Re: [ccp4bb]: Detwinning of anomalous signal

[Roman Hillig <roman.hillig@charite.de>]

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Hi,

we had a case where we de-twinned SeMet data with anomalous signal from
a partially hemihedrally twinned crystal (Mol.Cell 3, 781). Due to the
small crystal size and weak isomorphous and anomalous signal, we could
not locate the Se site in difference Patterson maps but finally
identified them in anomalous difference Fourier maps using phases from
two Hg derivatives. 
The anomalous difference Fourier maps showed higher peaks for the SeMet
positions with de-twinned data compared to not twin-corrected data, so
the de-twinning was useful/successful.

In our case, the space group was I41, where the pair of twin-related
reflections is for instance I(h,k,l) / I(-h,k,-l).

Interestingly, the output of SCALEPACK or XDS does not list I(-h,k,-l)
or any of its symmetry-related reflections, but a friedel mate,
I(h,-k,l). 

So for data without anomalous signal, we took I(h,k,l) and I(h,-k,l) as
the twin-related pair, determined the twin fraction alpha vi the
fractional differnce H following Yeates (see e.g. Meth.Enz. 276, 344-358
(1997) and Acta Cryst. A44, 142-144 (1988)) and de-twinned it.

Now for the case with anomalous data, you end up with two twin-related
pairs:
The first is   I(+) (h,k,l) / I(-) (h,-k,l)
the second is  I(-) (h,k,l) / I(+) (h,-k,l).

With these two pairs we proceeded like in the normal case, determining
alpha via H and de-twinning both pairs. 

Hope this helps.

Roman



Peter.Burkhard@unibas.ch wrote:
> 
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> ***    CCP4 home page http://www.dl.ac.uk/CCP/CCP4/main.html    ***
> 
> Hi,
> 
> I would like to have my anomalous signal detwinned - which is not possible
> in the current version of the "detwin" program.
> 
> The scenario is as follows:
> I have native data with twin fraction about 15%.
> I have derivative data with twin fraction about 15%.
> I have a very good anomalous Patterson from untiwnned derivative data.
> I get a usable difference Patterson with untwinned data.
> I get a better difference Patterson with detwinned native.
> 
> => therefore I need to untwin my derivative (including the anomalous
> signal) as well!
> 
> Actually I have nearly the same scenario for a structure that I
> already solved (with untwinned derivative SAD phasing!)
> 
> Any hints and ideas are welcome!
> 
> Pete
> 
> ___________________________________________________________________________
> Dr. Peter Burkhard
> Abteilung Strukturbiologie
> Klingelbergstr. 70
> CH-4056 BASEL, Switzerland
> 
> Phone:  ++41 61 267 20 91
> Fax:    ++41 61 267 21 09
> E-mail: Peter.Burkhard@unibas.ch

-- 
_________________________________________________________________________

Roman Hillig, Dr., protein crystallographer

Institut fuer Immungenetik
Charite
Spandauer Damm 130, Haus 31
D - 14050 Berlin, Germany 

phone ++49-(0)30-450 53019   e-mail roman.hillig@charite.de 
fax   ++49-(0)30 450 53953
_________________________________________________________________________