If you really have the systematic absences for P41 or P43 then you must
have (at least) four molecules in the cell, twinning cannot reduce this.
Since a solvent content of 16% is too low, the most likely explanation
is that it is not twinned but that the protein is smaller than the one
you are expecting. With an unknown sequence and no experimental phase
information such as SAD or MAD, your only hope would be Arcimboldo, but
that requires data to about 2A or better (you don't mention the
resolution). As an even longer shot, you could search the PDB for
matching cells and space groups.
George
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-22582
On Fri, 1 Oct 2010, Clayton, Gina Martyn wrote:
> Hi there
>
> Just wondering if anyone has any suggestions how I can deal, if
> possible, with the following situation -. My first encounter with
> twinned data...
>
> which initially indexed as p43/p41 cell dimensions 58.4 58.4 61 90 90
> 90.
> Having seen the Matthews Coefficient for the solvent content of the unit
> cell as 16% I discovered the data are merohedrally twinned with twin
> fraction given as 0.1.
>
> I reprocessed the data as p1, p2 p21, c2221 (Rsymm etc indicates wrong
> space group, Mats Coeff 16%) p4 etc. In p2 the (pseudo) twin fraction is
> given as 0.43 (Mats Coeff 60% solvent 2.7 mol/ASU).
>
> I ran Detwin (ccp4) on the p2 p21 data with alternate operators as
> indicated by Xtriage in Phenix. I have had no molecular replacement
> solution i.e. Molrep rotational searches are not giving peaks and Phaser
> has not found a solution (nor with alternates of the search "model").
>
> Does anyone have any suggestions/best paper to consult etc based on
> their experience of twinned data (aside from sort the crystals out...)
> or should I "throw in the towell?"
>
> Thanks in advance for any information and advice
>
> Gina
>
>
>
>
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