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[ccp4bb]: I222 to P212121

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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