Hi,
If there's an NCS translation, recent versions of Phaser can account for it and give moment tests that can detect twinning even in the presence of tNCS. But I agree with Eleanor that the L test is generally a good choice in these cases.
However, the fact that you see density suggests that your crystal might be more on the statistical disorder side of the statistical disorder <--> twinning continuum, i.e. the crystal doesn't have mosaic blocks corresponding to one twin fraction that are large compared to the coherence length of the X-rays. So you might want to try refinement with the whole structure duplicated as alternate conformers.
Best wishes,
Randy Read
-----
Randy J. Read
Department of Haematology, University of Cambridge
Cambridge Institute for Medical Research Tel: +44 1223 336500
Wellcome Trust/MRC Building Fax: +44 1223 336827
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Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk
On 11 Mar 2014, at 14:10, Eleanor Dodson <[log in to unmask]> wrote:
> Sorry - hadnt finished..
> The twinning tests are distorted by NC translation - usually the L test is safe, but the others are all suspect..
>
>
>
> On 11 March 2014 14:09, Eleanor Dodson <[log in to unmask]> wrote:
> What is the NC translation? If there is a factor of 0.5 that makes SG determination complicated..
> Eleanor
>
>
> On 11 March 2014 14:04, Stephen Cusack <[log in to unmask]> wrote:
> Dear All,
> I have 2.6 A data and unambiguous molecular replacement solution for two copies/asymmetric unit of a 80 K protein for a crystal integrated
> in P212121 (R-merge around 9%) with a=101.8, b=132.2, c=138.9.
> Refinement allowed rebuilding/completion of the model in the noraml way but the R-free does not go below 30%. The map in the model regions looks generally fine but there is a lot
> of extra positive density in the solvent regions (some of it looking like weak density for helices and strands) and unexpected positive peaks within the model region.
> Careful inspection allowed manual positioning of a completely different, overlapping solution for the dimer which fits the extra density perfectly.
> The two incompatible solutions are related by a 2-fold axis parallel to a.
> This clearly suggests some kind of twinning. However twinning analysis programmes (e.g. Phenix-Xtriage), while suggesting the potentiality
> of pseudo-merohedral twinning (-h, l, k) do not reveal
> any significant twinning fraction and proclaim the data likely to be untwinned. (NB. The programmes do however highlight a
> non-crystallographic translation and there are systematic intensity differences in the data). Refinement, including this twinning law made no difference
> since the estimated twinning fraction was 0.02. Yet the extra density is clearly there and I know exactly the real-space transformation between the two packing solutions.
> How can I best take into account this alternative solution (occupancy seems to be around 20-30%) in the refinement ?
> thanks for your suggestions
> Stephen
>
> --
>
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> Dr. Stephen Cusack,
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