Antwort: [ccp4bb]: Tetragonal Twinning & Detwin

[Roman.Hillig@SCHERING.DE]

***  For details on how to be removed from this list visit the  ***
***          CCP4 home page http://www.ccp4.ac.uk         ***


Dear David

Perhaps packing considerations can help you out with your twinning problem:
In P4 there are 4 a.u. per unit cell, in P422 it would be 8 a.u./unit cell.  If
the true space group was P4 and you have a perfect twin, and assume you have one
protein molecule per asym. unit, then when you calculate Matthews parameters for
both P4 and P422, they would look alright for P4 and one molecule, but for P422
you would obtain a reasonable Matthews parameter only for 0.5 molecules per a.u.
The other way around: the wrong assumption of P422 caused by perfect twinning
means that the lattice is too small to accommodate the number of molecules
required by this space group. Think this was what made Luecke et al. suspicious
about the possibility of twinning in the case of bacteriorhodopsin ( Luecke, H.,
Richter, H.T., and Lanyi, J.K. (1998). Proton transfer pathways in
bacteriorhodopsin at 2.3 angstrom resolution.  Science 280, 1934-1937.)

It would become a bit more difficult when the true space group is P4, and you
have 2 molecules in the asym. unit, connected by two-fold NCS. Then you obtain
normal Matthews parameter for the true space group and  2 mol. per asym. unit,
but also for the wrong sp.gr. P422 with 1 mol. per a.u. However, if you are
lucky and the NCS axes do not run parallel to the crystallographic axes, you
should then be able to differentiate between NCS and pseudo-crystallographic
two-fold axes (caused by perfect twinning) by examination of the self rotation
function. The self rotation peaks of data processed in P4 should be at kappa
180, omega 90, and phi _exactly_ at 0°,45°,90° etc. only in the case of perfect
twinning. If they are off 0°, then it is NCS and thus not perfect twinning.


I hope all these thoughts are correct ...


Roman




----------------------------------------------------------------------------------

Dr. Roman C. Hillig

Schering AG
Enabling Technologies
Structural Biology
D-13342 Berlin

email roman.hillig@schering.de
phone (+)49 (0)30 468 18451, fax (+)49 (0)30 468 98451
----------------------------------------------------------------------------------