If the data set had P6 symmetry before anisotropic scaling it would
keep that symmetry afterwards. If it was only P2 symmetry before, it
certainly would not have P6 afterwards. Any anisotropic scaling I've
seen constrains the anisotropy to the lattice symmetry so symmetry
cannot be degraded via its application.
If your data set had, in principle, P6 symmetry but was expressed in
a lower symmetry asymmetric unit and contained nonsymmetry-conforming
noise before anisotropic scaling it would also contain broken symmetry
afterwards. The higher symmetry was not lost, it was never there to
begin with.
Dale Tronrud
On 4/28/2012 12:06 AM, Zhijie Li wrote:
Hi,
My first thought was same with David: the truncation won't change the crystal's
space group. The symmetry of the crystal is
reflected by the symmetry of the amplitudes of many many reflections across all
resolutions. Ellipsoidal truncation itself only
removes some very weak reflections from the outer shells. The remaining
reflections will still have a good number of reflections
carrying the symmetry of the crystal.
However a second thought on the anisotropic scaling and B-factor correction led
me to this scenario: suppose we have a crystal
that's really P6, but we have cowardly indexed it to a lower space group P2,
with the 2-fold axis, b, coinciding the real 6-fold
axis. By losing the a=c restrain, the anisotropic scaling along H and L now may
not be strictly equal (for example, could be caused
by outliers that would have been identified and filtered out if indexed
correctly as P6), resulting in the loss of the 6-fold
symmetry in the scaled dataset. Apparently this is an artifact due to an
improper SG assignment before the anisotropic scaling and
B-factor correction.
Just some crazy thoughts. Please correct me if I am wrong.
BTW, to Theresa: an very informative introduction on ellipsoidal truncation and
anisotropic scaling can be found here:
http://services.mbi.ucla.edu/anisoscale/
--------------------------------------------------
From: "Theresa Hsu" <theresah...@live.com>
Sent: Friday, April 27, 2012 3:18 PM
To: <CCP4BB@JISCMAIL.AC.UK>
Subject: [ccp4bb] Anisotropic diffraction
Dear crystallographers
A very basic question, for anisotropic diffraction, does data truncation with
ellipsoidal method change the symmetry? For example,
if untruncated data is space group P6, will truncated data index as P622 or P2?
Thank you.
Theresa
