Twinning

Kay Diederichs dikay at sun2.ruf.uni-freiburg.de
Mon Feb 13 09:50:02 EST 1995


Satish Nair (satish at a.chem.upenn.edu) wrote:


: Hello,

: I'd appreciate hearing from folks who have dealt with twinned crystals
: (not the obvious sort but the more subtle kind).  In particular, how
: to tell if a particular crystal form is twinned, and any success in
: deconvoluting twinned data.

Hi,

I have successfully dealt with such a problem. The twinning fraction was
randomly distributed between (approx.) 0 and 0.5 . 
In short:

1) find out about the problem by thinking about possible 'twinning laws'
   In my case, it was space group P3sub2 with twinning law
   h'=k, k'=h, l'=-l . The reflections with indices transformed by this
   law have a correlation greater than 0.3 (up to 1 for alpha=0.5).
2) find the twinning fraction:
   a) take the data set with the lowest alpha (i.e. lowest correlation
      of twin-related refl.) as a reference, define this as alpha=0 (say).
   b) evaluate the twinning fraction of all other data sets with respect
      to this reference. You do this with high accuracy by 'forward-twinning'
      of the reference and calculating R-factors or correlation to the other
      data sets. DO NOT detwin the other data sets to look for best R-factor
      against the reference, the results are not as good.
3) detwin the other data sets and proceed as usual, i.e. calculate
   difference Pattersons to find heavy atom positions, and so on.
   A small remaining twinning fraction, resulting from the fact that your
   reference does not have alpha=0, does not hurt.

Hope this works for you!
Kay
--
Kay Diederichs                      email: dikay at ruf.uni-freiburg.de 
Universitaet Freiburg, Institut fuer Biophysik, Albertstr. 23 
D-79104 Freiburg, Germany   Tel. +49/(0)761/2035391 FAX +49/(0)761/2035016




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