Further Qs.
Do you have a noncryst translation parallel to the b axis (ctruncate
will list any such translation..)
If the b shift is 0.5 then the 0k0 "absences" will be present whether
the spacegroup is P2 or P21.
How many Xe sites do you expect? If there is only one then phasing is
more difficult in monoclinic SGs - you have to break the centrosymmetry
of the heavy atom distribution.
Do you have native data without Xe?
I always check for peaks which are consistent in the isomorphous and
anomalous difference pattersons.
The NCS symmetry will doubtless make the exptl phasing more complicated,
but it should help you with averaging later!
Eleanor
On 10/03/2011 03:33 PM, Marta Ferraroni wrote:
> Dear all,
>
> We collected some Xenon derivatives of a protein (an heterotetramer
> α2β2)that seems to crystallize in P21 since the 0k0 reflections with
> k=2n+1 are not present. However in the anomalous Patterson we found
> strong peaks in the section v=0 yet none in the Harker section v=1/2.
> Furthermore we weren't able to solve the structure both in P2 and P21
> using these derivatives with the most commonly used programs.
>
> The cell is 89 125 90 90 102 90 so a is approximately equal to c that
> could permit pseudomerohedral twinning albeit the tests (Padilla-Yeates
> and Britton) estimate a fraction of twinning around 0.05.
>
> Data cannot be scaled as C orthorhombic even if the data reduction
> programs (XDS and imosflm) assign a higher score to the related
> orthorhombic cell with dimensions 112 139 124 90.00 90.00 90.00. In the
> native Patterson map there are not strong peaks. According to the
> Matthews coefficient the asymmetric unit contains 2 heterotetramers and
> the self rotation function may indicate a 222 non crystallographic
> symmetry with one two-fold axis perpendicular to the crystallographic
> one (see figure attached).
>
> Our questions are:
>
> 1) why the anomalous Patterson is not consistent with the space group?
>
> 2) Is there the possibility that the NCS could hamper the determination
> of the correct space group and eventually determine a lower estimate of
> the twinning fraction?
>
> thanks in advance for your help
>
> Regards,
>
>
> Marta Ferraroni
> Dept. of Chemistry
> University of Florence
> Italy
>
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