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Re: twinning

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Dear Titia

I solved a structure from partially hemihedrally twinned xtals by MIRAS.
(Published in Hillig, R.C., Renault, L., Vetter, I.R., Drell, T.,
Wittinghofer, A., and Becker, J. (1999). The crystal structure of rna1p:
A new fold for a GTPase-activating protein.  Molecular Cell. 3,

I know of one more MIR-twin-paper, but with very low twin fraction. And
with a special case, NCS axis close to crystallographic axis, therefore
using a differnt way to calculate twin fraction alpha:
Forst, D., Welte, W., Wacker, T., and Diederichs, K. (1998). Structure
of the sucrose-specific porin scry from salmonella typhimurium and its
complex with sucrose.  Nature Structural Biology 5, 37-46.

In my case, we managed to find a relatively low twinned native. But
among the derivatives, a highly twinned one (24%) gave the by far best
phases. These were very much improved by detwinning the data.

So in the end, partial hemihedral twinning should not hinder MIR
structure determination, given that your data are not twinned higher
than 25-30 %. (Data from a crystal with a twin fractions of 33% were not
correctable any more.)

In my case, the space group was I41, so I am now familiar with twinning
in tetragonal space groups. And my little program so far is just dealing
with tetragonal symmetry. But there is now other software to detwin. One
program you can get from S. Ramaswamy (cephalosporin synthase paper).
Then there is DETWIN in CCP4. As far as I know, you have to input a twin
fraction in DETWIN. You are supposed to find the true value by testing
different twin fractions and then checking in TRUNCATE/XLOGGRAPH at
which twin fraction your cumulative intensity distribution looks most

Good luck,



Dr. Roman Hillig

Institute for Immunogenetics
Spandauer Damm 130 
D - 14050 Berlin, Germany 

phone ++49-030-450 53019   e-mail roman.hillig@charite.de 
fax   ++49-030 450 53953