issues/1999/07/00/li0324/) and for the
theoretical background Weckert and Huemmer (1997) Acta Cryst. A53,
108-143.

Meanwhile we would like to investigate whether this method can also be
applied to unknown macromolecular crystal structures. Since the main
interest of our institute is small molecule crystallography and
diffraction physics we do not have any 'biological' infrastructure.
Therefore, we are looking for groups that might have crystals suitable
for phasing by three-beam diffraction.

The advantage of the method is that we need only native crystals and
an intensity data set as complete as possible. (Could be collected by
ourselves in the extrem low resolution part).

The crystals have to be of very low mosaicity and of reasonable
stability in let's say an unfocused monochromatic beam of the bending
magnet of a third generation synchrotron source. Since cryogenic
cooling increases in general the mosaicity, we have to measure close
to room temperature. The problem of the mosaicity is discussed in the
respective section of the Acta D paper cited above. (The values given
by area detector integration programs are not representative in most
cases !!)

So far good experiences exist for molecules up to a MW of about 25000.
For larger molecules crystals have to be in addition quite large
(>400-500 micro meter) otherwise the individual reflections are too weak
for this method.

If someone has crystals of an unknown protein structure that fulfill
this criteria I would be happy if we could get in contact.

Since we have already investigated quite a number of macromolecular
compounds of known crystal structure we are not interested in
structures already solved except if someone has really nice crystals
of something large (eg. unit cell > 1e6 A^3) for systematic studies on
larger systems. The constrains on mosaicity are quite high, therefore,
it is probably not worth trying crystals where derivatives are already
available or where the Se-Met.-(MAD)-method can be applied easily.


 Thanks in advance,
 
  Edgar Weckert


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Edgar Weckert			    
Institut fuer Kristallographie      
Universitaet Karlsruhe (TH)           
Kaiserstr. 12                       email: Edgar.Weckert@physik.uni-karlsruhe.de
D-76128 Karlsruhe                   phone: +49 (0)721 608 6045 (3320)
Germany                               fax: +49 (0)721 608 7264
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