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
