Re: [ccp4bb]: I222 to P212121

[Bart Hazes <bhazes@ualberta.ca>]

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Your original crystal has one molecule per asymmetric unit and there are 8 of
those in I222. In P212121 there are only 4 asymmetric units so it is likely
that you have two monomers in the asymmetric unit. It is possible that small
differences from a "special position" can change these monomers from obeying
either crystallographic symmetry (giving I222) or non-crystallographic
symmetry (giving P212121), you will have to check the int. tables to see if
that is an option. If this is your case you may still be able to detect a
pseudo-I222 symmetry. As Eleanor suggested doing a self-Patterson is the way
to find out. Also try one at low resolution (data up to 8 Angstrom and lower
for instance) to still allow detection of stronger deviations of true
symmetry.

One puzzle is that even if you have some strange special situation or
possibly an incorrect space group assignment, the error or deviation is
probably going to be translational in nature. Therefore I would expect the
rotation function to still work, especially since you have a perfect search
model, assuming there are not large motions between domains. If you have a
multi-domain protein and hinge motions are likely then try to cut the search
model into domains and repeat the rotation function with each domain
seperately.

Bart Hazes

On Mon, 23 Apr 2001, [big5] °K¤pÖr Hanna S. Yuan wrote:

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> ***    CCP4 home page http://www.dl.ac.uk/CCP/CCP4/main.html    ***
> 
> Dear All,
> 
> I have a question not directly related to CCP4 but may be interesting to
> most crystallographers. We have a protein crystallized in I222 space
> group.  The structure was solved by MIR with one molecule per asymmetric
> unit.  Recently we crystallized the same protein in a very similar
> condition, but the space group is P212121. The unit cell dimensions of
> the P212121 cell are almost identical to those of I222. So the only
> difference is that the reflections with h+k+l=2n+1 are now present!
> 
> We thought this is an easy problem that we just need to solve the
> structure by molecular replacement methods. But we did not find obvious
> solutions. The chance that a protein packs differently but resulting in
> exactly the same size of unit cell should be rare! So is it possible
> that there are two crystals and one is mis-indexed by one, so the
> combination of the two I222 gives a diffraction pattern of P222? Has any
> one dealed with this type of problem before, changing of space group but
> not unit cell dimensions? What is the explanation?
> 
> By the way, the Rsym is quite low (around 5%).
> 
> Hanna S. Yuan
> ************************************************************************
> 
> Hanna S. Yuan
> Institute of Molecular Biology
> Academia Sinica                     Tel: 8862-27884151
> Taipei, Taiwan 11529              Fax: 8862-27826085
> E-mail: hanna@sinica.edu.tw      WWW: http://hyuan.imb.sinica.edu.tw
> 
> 

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