Re: Twinning & Anomalous

[Kay Diederichs <Kay.Diederichs@uni-konstanz.de>]

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Frank von Delft wrote:
> 
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> Dear All
> 
> I got stuck on this one:
> 
> Can an anomalous signal be reliably extracted from twinned data?   And
> if, why?
> 
> My first guess was that one could take the anomalous differences from
> the twinned Fs and just detwin them like one would the merged Fs, but
> I think I see all sorts of holes in this reasoning.
> 
> Another question concerns Refmac:  does it take anomalous scattering
> into account when calculating Fc?  I would think this applies especially
> to high resolution (better than 1.6A?), high-redundancy, high quality
> data sets, where the sulphurs at least give detectable anomalous
> scattering (as in Raxis's much-toted advert).
> 
> Thanks for your thoughts
> Frank
> 
> Dept Biochem
> Univ Cambridge

Frank,

I'd say you can detwin your observed intensities (not Fs!) in the case
of anomalous signal like you would if there were no anom signal, i.e.
- you need to know that it's merohedral twinning
- you need to know the twinning law
- you need to know the twinning fraction
Then, after detwinning, you can convert Is to Fs and take anom
differences.

A useful reference is 'Protein Crystals and their evil twins' by TO
Yeates and BC Fam, Structure 7, R26-R29 (1999) (, although they do not
mention that you can get an accurate value for the twinning fraction by
taking a model's intensities, then simulate twinning on these as a
function of alpha and comparing R-factors/correlation coeffs to the
measured data, like in Forst et al (1998), Nature Structural Biology 4,
34-45).

Kay
-- 
Kay Diederichs         http://strucbio.biologie.uni-konstanz.de/~kay 
email: Kay.Diederichs@uni-konstanz.de  Tel +49 7531 88 4049 Fax 3183
Fakultaet fuer Biologie, Universitaet Konstanz 
Box M656, D-78457 Konstanz, Germany