Re: [ccp4bb] MAD and twinning

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CCP4bb <-- 2007 <-- April 2007 <-- 24 April 2007

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Subject: Re: MAD and twinning
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2007-04-24

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Subject: Re: MAD and twinning
From: Petrus H Zwart PHZwart {- at -} LBL {- dot -} GOV
Date: 2007-04-24

Subject: Re: MAD and twinning
From: Philippe DUMAS p {- dot -} dumas {- at -} IBMC {- dot -} U-STRASBG {- dot -} FR
Date: 2007-04-24

Hi,

Apparently, you are in a "bona fide" twinning situations, and yet you can't
detwin properly the data. Would it be possible that, while your crystal was
rotating during data collection, different volumes of the two twin crystals
were irradiated. Therefore, depending on the phi angle, the twinning
fraction would have varied. Of course, in such a (hypothetical...)
situation, merging everything would obscure the signal.

So one obvious questions: was your crystal fully bathed in the beam ?
If not: would be interesting to try having a look at the unmerged data...
Well, easier to suggest than to do...

By the way, there may well exist papers on such problems.

Philippe Dumas
IBMC-CNRS, UPR9002
15, rue René Descartes 67084 Strasbourg cedex
tel: +33 (0)3 88 41 70 02
p.dumas@ibmc.u-strasbg.fr

-----Message d'origine-----
De : CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK]De la part de
Jordi Benach
Envoyé : mardi 24 avril 2007 17:56
À : CCP4BB@JISCMAIL.AC.UK
Objet : [ccp4bb] MAD and twinning

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 <-- 24 April 2007

Previous message:
Subject: Re: MAD and twinning
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2007-04-24

Next message:
Subject: Re: MAD and twinning
From: Petrus H Zwart PHZwart {- at -} LBL {- dot -} GOV
Date: 2007-04-24