There is something funny visible in the scala loggraphs for the axial
reflections. The I/SigI for l=2n are significantly higher than you would
expect for systematic absences.
You dont say if there is a non-crystallographic translation in your
data - the newest 6.1.1 truncate would tell you that, or you can run
SFCHECK to analyse the data.
If you do have a translation between the 2 molecules that can distort
the moment calculations and disguise twinning.
( When the point group appears to be P422 but there is twinning, it
usually means the true space group is P4i with perfect twinning and
twinning operator k,h,-l)
But there are other puzzles - why has the c axis shrunk so much?
No real answers!
Eleanor
Brett Collins wrote:
> Dear ccp4 gurus,
>
> (apologies to Phil who I already pestered with this). We have a wildtype and
> point mutant protein that have crystallised in the same condition but seem to
> have different unit cell parameters.
>
> Both appear to be P41212 based on structure solution by MR using phaser (2
> mol/ASU). However the mutant has cell constants
> 118, 118, 216, 90 90 90 (Rmerge 0.07(0.55), redundancy 8.6, Wilson B ~60)
> and the wildtype has constants
> 118, 118, 193, 90, 90, 90 (Rmerge 0.06(0.36) redundancy 6.5, Wilson B ~60-
> ***(SCALA logfile "19_scala.log" attached)
>
> As for systematic absences, the mutant has h = 2n and l = 4n as expected. After
> structure refinement, the R/Rfree for the mutant at 2.4 A resolution is
> 20.0/23.9. All is happy.
>
> However wildtype has h=2n and l=2n which doesn't match p41212 but could match
> p42212. However phaser can find no solution for P42212. For the wildtype in
> p41212 at 2.6 A resolution refinement is stuck at 25.6/32.0 and we can't seem to
> get it any lower. There is evidence of twinning from the cumulative intensity
> distribution out of truncate, but not really from the moments of E. But P41212
> has no twinning operator SO...
>
> If I take the other top Mosflm indexing solution of P212121 where a and b cell
> lengths are essentially identical (118.12, 118.17, 193, 90, 90, 90 SCALA logfile
> "32_scala.log") which satisfies the systematic absences at least and refine
> against this data after solving with phaser (4 mol/ASU) I now get R/Rfree of
> 24.8/29.8 - a drop of ~2%. Still the cumulative intensity shows evidence of
> twinning though...
>
> Any thoughts from people with bigger brains than me would be appreciated.
>
> Cheers,
> Brett
>
>
> ------------------------------------------------------------------------
>
>
> ################################
> Brett Collins, PhD
> Group leader
> Institute for Molecular Bioscience
> Level 3 North
> Queensland Bioscience Precinct
> The University of Queensland
> St. Lucia, 4072, Qld,
> AUSTRALIA
>
> e-mail: [log in to unmask] <mailto:[log in to unmask]>
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>
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>
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>
>
>
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