Nat and Misha, Thank you for the suggestions.
Xtriage does indeed detect twinning in P1, reporting similar values for <|L|>, <L^2>, and twin fraction as in P212121. The unit cell dimensions for the 2.0-A structure (P1) are: 72.050 105.987 201.142 89.97 89.98 89.94 P 1 The unit cell dimensions for the 2.8-A structure (P212121) are: 75.456 115.154 202.022 90.00 90.00 90.00 P 21 21 21 I have been processing in HKL2000, which only recognizes one set of unit cell parameters for each Bravais lattice (does anyone know how to change this?). Specifically, for a primitive monoclinic unit cell it estimates: 104.53 71.82 200.99 89.86 91.80 91.16 This is the unit cell which refined to Rwork/Rfree ~ 27%/34%. Indexing in mosflm gives three options for primitive monoclinic: 105.6 71.7 200.9 90.0 90.1 90.0 71.7 105.6 201.0 90.0 89.9 90.0 71.7 200.9 105.6 90.0 90.3 90.0 Attempting to integrate in any of these space groups leads to a fatal error in subroutine "MASKIT". I can also use the "index multiple lattices" feature to get a whole slew of potential space group; however, integrating reflections leads to the same fatal error. Finally, Zanuda tells me that P212121 is the best space group, according to R-factors. However, I do not believe P212121 is the correct assignment. Best, Chris On 7/10/14, Isupov, Michail <m.isu...@exeter.ac.uk> wrote: > I would recommend to run ZANUDA in the default mode from ccp4i or on CCP4 > web server. > ZANUDA has resolved several similar cases for me. > > Misha > > ________________________________________ > From: CCP4 bulletin board [CCP4BB@JISCMAIL.AC.UK] on behalf of Chris Fage > [cdf...@gmail.com] > Sent: 10 July 2014 01:14 > To: CCP4BB@JISCMAIL.AC.UK > Subject: [ccp4bb] Proper detwinning? > > Hi Everyone, > > Despite modelling completely into great electron density, Rwork/Rfree > stalled at ~38%/44% during refinement of my 2.0-angstrom structure > (P212121, 4 monomers per asymmetric unit). Xtriage suggested twinning, > with <|L|> = 0.419, <L^2> = 0.245, and twin fraction = 0.415-0.447. > However, there are no twin laws in this space group. I reprocessed the > dataset in P21 (8 monomers/AU), which did not alter Rwork/Rfree, and > in P1 (16 monomers/AU), which dropped Rwork/Rfree to ~27%/32%. Xtriage > reported the pseudo-merohedral twin laws below. > > P21: > h, -k, -l > > P1: > h, -k, -l; > -h, k, -l; > -h, -k, l > > Performing intensity-based twin refinement in Refmac5 dropped > Rwork/Rfree to ~27%/34% (P21) and ~18%/22% (P1). Would it be > appropriate to continue with twin refinement in space group P1? How do > I know I'm taking the right approach? > > Interestingly, I solved the structure of the same protein in P212121 > at 2.8 angstroms from a different crystal. Rwork/Rfree bottomed out at > ~21%/26%. One unit cell dimension is 9 angstroms greater in the > "twinned" dataset than in the "untwinned". > > Thank you for any suggestions! > > Regards, > Chris >
