Hi Pat,
On top of Phil's suggestion for invoking TLS, what about the NCS? Is there anything peculiar about them (tNCS or the like)? If there isn't, are you imposing NCS during refinement? perhaps you should relax the NCS restraints or not impose any at all and see what happens then?
My 2p thoughts.
Boaz
Boaz Shaanan, Ph.D.
Dept. of Life Sciences
Ben-Gurion University of the Negev
Beer-Sheva 84105
Israel
E-mail: [log in to unmask]
Phone: 972-8-647-2220 Skype: boaz.shaanan
Fax: 972-8-647-2992 or 972-8-646-1710
________________________________________
From: CCP4 bulletin board [[log in to unmask]] on behalf of Patrick Loll [[log in to unmask]]
Sent: Saturday, April 27, 2013 12:38 AM
To: [log in to unmask]
Subject: [ccp4bb] refinement hanging--what am I missing?
Hi all,
Here is a problem that's been annoying me, and demanding levels of thought all out of proportion with the importance of the project:
I have two related crystal forms of the same small protein. In both cases, the data look quite decent, and extend beyond 2 A, but the refinement stalls with statistics that are just bad enough to make me deeply uncomfortable. However, the maps look pretty good, and there's no obvious path to push the refinement further. Xtriage doesn't raise any red flags, nor does running the data through the Yeates twinning server.
Xtal form 1: P22(1)2(1), a=29.0, b=57.4, c=67.4; 2 molecules/AU. Resolution of data ~ 1.9 Å. Refinement converges with R/Rfree = 0.24/0.27
Xtal form 2: P2(1)2(1)2(1), a=59.50, b=61.1, c=67.2; 4 molecules/AU. Resolution of data ~ 1.7 Å. Refinement converges w/ R/Rfree = 0.21/0.26
As you would expect, the packing is essentially the same in both crystal forms.
It's interesting to note (but is it relevant?) that the packing is quite dense--solvent content is only 25-30%.
This kind of stalling at high R values smells like a twin problem, but it's not clear to me what specific kind of twinning might explain this behavior.
Any thoughts about what I might be missing here?
Thanks,
Pat
---------------------------------------------------------------------------------------
Patrick J. Loll, Ph. D.
Professor of Biochemistry & Molecular Biology
Director, Biochemistry Graduate Program
Drexel University College of Medicine
Room 10-102 New College Building
245 N. 15th St., Mailstop 497
Philadelphia, PA 19102-1192 USA
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