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.
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