[phenixbb] P43212 spacegroup change to C2221?

Francis E Reyes Francis.Reyes at Colorado.EDU
Tue Aug 25 10:07:41 PDT 2009

Your ASU in P43212 contains a single homodimer?

P422 can be a perfect twin for P4. Or you may have an NCS operator  
that's nearly parallel to a crystallographic axis. Or you may have both.

I would try scaling into P43 and molecular replace into that data.  
Take the solution and rigid body refine (if reducing the symmetry  
generates another molecule treat them as independent rigid bodies)  
with and without NCS and with and without twinning as well as with  
twinning and NCS.

A drop in R-factor in any of those rigid body refines could be  
indicative of twinning.  Depending on the resolution you will want to  
remove NCS (but keep twinning) for final refinements.


On Aug 25, 2009, at 10:47 AM, 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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> phenixbb at phenix-online.org
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Francis Reyes M.Sc.
215 UCB
University of Colorado at Boulder

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