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*To*: ccp4 <ccp4bb@dl.ac.uk>, hygroup@gate.sinica.edu.tw*Subject*: Re: [ccp4bb]: I222 to P212121*From*: "=?big5?B?sEukcNZy?= Hanna S. Yuan" <mbyuan@gate.sinica.edu.tw>*Date*: Thu, 26 Apr 2001 11:09:24 +0800*Organization*: IMB*References*: <Pine.LNX.4.21.0104241826330.19126-100000@XRAY.bmc.uu.se>*Reply-To*: hanna@sinica.edu.tw*Sender*: owner-ccp4bb@dl.ac.uk

*** For details on how to be removed from this list visit the *** *** CCP4 home page http://www.dl.ac.uk/CCP/CCP4/main.html *** 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 again. 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

**References**:**Re: [ccp4bb]: I222 to P212121***From:*"Gerard \"DVD\" Kleywegt" <gerard@xray.bmc.uu.se>

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