Hi John!
I have recently encountered strong merohedral twinning in spacegroup
P6(3)
which made the data look like P6(3)22. Nevertheless, the structure could
be
refined to 1.55 A with excellent maps and stereo chemistry.
You may want to take a look at
http://anx12.bio.uci.edu/~hudel/br/twinning/index.html
Todd's server does a great job of identifying the twinning. I think
your cross-rotation function during MR should have resulted in
multiple non-redundant solutions as well. You can also try self-rotation
functions which should show 2-folds in the plane of twinning.
I have found that Todd's server comes up with estimates of the twinning
fraction that are slightly lower when compared to the refined twinning
fraction
from SHELXL.
I would recommend using SHELXL to refine against the twinned raw data
(intensities) rather than data-based and twin-ratio-based detwinning of
your raw data.
Good luck, Hudel
John R. Horton wrote:
> Quickly...
>> I have been trying to refine a protein structure derived from a
> molecular replacement solution for some time...while the R factor went
> down easily, the Rfree would stay the same or go up whatever I did...
> Recently, I surmised and discovered that I have merohedrally twinned
> data...I used the new crystal twinning server at UCLA at
>>http://www.doe-mbi.ucla.edu/Services/Twinning/>> Anyway, I am hoping someone can give me some insight:
>> The space group is P31 and the server gives me the following results:
>> -------------
> Partial Merohedral Twinning Test Results
> No. Total Data Read = 17264
> No. Acentric Data = 17264
> No. Data Within Resolution Range = 9390
>> Testing all possible twin laws for the given space group
>> 2 along a, b
>> No. Twin Law Related Reflections = 4270 (pairs)
> No. Twin Law Pairs Considered = 2135
>> <H> = 0.372939
> <H2> = 0.198790
>> Twin Fraction = 0.120468 +/- 0.006593
>> 2 along a*, b*
>> No. Twin Law Related Reflections = 4375 (pairs)
> No. Twin Law Pairs Considered = 2187
>> <H> = 0.363768
> <H2> = 0.193497
>> Twin Fraction = 0.127641 +/- 0.008591
>> 2 along c
>> No. Twin Law Related Reflections = 4350 (pairs)
> No. Twin Law Pairs Considered = 2175
>> <H> = 0.044386
> <H2> = 0.003633
>> Twin Fraction = 0.451707 +/- 0.003908
>> ------------
>> Can the data be used and successfully detwinned??
>> --
>> ------------------------------------------------------------------
> John R. Horton Ph.D E-mail: jrhorto at emory.edu> Assistant Professor (Research) Voice Mail: 404-727-8492
> Emory School of Medicine FAX: 404-727-3746
> Department of Biochemistry
> 1510 Clifton Road, Suite G239
> Atlanta, GA 30322
> ------------------------------------------------------------------
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