Re: [ccp4bb]: Monoclinic twinning?

[Christopher Colbert <colbc@chop.swmed.edu>]

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Hi Phil,

You are correct that straight scaling does not give a true indication of
perfect twinning (twin fraction = 0.5).  However, given the twin analysis
numbers given by Matt.  Scaling will clearly give away partial twinning.
I think my main point is similar to what your examples indicate.  That
monoclinic can masquerade as orthorhombic when the Beta angle approaches
ninety and that careful, complete analysis will reveal what is truly
happening.  Agreed?  

Later,

Chris

On Thu, 6 Feb 2003, Phil Jeffrey wrote:

>On Wed, 5 Feb 2003, Christopher Colbert wrote:
>
>> I don't think you have twinning.  How do your scaling statistics vary
>> between P222 and P2 with each axis being the unique axis?  If the
>> R-factors drop considerably from P222 to P2, then you most likely have P2
>> symmetry.  If they only change by a little ([Flame retardent on] a
>> specific number is a topic of heated debate [Flame retardent off]), then
>> you have the higher symmetry of P222 and should re-evaluate the screw axes
>> by careful inspection of the appropriate zones for systematic absences.
>
>Unfortunately, this is not a definitive test for twinning.  One of the p53
>xtal forms was monoclinic with a 90 degree beta angle.  One dataset we
>obtained had a twin fraction of ~0.5, therefore it scaled in primitive
>monoclinic and primitive orthorhombic with very similar statistics in
>Scalepack (including Chi**2, which might be a better test than R-symm).
>
>However since we have other datasets with lower twin fractions we knew
>a priori that the true space group was P21.
>
>One of the Brca2 crystal forms was also perfectly-twinned monoclinic
>masquerading as primitive orthorhombic, and in fact we believed it was
>P2x2x2x and collected several datasets on it as such, until molecular
>replacement revealed that it was not consistent with any orthorhombic
>space group.
>
>Regards
>Phil Jeffrey
>-------------------------------------------------------------------------------
>| Phil Jeffrey                                  |                             |
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>

Christopher L. Colbert, Ph.D.
Research Associate
Howard Hughes Medical Institute                   Phone: (214) 648 5002
University of Texas Southwestern Medical Center	  FAX:   (214) 648 5095
5323 Harry Hines Boulevard
Dallas, TX 75390