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Leigh, 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. -Andy 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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