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Hi Markus, Juergen,
I doubt slippage would result in a pseudo translational vector.
In some circumstances, two crystals sitting on top of each other can
however produce this effect, it is called non-merohedral twinning.
trying not to embarrass myself, see here an (2D) unit cell, with *'s
denoting the corners:
*-----------*
| |
| |
*-----------*
Now be creative and stick two together:
*-----------*
| |
| |
*-----+-----*
| |
| |
*-----------*
Note that this set of two unit cells has a higher symmetry than the
single cell: a(n approximate) four fold axis is present at the +. If you
rotate the two unit cells (or sublattices) you get
*-----------* *-----*-----*
| | | | |
| | | | |
*-----------* * | *
| | | | |
| | | | |
*-----------* *-----*-----*
overlaying these two you get something like:
*-----*-----*
| | |
| | |
*-----|-----*
| | |
| | |
*-----*-----*
If you have a situation like this, and if it is indexed like the small
unit cell, you will have a large number of unaccounted spots, so you
actually might end up with an indexing solution that has the 'super
cell' shown above, albeit with a number of indices that have no
significant intensity (given rise to large peaks in the Patterson: your
pseudo translational symmetry)
Not sure what to do with non merohedrally twinned data, besides panick,
scream and/or hide (especially on Halloween). I believe shelxl is (of
course) able to deal with it during refinement.
There are two papers that inspired my explanation (or blatant copy) as
seen above:
Parsons, Acta Cryst. (2003). D59, 1995-2003
(above example, but nicer and more thorough explanation)
Dauter, Acta Cryst. (2003). D59, 2004-2016
(general twinning stuff and nice images of non-merohedral
diffraction patters)
Note that the presence of pseudo translational symmetry is not that
uncommon, and the excursion into non merohedral twinning as seen above
might be simple paranoia....
Cheers,
Peter
Juergen Bosch wrote:
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Hi Markus,
since you are in desperate mode, I would recommend XDS for integration
to rescue your dataset and hopefully come up with something decent to
work with.
Regarding the pseudotranslation I would tend to say you can't get this
from a split crystal or from two crystals in the loop - but I'm sure
some expert will prove me wrong with my statement or perhaps not :-)
Juergen
Markus Rudolph wrote:
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This may be a PCI question:
Is it possible to introduce pseudo-translation by integrating a
diffraction pattern from a split crystal or when two crystals in a
loop with a defined orientation have been collected on? It might be
that during integration the mask slips from one pattern to another.
Some sharp reflections are integrated single when the box size is
small enough while others are split so little that they are integrated
as a single reflection. Is there software that could help in
integrating and separating such a split pattern (Bruker software does
not take marccd images)?
Ah, and no, there is no other dataset, no other crystal, no more protein.
Thanks for any input,
Markus
___________________________________________________________________________________
Markus Rudolph phone: +49 551 39-14088
Dept. Mol. Struct. Biol. fax: +49 551 39-14082
Georg-August University e-mail: [EMAIL PROTECTED]
Justus-von-Liebig-Weg 11
___________________________________________________________________________________
