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Dear all,
How would one properly select reflections for R-free in these situations?  Presumably if the selection is done in P1 then it mimics twinning or high NCS, such that reflections in both the work and free set will be (potentially?) related by symmetry.
-Christina

From: Mohinder Pal <[log in to unmask]>
To: [log in to unmask]
Sent: Thu, October 21, 2010 7:05:42 AM
Subject: [ccp4bb] Regarding space group P1, P21

Dear CCP4BB members,

I have solved a protein-drug complex structure in P21212 space group.  In this structure, the drug molecule is  falling on the two-fold symmetry axis having averaged electron density  with 0.5 occupancy. We tried a lot to crystallize this protein-drug complex in different space group but no success so far.  I have tried to solve the same data  in space group P1 (statistics are fine as I have collected data for 360 degree). The map looks even better with one conformation for a drug. Interestingly, then I reprocessed the same data using imosflm in P21 space group which have penalty 1 compared to 4 for P21212.  The structure in P21 is  also refining well (with one conformation of the drug compound without symmetry axis at the ligand position). The question is , is it a good practice to solve this structure in P1 and P21 even if the data has higher symmetry?

Secondly, I have been advised that I have to be careful to refine structure in P1 as there will be problem regarding observation/parameter ratio if I add too many water molecules. What will be the case if the electron density present  for water molecules? 

I can put restrains to protein structure  but  I am just curious to know one restrain equals how many observations.

I look forward to hear your suggestions.

Kind regards,

Mohinder Pal