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[ccp4bb]: strange diffraction pattern



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Dear all,

We have now observed for the second time within a few months a strange
diffraction behaviour of two totally unrelated proteins which
crystallize in space group R32. Whereas for the first protein (sex
hormone-binding globulin) we did get nice diffracting R32 crystals with
one steroid bound, we now keep getting these misordered crystals with a
different steroid. For the second protein, we only get these misordered
crystals.

To make the discussion easier, I did put JPEG files of the diffraction
pattern onto our WEB-server. The three images can be accessed by the
following link. The last image is annotated in order to make the points
below more clear.

http://www.mdc-berlin.de/~crystal/struc/structurelist.html

The major characteristics of these images are, that planes with nicely
shaped spots do alter with planes which have smeared reflections.
Basically planes with l=3n (hexagonal setting) do alter with planes
(l=3n+1, l=3n+2) with smeared reflections.

I thought about reticular merohedral twinning, but then I would expect
twice the number of reflections on the planes (l=3n+1, l=3n+2) but still
nice spots. Maybe not ??

One other characteristics is that the smeared spots on these planes do
not fall on the R32 lattice but are slightly displaced. If you draw a
line on the 1 oscillation images between the nice spots, the smeared
spots do not fit onto the line (See image 3). I therefore tried to index
with a P3 lattice instead of the R3 lattice. In that case the
reflections are fitted better but of course there is an are enormously
high number of predicted spot positions with no spots to be seen.

What intrigues us is that we observed this now for two different
projects. Probably we will just have to look for better crystallisation
conditions, however it would help us if we could understand the problem.

Therefore any help is greatly appreciated and in case the solution to
the problem is not too embarrassing for me, I will of course post a
summary.

Best regards
Yves

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Yves Muller
Forschungsgruppe Kristallographie
Max-Delbrueck-Centrum fuer molekulare Medizin  Tel: +49-30-9406-2725
Robert-Roessle-Str. 10                         Fax: +49-30-9406-2548
D-13122 Berlin                             E-mail: yam@mdc-berlin.de
                   http://www.mdc-berlin.de/~crystal/cowork/yam.html
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