What were the twinning tests like in p42212? I suspect that it is really that spacegroup, the low Rfree is artificial (33% is not too bad at 3.9 Ang) and the electron density is bias.
JPK From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Andre Luis Berteli Ambrosio Sent: Tuesday, December 06, 2016 9:17 AM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] Choosing test set for twin refinement, with multiple operators Dear all, We are currently refining a low resolution model (3.9 A max), obtained by MR. Dataset was collected from a single crystal with one long cell axis (~620 A) and high solvent content (74%). Best refinement results (by far!) are obtained in Refmac, by imposing P21 sg (20 multidomain monomers/AU) and allowing for amplitude-based twin refinement. Accordingly, Refmac identifies four twin operators, which I understand have considerable fractions, as follows: **** Twin operators with estimated twin fractions **** Twin operator: H, K, L: Fraction = 0.249; Equivalent operators: -H, K, -L Twin operator: -H, -K, L: Fraction = 0.260; Equivalent operators: H, -K, -L Twin operator: -K, -H, -L: Fraction = 0.235; Equivalent operators: K, -H, L Twin operator: K, H, -L: Fraction = 0.257; Equivalent operators: -K, H, L I suspect that the low Rfree (~25%) may be artificial and results from an inappropriate selection of the test due to the multiple operators. However, I cannot figure out how to properly select the test set when such a high number of operators must be considered. FYI, space group P42212 resulted in the best processing statistics (5 monomers/AU); however, model never improved from an Rfree of 33%, with the quality of the electron density distribution for some of the domains being heavily compromised. I honestly apologize if I am missing some obvious points here and will be happy to provide more info, including statistics on data processing. Since is a multidomain protein, is it also appropriate to include TLS refinement at this low resolution? I thank you all in advance. Best, Andre.
