[ccp4bb]: Non-merohedral twinning......

[raji <raji@lamar.colostate.edu>]

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Hi Everyone,
I need help with determining whether my crystal is non-merohedrally twinned or 
not. And, if my crystal is twinned, can I find a program that can detwin 
non-merohedrally twinned data?
Here's what I find so far-
The crystals by themselves are not microscopically twinned. However, the 
diffraction pattern has some reflections adjacent to some main reflections. 
These reflections are NOT being picked up by the indexing routine.
I am able to index, scale my data fine. The spacegroup is p212121 and the 
scaled data is very consistent with p212121. I'd like to point out that my a 
and b axis are almost close to one another (105 109). The R-merges are as low 
as I have seen with other related data sets and the completeness is excellent. 
I performed merohedral twinning tests and the crystals are untwinned according 
to the Yeates test.
I performed Molecular Replacement on my data and got good solutions for 
cross-rotation and translation searches. My maps dont look abnormal at all 
(although I am not experienced at all to determine what one can find out about 
twinning from a map).
Is it possible to have completely non-overlapping twins and if so, can one 
ignore the 2nd lattice? Or, do I have some kind of crystal splitting ?? Is 
there a way to distinguish between splits and twins ? 
Since, so far I am unable to decide what's going on, I'd like some suggestions 
as to how I can either establish or rule out non-merohedral twinning...
Thank you very much.
Raji

Rajeswari Edayathumangalam
Dept. of Biochemistry & Molecular Biology
Colorado State University
Fort Collins, CO 80523 USA