Dear Jacob,
For the molecular replacement part, twinning is not a great barrier. We've generally found that, as long as you get the space group right (which can be an issue with twinned crystals), the signal in the MR search is not that badly degraded. You could easily set up a search using different possible calmodulin conformers as alternatives and see which gives some signal (if any).
The signal in SAD phasing is probably affected worse, but there are still plenty of examples where people have succeeded with twinned crystals. I don't have a list of references at hand, but people on the BB probably can point you at some of them. However, if you're able to get a convincing MR solution then MR-SAD should work.
You didn't mention whether or not your crystal suffers from translational NCS. With the new version of Phaser that handles tNCS this is arguably an advantage, as there are fewer truly independent copies to find. By coincidence, we published a paper last year, together with Mariusz Jaskolski, Zbyszek Dauter and their colleagues, on a structure that combined tetartohedral twinning and 7-fold tNCS: http://journals.iucr.org/d/issues/2014/02/00/yt5061/index.html.
Best wishes,
Randy Read
On 2 Feb 2015, at 16:16, Keller, Jacob <[log in to unmask]> wrote:
> Dear Crystallographers,
>
> Is there any software in CCP4 which can solve twinned structures? I have several datasets which appear to be tetartohedrally twinned, with possible spacegroup of p31/p32 masquerading as p6222/p6422. I think this is approximately equivalent to 1/8-fold NCS?
>
> The data quality (Rpim, CC1/2) is great and resolution is good at 1.9 Ang, although cut off by detector edge I/sig = ~4. Also, I have a possible SAD dataset with CsI soaked in, measured at CuKa, as well as [way too many] possible MR models (calmodulin, which has a ton of different conformers in the PDB). A major problem is that the cell is pretty big, ~100x100x130, so this means ~8 copies, and with tetartohedral twinning, this is more like 64 monomers I think. I am thinking to try MR-SAD, with some brute-force search-model testing, but I am not sure whether/where this could be done in CCP4. Any suggestions in terms of approaches and/or software?
>
> BTW, additive screens to try to grow untwinned crystals did not pan out, and I am pretty much out of protein. I do have a few more nice crystals to play with, but they all have the same morphology.
>
> Jacob
>
>
> *******************************************
> Jacob Pearson Keller, PhD
> Looger Lab/HHMI Janelia Research Campus
> 19700 Helix Dr, Ashburn, VA 20147
> email: [log in to unmask]
> *******************************************
------
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
Hills Road E-mail: [log in to unmask]
Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk
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