[phenixbb] P43212 spacegroup change to C2221?

Andy Torelli att29 at cornell.edu
Tue Aug 25 10:50:55 PDT 2009


	Twinning is a complicated issue and I'm not an expert, but here is some 
information that I hope is useful.

	Which test in xtriage indicated twinning, and what was the fraction? 
Be forewarned, in general the H-test and Britton test are blind to 
"over-reducing" your data.  In other words, after you reduce your data 
in a given space group these tests won't be able to tell you if you 
actually have a lower-symmetry space group plus twinning that 
artificially make the data appear to have higher symmetry than they do 
(crystallographic symmetry that is).  You will often hear the advice to 
scale your data in P1 and run xtriage for exactly this reason.  The 
L-test however, operates via a different statistical analysis of the 
intensity distribution and does not have this caveat.  Both types of 
tests are useful/complementary, but be aware of the caveats.

	Secondly, twinning in P21 data can appear C2221.  I can't exactly 
visualize how the twinning might make a P21 space group appear to be 
P43212 and then ultimately C2221 in your case, but I think your 
description makes this a suspicious coincidence.  I agree with other 
advice to examine lower symmetry space groups and you may need to take a 
step back to really nail down the true space group.


On 8/25/2009 12:47 PM, Leigh Allen wrote:
> Hello Phenix users.
> I recently solved the structure of my enzyme using SAD to a resolution 
> of 1.9A.  The R values were 0.19/0.25 and the space group was P43212 
> with a unit cell ~80, 80, 220. 
> I then starting collecting datasets cocrystallized with ATP analogs and 
> noticed that a lot of my datasets have a different unit cell when scaled 
> in P43212.  It comes out to be ~60, 60, 220 and I can NOT get a solution 
> in Phaser with this data.  If I scale it in C2221, then the unit cell is 
> the same (80, 80, 220), but upon refinement, the R factors don't go much 
> below 0.26/0.32.  Has anyone ever seen something like this happen?  I'm 
> unsure if I should feel like the solution is C2221 since there are space 
> groups of higher symmetry with similar residuals that seem plausible in 
> indexing just the smaller unit cell. Should I keep trying to get a 
> solution out of the data that leads to different unit cell parameters, 
> but has higher symmetry?
> Xtriage seems to find twinning, but I think it's from NCS since my ASU 
> contains a homodimer.  I could also be wrong about this...
> Thanks in advance for any advice!
> *******
> Leigh Allen
> Ph.D Candidate
> McCafferty Lab
> Duke University
> Department of Chemistry
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