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[ccp4bb]: Disorder about a crystallographic twofold

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Dear colleagues,

I am working on a crystal structure in space group C2 that
contains three molecules in the asymmetric unit. The functional
unit of the protein is a dimer, and consequently I find 1.5 dimers
in the asymmetric unit. The complete dimer is well defined in the
electron density, but the half dimer seems to be disordered about
the crystallographic twofold axis. 

Molecular replacement gave two solutions for the half dimer in the
rotation, approximately 30 degrees apart. Each of the two solutions
refines alright together with the complete dimer, but the refinement
gets stuck at about R=29% and Rfree=33% (resolution 1.9 A) and the 
electron density for the half dimer looks ok for about 30% of the
molecule and pretty bad for the rest. If I refine the complete dimer
together with both half dimer coordinate sets at occupancies of 0.5
for both orientations, the structure refines to R=24% Rfree=30%, so 
there seems to be some information in both solution. Of course the
electron density does not look great still.

My question is, has anybody encountered a similar problem, or does
anybody know of a paper dealing with such a thing? And how do you
deal with this? I could of course just forget about the disordered
half dimer, and describe just the well-ordered dimer, but I'd prefer
to do a proper job.

By the way, the data scale well, the merging R is about 4% overall
with an I/sigma of about 3.0 at the high resolution end, and a
data redundancy of about 4.5.

Any hint is appreciated.

Cheers, Manfred.


 Manfred S. Weiss, Dr.
 Institute of Molecular Biotechnology
 Department of Structural Biology and Crystallography
 Beutenbergstrasse 11                                 Fon: +49-3641-65-6081
 Postfach 100813                                      Fax: +49-3641-65-6062
 D-07708 Jena                                    Email: msweiss@imb-jena.de
 GERMANY                        Web: http://www.imb-jena.de/www_sbx/manfred