Pseudo-merohedral twinning refinement in C2, when beta = 94 degrees
Hi all, I currently have a dataset that scales well in C2. (a = 224,b = 129,c = 392. beta= 94.5 degrees). Previously, I had thought that these crystals were in P6322 (posted this a few months ago), but I've since grown better crystals, where the beta angle is now 94.5, and not close to 90.0. I'm reasonably confident with the scaling - however, it has always been really difficult to index these crystals in HKL2000. Like the previous crystals, this new C2 dataset looks like it might be perfectly twinned, based on Xtriage's L test. I've solved the structure by MR, and refined it to 3.0 angstroms to R=30%, Rfree=33%, but I am running out of things to fix. I'd like to perform twinning refinement, based on this Xtriage L test result. The problem is that Xtriage cannot find any pseudomerohedral twinning laws, likely because the beta angle is too far from 90 degrees?? For kicks and giggles, I started to enter arbitrary beta angles in Xtriage, and at a beta angle below 93.4, Xtriage can find two twinning laws (closer to 90 degrees, Xtriage can find 5 twin laws). Using "-1/2*h+3/2*k, 1/2*h+1/2*k, -l" in Phenix refine (the beta angle is still 94.5 degrees, I haven't changed this) results in the R and Rfree dropping a lot to 25.5 and 28.6, respectively. Used the same random seed as previous runs to generate the test set from the scalepack file, with the "use lattice symmetry to generate test set" flag enabled. The maps look a lot noisier. So my questions are: 1. Is this twinning refinement valid? I'm really thinking no - since the maps didn't improve. Suppose the maps didn't get worse though - would this twinning refinement then be valid? 2. How do I go about reindexing this dataset in I2? i.e. to see if the beta angle with this unit cell selection is closer to 90 degrees. Would this even help, with respect to pseudomerohedral twinning refinement? Thanks!
Hi John,
Did you try and process the data in P1?
When 50% twinned and no twin laws, one typically tries to go down in
symmetry ...
P
On 20 September 2011 00:20, John Pak
Hi all,
I currently have a dataset that scales well in C2. (a = 224,b = 129,c = 392. beta= 94.5 degrees). Previously, I had thought that these crystals were in P6322 (posted this a few months ago), but I've since grown better crystals, where the beta angle is now 94.5, and not close to 90.0. I'm reasonably confident with the scaling - however, it has always been really difficult to index these crystals in HKL2000.
Like the previous crystals, this new C2 dataset looks like it might be perfectly twinned, based on Xtriage's L test.
I've solved the structure by MR, and refined it to 3.0 angstroms to R=30%, Rfree=33%, but I am running out of things to fix. I'd like to perform twinning refinement, based on this Xtriage L test result.
The problem is that Xtriage cannot find any pseudomerohedral twinning laws, likely because the beta angle is too far from 90 degrees?? For kicks and giggles, I started to enter arbitrary beta angles in Xtriage, and at a beta angle below 93.4, Xtriage can find two twinning laws (closer to 90 degrees, Xtriage can find 5 twin laws). Using "-1/2*h+3/2*k, 1/2*h+1/2*k, -l" in Phenix refine (the beta angle is still 94.5 degrees, I haven't changed this) results in the R and Rfree dropping a lot to 25.5 and 28.6, respectively. Used the same random seed as previous runs to generate the test set from the scalepack file, with the "use lattice symmetry to generate test set" flag enabled. The maps look a lot noisier.
So my questions are:
1. Is this twinning refinement valid? I'm really thinking no - since the maps didn't improve. Suppose the maps didn't get worse though - would this twinning refinement then be valid?
2. How do I go about reindexing this dataset in I2? i.e. to see if the beta angle with this unit cell selection is closer to 90 degrees. Would this even help, with respect to pseudomerohedral twinning refinement?
Thanks! _______________________________________________ phenixbb mailing list [email protected] http://phenix-online.org/mailman/listinfo/phenixbb
-- ----------------------------------------------------------------- P.H. Zwart Research Scientist Berkeley Center for Structural Biology Lawrence Berkeley National Laboratories 1 Cyclotron Road, Berkeley, CA-94703, USA Cell: 510 289 9246 BCSB: http://bcsb.als.lbl.gov PHENIX: http://www.phenix-online.org SASTBX: http://sastbx.als.lbl.gov -----------------------------------------------------------------
Peter Zwart, Thanks for the suggestion to try P1. Indexing in P1 reveals the twin law "-k,-h,-l" at near 50%, as revealed by Xtriage. So I will explore refining in this space group +/- twinning. Apologies if other people have offered advice - I can only see responses on the phenixbb archive site at the moment! On 20/09/2011 12:20 AM, John Pak wrote:
Hi all,
I currently have a dataset that scales well in C2. (a = 224,b = 129,c = 392. beta= 94.5 degrees). Previously, I had thought that these crystals were in P6322 (posted this a few months ago), but I've since grown better crystals, where the beta angle is now 94.5, and not close to 90.0. I'm reasonably confident with the scaling - however, it has always been really difficult to index these crystals in HKL2000.
Like the previous crystals, this new C2 dataset looks like it might be perfectly twinned, based on Xtriage's L test.
I've solved the structure by MR, and refined it to 3.0 angstroms to R=30%, Rfree=33%, but I am running out of things to fix. I'd like to perform twinning refinement, based on this Xtriage L test result.
The problem is that Xtriage cannot find any pseudomerohedral twinning laws, likely because the beta angle is too far from 90 degrees?? For kicks and giggles, I started to enter arbitrary beta angles in Xtriage, and at a beta angle below 93.4, Xtriage can find two twinning laws (closer to 90 degrees, Xtriage can find 5 twin laws). Using "-1/2*h+3/2*k, 1/2*h+1/2*k, -l" in Phenix refine (the beta angle is still 94.5 degrees, I haven't changed this) results in the R and Rfree dropping a lot to 25.5 and 28.6, respectively. Used the same random seed as previous runs to generate the test set from the scalepack file, with the "use lattice symmetry to generate test set" flag enabled. The maps look a lot noisier.
So my questions are:
1. Is this twinning refinement valid? I'm really thinking no - since the maps didn't improve. Suppose the maps didn't get worse though - would this twinning refinement then be valid?
2. How do I go about reindexing this dataset in I2? i.e. to see if the beta angle with this unit cell selection is closer to 90 degrees. Would this even help, with respect to pseudomerohedral twinning refinement?
Thanks!
-- John E. Pak, Ph.D. Postdoctoral Associate, Stroud Lab Department of Biochemistry& Biophysics UCSF MC2240 Genentech Hall Room S414 600-16th St. San Francisco, CA 94158-2517 Lab #: 415-476-3937 Fax #: 415-476-1902 Cell #: 415-215-0048
participants (2)
-
John Pak -
Peter Zwart