Unrefinable structure with strange symmetry clash. Possible Merohedral twin?

Kerim Babaoglu kerim.babaoglu at stjude.org
Sat Jun 14 14:26:40 EST 2003


I have recently solved a structure by MR which I began to suspect was
a twin due to my inability to refine it.  I have collected and solved
the structure twice on 2 different data sets, one at home and one at
the APS.  The data indexes integrates and scales well (Rsym 7%) to
2.0A as P6(2)22.  I have a rather large unit cell (87 87 263)
calculated to contain 24 molecules which we suspect is 12 dimers. I
was able to get a molecular replacement solution and the unbiased
2fofc omit maps look beautiful so I believe that this is where the
dimeric ASU is located.  The problem is when I generate the symmetry
related molecules a huge clash at one of the 2-fold symmetry planes
causing a helix of one of the monomers in my dimeric ASU to overlap
with itself in the sym-mate, however as I stated the maps for the rest
of the entire dimer look absolutely correct.  Refining with or with
out this helix has little effect on the R values which are hovering in
the 48-53 range.  We have doubled checked the space group and the unit
cell parameters repeatedly on a number of different crystals (from the
same conditions) and are confident they are are correct.  Upon running
out of explanations for why this was occuring I started to investigate
twinning as a possibility.

I know that the hexagonal space groups are suspect twinners, and
unexplained inability to refine is a warning sign of twinning, so I
ran the CNS script detect_twinning.inp which I believe is similar to
the Yeates web-based test (Is that correct?) I got a very strange
result.  My <|I|^2>/(<|I|>)^2 which is supposed to fall in between 1.5
and 2 is 4.9.  And my (<|F|>)^2/<|F|^2> which is supposed to be
between 0.785 and 0.865 is 0.685.  I have double checked this a number
of times to make sure I haven't screwed anything up but the input file
is fairly short and straight-forward.  Does anyone know what this
might mean?

Also any other suggestions on what else may be going on would be
greatly appreciated.

Kerim Babaoglu

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