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Re: [ccp4bb]: I222 to P212121

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Dear All,

I would like to thank all the replies to my question of how a I222
changes to a P212121 space group. We have solved the structure and let
me summarize how this is happening.

Eleanor suggested to check if h+k+l=2n+1 reflections in P212121 cell are
weak and do the native Patterson to see if there is a peak closed to
(0.5, 0.5, 0.5). This is to find out if we have a pseudo-I-center cell.
In our case h+k+l=2n+1 reflections are not weak that they have an
average F of about 10% smaller than that of 2n reflections. However, we
indeed see a strong native Patterson peak at (0.5, 0.5, 0.5) with an
ellipsoid shape but not a perfect sphere like what we observed for the
I222 case. So it is likely that our P222 cell has a pseudo-I symmetry
but (x, y, z) is not translated exactly to (x+0.5, y+0.5, z+0.5).

Gerard showed a similar example of changing from a I222 (1cel) to a
P21212 (3cel) with the same unit cell dimension.  In this case, the
single molecule in I222 is located at (x, y, z) and the two molecules in
P21212 are located at (x, y, z-0.25) and (x-0.5*, y-0.5*, z-0.75). 0.5*
indicates a value closed to 0.5. The translation between the two
molecules in P21212 cell is (0.46, 0.5, 0.5), so it is transformed from
I to P.

In our case, although h+k+l=2n+1 are strong, all the 2n+1 reflections in
the axes are absent. So we thought the new cell is a P212121. But it
turns out that the correct space group is P21212.  It is a little bit
more complicated for us, since the 2-fold is along b-axis, so we have to
move it to the c-axis. The result is similar to the cel case that the
two molecules in P21212 is separated by (0.49, 0.52, 0.50) and the
origin of I222 cell is moved to (0, 0, 0.25) in the new but the similar
packing P21212 cell.

Isn't this interesting? Thanks for all the helpful suggestions once

Hanna 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