Re: [ccp4bb] Perfect twins.

- Protein crystallography

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CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

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Subject: Re: Wilson plot from truncated.mtz
From: Phil Evans pre {- at -} MRC-LMB {- dot -} CAM {- dot -} AC {- dot -} UK
Date: 2008-08-23

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Subject: Thanks for the many responses concerning protein concentration
From: Mark Hilge hilge {- at -} NMR {- dot -} RU {- dot -} NL
Date: 2008-08-24

Subject: Re: Perfect twins.
From: James Irving james {- dot -} irving {- at -} MED {- dot -} MONASH {- dot -} EDU {- dot -} AU
Date: 2008-08-23

Hi Francis,

Detwinning of perfectly twinned data is done with reference to Fcalc
(determined from from the model): it is not possible to arithmetically
deconvolute the contribution of the twin domains to the perfectly twinned
reflection data.

Because your model is incomplete you will be introducing (or subject to)
significant bias by refining against the resulting incompletely/inaccurately
detwinned data. The solution is to use minimize_twin.inp/anneal_twin.inp
(CNS) or phenix.refine twinning.twin_law="xxx" (PHENIX) to perform the
refinement in the "true" space group on the original data, taking into
account the twin operator. The map produced by model_map_twin.inp (and
presumably phenix.refine) will still be based on data detwinned with
reference to the model, but at least this occurs post-refinement.

One way to limit a molecular replacement search to one twin domain is to
perform the rotation search in the "apparent" space group and the
translation search in the "true" space group. If the result is more than
one molecule in the asymmetric unit, NCS is certainly useful because
perfectly twinned data halves the number of independent reflections (and
hence the data-to-parameter ratio). Test set (rfree) reflections should
also be chosen taking into account the twin operator, for example with
make_cv_twin.inp in CNS.

HTH,
James

CCP4bb navigation

CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

Previous message:
Subject: Re: Wilson plot from truncated.mtz
From: Phil Evans pre {- at -} MRC-LMB {- dot -} CAM {- dot -} AC {- dot -} UK
Date: 2008-08-23

Next message:
Subject: Thanks for the many responses concerning protein concentration
From: Mark Hilge hilge {- at -} NMR {- dot -} RU {- dot -} NL
Date: 2008-08-24