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 P4_1 22 (I'm not clear on
how it distinguishes P4_1 22 and P4_3 22..)
Molecular replacement in Phaser, checking all the possible spacegroups,
gives two outstanding solutions
LLG Z-score
P4_3 22 1972 41 (1mol/asu)
P4_3 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/C222_1 ...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/C222_1
....systematic absences again suggest a 4 fold...the curves calculated
from the cumulative intensity distribution suggest partial twinning
(when inputting C222_1 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
P4_3 22 1317 35 (1mol/asu)
C222_1 2237 46 (2mols/asu, ASU too full warning !)
Rigid body refinement of the solutions give (C222_1 ) Rfree 43%, Rfac
42% and ( P4_3 22 ) 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/C222_1 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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