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Re: [ccp4bb]: Disorder about a crystallographic twofold

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On Wed, 28 Jun 2000, Manfred Weiss wrote:

> 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. 
> ...

this sounds remarkably like another case with 1.5 dimers in the asymmetric

Barycki, JJ; O'Brien, LK; Birktoft, JJ; Strauss, AW; Banaszak, LJ.  Pig
heart short chain L-3-hydroxyacyl-CoA dehydrogenase revisited: sequence
analysis and crystal structure determination. Protein Science, 1999 Oct,

in that case the dimer in the special position was partially disordered,
and the source of the disorder was not completely understood. theirs was
in space group C222(1).

this is so unusual that its going to be hard to come up with any generic
advice that is useful.  the first thing i would check is to make 
absolutely sure of the space group assignment; perhaps some of the missing
reflections from the centering aren't really missing?  if that checks out,
and the multiple positions seem real, your use of alternate positions
seems like a reasonable approach. i think you are going to have to depend
heavily on the free R factor.

"Time is an illusion.  Lunchtime doubly so." - Ford Prefect
                        David J. Schuller
                        modern man in a post-modern world
                        University of California-Irvine