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I agree here. The 50 % could be a good indicator.

I am finishing a new structure which was about 42% twinned in the monoclinic setting (b = ~90) for an Se-Met dataset. When I went to refine it with the native dataset, the twin fraction increased to 50%, figured something wasn't right. fortunately, the native was much better behaved / untwinned in an orthorhomic crystal form with a near identical unit cell.

-tg


From: CCP4 bulletin board [[log in to unmask]] on behalf of Eleanor Dodson [[log in to unmask]]
Sent: Tuesday, December 16, 2014 2:36 PM
To: [log in to unmask]
Subject: Re: [ccp4bb] P31 or P32

But are you sure it is twinned and not really P312? 

Ltest etc is a pretty reliable indicator but the results can be biased by non-crystallographic translation, etc

And do the HA sites obey the twinning symmetry?



On 16 December 2014 at 16:36, Keller, Jacob <[log in to unmask]> wrote:
I would say try to get better crystals and data, and also do a MAD experiment. But...perfect twins are really hard, I think. Maybe vary the crystallization conditions a bit, additive screen, seeding, etc.

JPK

-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of RHYS GRINTER
Sent: Tuesday, December 16, 2014 10:37 AM
To: [log in to unmask]
Subject: Re: [ccp4bb] P31 or P32

Hi Eleanor,

The data is perfectly twinned, with the original auto processing assigning a point group of P312,  but I reprocessed it in P3 and it seems to be behaving okay.
Are there any additional precautions I should take at the phasing stage with twinned data?

BW

Rhys
________________________________________
From: CCP4 bulletin board [[log in to unmask]] On Behalf Of Eleanor Dodson [[log in to unmask]]
Sent: 16 December 2014 15:13
To: [log in to unmask]
Subject: Re: [ccp4bb] P31 or P32

Sorry - of course it can.. Sorry.
 Only if the twinning is perfect then you apparently get a higher symmetry..
Eleanor

On 16 December 2014 at 14:26, Tim Gruene <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Dear Rhys,

I would try to place idealised secondary structure elements with coot into the density - at this resolution they probably fit both hands, but you may see a difference when you do e.g. rigid body refinement.

Best,
Tim

On 12/16/2014 10:39 AM, RHYS GRINTER wrote:
> Hi All,
>
> This will no doubt show something of my ignorance with experimental phasing, however I'm currently working on solving a 3.8A SAD dataset with Pt anomalous signal. Both Shelx and Xtriage see reasonable anomalous signal to about 7A and seem to get  statistics suggesting a solution when I run SAD phasing using Autosol in Phenix. After density modification I get pretty nice looking maps with clear solvent channels and interconnected density for protein, however there is very little difference in map R-factor between the hands and the maps from both the P31 and P32 solution appear comparable in structure.
> Obviously DM is struggling to break the hand ambiguity, however can some one tell me if what I'm seeing represents a definite solution? And what is the best way to proceed from here?
>
> Cheers,
>
> Rhys
>

--
Dr Tim Gruene
Institut fuer anorganische Chemie
Tammannstr. 4
D-37077 Goettingen

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