[phenixbb] pseudo-merohedral twinning in P212121

Schneider Sabine paxss2 at nottingham.ac.uk
Wed Sep 12 02:41:34 PDT 2007

Hi everyone,


I have a data set to 2.8A, with most likely 2 molecules in the asu and a
molecular replacement model with 40% sequence identity. (Cell 71.3 71.3
227.9 90 90 90)


Initially the data appeared to be either P4322 or P43212 (Rsym = 7.4%;
Cell 71.3 71.3 227.9 90 90 90), but the molecular replacement solutions
had major clashes with the symmetry related molecules and a lower LLG
for the solution of the second molecule. No twinning was detected by


Processing the data in P212121 (Rsym 5.6%, cell 71.3 71.4 228.2 90 90
90) gave me a better molecular replacement solution (LLG 121/373), but I
had 5 clashes with the second molecule in the asu. (I tested all
alternative spacegroups) 


I also have a MAD dataset (very low quality!) and I combined the phases
with the one obtained from molecular replacement in Sharp. After NCS
averaging the maps had improved (still not great). I tried to improve
things in the model and run Phaser again and the solution had better
statistics (LLG 338/1223) and the clashes were reduced to 2. 

Xtriage detected pseudo-merohedral twinning with a twin fraction of 43%
with twin operator -k, -h, -l. 

So I tried refining the model with phenix telling it the twinning
operator and fraction, but R/Rfree are stuck around 45%. I also tried
simulated annealing but that gave even higher R/Rfree. 


Is there anything I can do, or is it a hopeless case? 


Thanks for your help in advance!



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