Re: [ccp4bb] MAD and twinning

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CCP4bb <-- 2007 <-- April 2007 <-- 25 April 2007

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From: Jeff Speir speir {- at -} SCRIPPS {- dot -} EDU
Date: 2007-04-24

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Subject: Re: MAD and twinning
From: "George M {- dot -} Sheldrick" gsheldr {- at -} SHELX {- dot -} UNI-AC {- dot -} GWDG {- dot -} DE
Date: 2007-04-25

XPREP may well be the only program that can read a .sca file, detwin it as
you suggest, and write another .sca file directly.

Since it is a MAD experiment and the true space group is probably P31 or
P32, you must be careful to check that the different wavelengths have been
indexed consistently. Even if you get the same twin fraction for each
wavelength independently and the merging R-index is not too bad, they
could still be indexed inconsistently!

George

Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-2582

On Tue, 24 Apr 2007, Jordi Benach wrote:

> Hi,
>
> I just got a MAD dataset (~2.5A) of a crystal that so far seems to
> belong to P31 or P32 space group and that also scales pretty well
> in P3121 or P3221. According to the UCLA twinning server, the
> P31/P32 crystal is merohedrally twinned with a twinning fraction of
> 0.33 and following a twinning operation h,-h-k,-l. CNS gives a
> twinning fraction of 0.42.
>
> Although we have solved twinned crystals in the lab with similar
> twinning fractions/resolution in the past using SOLVE or BnP/SHELX,
> only realizing that the crystal was twinned during refinement when
> the R values wouldn't go down.
>
> In this P31/P32 case, however, I can't solve the twinned data with
> any program, or at least I can't get interpretable electron density
> maps. Any ideas, clues? Is there a way to get a program that will
> detwin raw intensity data (like *.sca files directly from
> scalepack)?
>
> Thanks,
>
> Jordi
>

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CCP4bb <-- 2007 <-- April 2007 <-- 25 April 2007

Previous message:
Subject: Re: Mac Pro questions
From: Jeff Speir speir {- at -} SCRIPPS {- dot -} EDU
Date: 2007-04-24

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Subject: R and Rfree
From: U Sam us_x {- at -} HOTMAIL {- dot -} COM
Date: 2007-04-25