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Re: [ccp4bb]: Monoclinic twinning?
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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
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| Phil Jeffrey | |
| Crystallography Facility Manager | If you lie to the compiler, |
| Memorial Sloan-Kettering Cancer Center, NYC | it will get its revenge |
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