Hi, I was just going to make the same point! The only thing to add is that, if there really is translational NCS (which is certainly possible with 4 copies in the a.u.), then it’s essential both to account for it (which current versions of Phaser should do automatically, if you search for all 4 copies in one job) and to try all possible space groups. The situation Craig describes, in which it’s not immediately obvious whether your crystal has a crystallographic 2(1) and a pseudosymmetric non-crystallographic 2-fold or the reverse, is not uncommon. However, we’ve found that the likelihood score accounting for the effect of tNCS is pretty good at discriminating the two possibilities.
Best wishes, Randy Read ----- Randy J. Read Department of Haematology, University of Cambridge Cambridge Institute for Medical Research Tel: +44 1223 336500 Wellcome Trust/MRC Building Fax: +44 1223 336827 Hills Road E-mail: rj...@cam.ac.uk Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk > On 18 Mar 2017, at 06:12, CRAIG A BINGMAN <cabing...@wisc.edu> wrote: > > You really need to approach such situations with caution. Examination of the > relatively small number of axial reflections probably show that there might > be twofold screw axes in all three directions. But a non-crystallographic > microscopic translation of nearly 0.5 in the direction of a crystallographic > axis will give the same pattern of strong and weak reflections as a > crystallographic twofold screw axis. If I were you, I would be very sure to > try molecular replacement in all possible orthorhombic space groups. Several > programs, including Phaser, will organize that exhaustive search across all > eight possibilities for you. > >> On Mar 17, 2017, at 11:56 PM, Polisetty Satya Dev <pvss...@gmail.com> wrote: >> >> Hi, >> >> We checked all possible space groups of orthorhombic crystal system using >> Scala and Pointless but the statistics show that P212121 is the possible >> space group. >> >> Thank You, >> Satya Dev >> >> On Fri, Mar 17, 2017 at 8:03 PM, Teplyakov, Alexey [JRDUS] >> <atepl...@its.jnj.com> wrote: >> Check the space group. It may be orthorhombic with a pure rotational axis >> (e.g. P21212) or even monoclinic. >> >> >> >> From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of >> Polisetty Satya Dev >> Sent: Friday, March 17, 2017 9:51 AM >> To: CCP4BB@JISCMAIL.AC.UK >> Subject: [EXTERNAL] [ccp4bb] No improvement in R-factor after Refmac. >> >> >> >> Dear all, >> >> I solved a structure at 2.0 A resolution with R-work and R-free values 0.25 >> and 0.32 respectively and I am stuck at Refmac step where there is no >> further reduction in R-factor. >> >> The above stated values were obtained after several rounds of manual >> refinement followed by refmac. There are also areas where electron density >> is missing around peptide backbone in one of the monomer in ASU. >> >> Can anyone please tell me how can I improve the electron density and >> R-factor. >> >> >> The structure solution was obtained using Phaser MR and here are the data >> statistics: >> >> >> >> Average unit cell: 81.95, 100.40, 156.96, 90.00, 90.00, 90.00, >> Space group: P212121, >> Completeness 99.5, >> Multiplicity 6.4, >> Four monomers per ASU. >> Solvent content: 47%. >> >> Thank you everyone, >> Satya Dev, >> JNCASR. >> >
