Dear All,

I am trying to refine a 1.9 A structure that indexed with excellent statistics 
(XDS) into a primitive monoclinic cell with cell lengths a=81.33, b=90.11, 
c=113.25 and beta angle =102.15. Data analysis (Xtriage) indicated strong 
pseudo-symmetry with an off-origin peak (53% of origin peak) at fractional 
coordinates (0.07, 0.5, 0.19). The intensity statistics did not indicate 
twinning and Pointless picked P2(1) as the space group with a 94% probability. 
Molecular replacement (Phaser) gave a solution with 4 molecules in the 
asymmetric unit with appropriate packing of symmetry-related molecules and 
NCS-restrained refinements with TLS led to very good quality maps. However, the 
Rfree for the completed model has stalled at 28% (Rwork is at 24%).

I could not solve the structure in P2. Data processed in P1 has almost the same 
dimensions as the monoclinic cell (a=81.37, b=90.14, c=113.29) and MR with 
Phaser located 8 molecules in the asymmetric unit. The data scaled in P1 also 
has translational pseudo-symmetry at (0.06, 0.5, 0.185). Intensity statistics 
do not indicate twinning but Xtriage detects the two-fold as a 
pseudo-merohedral twin (-h,k,-l) and suggests the likely point group is P2. 
Initial refinement indicates the Rfree will likely remain high for the 
structure in P1 as well (Rfree ~37% and Rwork at 34%).

I would really appreciate advice regarding the stalled Rfree and finding the 
true space group.

Many thanks,


Renu Kadirvelraj
Biochemistry and Molecular Biology
120, Green Street
University of Georgia
Athens, GA 30605
Tel: (706) 583 0303

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