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[log in to unmask]" type="cite">Dear all,
I find myself posed with a rather interesting if somewhat confusing problem.
Two crystals grown from the same conditions, let's call them A and B..
A:
Resolution 2.1A
Spacegroup P4?
Rmerge 0.137 (0.324)
Mean((I)/sd(I)) 41.0 (17.8)
Completeness 100 (100)
Multiplicity 53.6 (56.3)
4/mmm is clear from indexing...systematic absences show a clear 4 fold screw-axis..Pointless gives the most likely as P4122 (I'm not clear on how it distinguishes P4122 and P4322..)
Molecular replacement in Phaser, checking all the possible spacegroups, gives two outstanding solutions
LLG Z-score
P4322 1972 41 (1mol/asu)
P43 3801 54 (2mols/asu, ASU too full warning !)
Solutions in other spacegroups had negative LLGs and/or were rejected for poor packing...the P1 solutions have LLGs of around -22000
I rebuilt both solutions in ARP/wARP both giving Rfree ~32% and Rfac ~23%...rebuilding (most residues accounted for), adding ligands and water makes no difference.
Different iterations of refinement/rebuilding eg. cutting resolution make no difference...the maps are really well defined and packing is very reasonable with no clashes in either spacegroup.
B:
Resolution 2.3A
Spacegroup C222?
Rmerge 0.187 (0.402)
Mean((I)/sd(I)) 11.8 (4.8)
Completeness 99.4 (98.8)
Multiplicity 6.8 (6.6)
Mosflm:
11 144 mC 255.61 64.32 63.97 90.0 90.3 76.1 C2
10 7 oC 90.69 90.74 124.09 90.3 90.7 89.7 C222,C2221
9 7 tP 63.97 64.32 124.09 90.7 90.3 90.0 P4,P41,P42,P43,P422,P4212,P4122,P41212,P4222,P42212,P4322,P43212
8 5 oP 63.97 64.32 124.09 90.7 90.3 90.0 P222,P2221,P21212,P212121
7 5 mP 63.97 124.09 64.32 90.7 90.0 90.3 P2,P21
6 4 mC 90.69 90.74 124.09 89.7 90.7 90.3 C2
5 4 mC 90.69 90.74 124.09 90.3 90.7 89.7 C2
4 3 mP 64.32 63.97 124.09 90.3 90.7 90.0 P2,P21
3 1 mP 64.32 63.97 124.09 90.3 90.7 90.0 P2,P21
2 0 aP 63.97 64.32 124.09 89.3 89.7 90.0 P1
1 0 aP 63.97 64.32 124.09 90.7 90.3 90.0 P1
This suggests pseudo-merohedral twinning to me...in C222/C2221...a and b are almost equivalent, so the 4/mmm symmetry would be apparent ?
The Rmerge in 422 (19.6%) is only slightly higher than C222/C2221 ....systematic absences again suggest a 4 fold...the curves calculated from the cumulative intensity distribution suggest partial twinning (when inputting C2221 into the 'old' server to calculate a twin fraction from the partial twin test it says there are no twin laws for that spacegroup...)
The outstanding solutions in Phaser this time are:
LLG Z-score
P4322 1317 35 (1mol/asu)
C2221 2237 46 (2mols/asu, ASU too full warning !)
Rigid body refinement of the solutions give (C2221 ) Rfree 43%, Rfac 42% and ( P4322 ) Rfree 44%, Rfac 43%....I'm thinking this is high and the maps from Phaser although fitting the placed molecules have poor connectivity (high Rmerge anything to do with this ?)
Going back to crystal A it turns out the same C222/C2221 is found but lower down in the list amongst the other solutions...
I have attached the Wilson plots for both crystals...A has a large spike at high resolution (which is why I cut the data to 2.4A to try and improve refinement, to no avail) and B looks horrid !
OK, I think that is all the information I have at the moment...have I completely missed the correct symmetry..the Rmerge does seem high..
I have not yet tried to detwin the data (if it truly is twinned) and perhaps that is impeding refinement ??
Any suggestions would be greatly appreciated.
Thank you,
Iain
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