Maybe it is really P43 but twinned, with probably only two molecules in the asymmetric unit. You can easily check this by putting it into SHELXL with the instructions TWIN 0 1 0 1 0 0 0 0 -1 and BASF 0.4, or with the new phenix refine (still beta test I think). Or wait for the next REFMAC.
George Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-2582 On Wed, 15 Aug 2007, Yanming Zhang wrote: > Hi , > Please help me on this structure: > Data 1.8A: cell 84.892 84.892 172.580 90 90 90 with tetrahedral index > Space group: P43212 or P43, Final refinement Rfree indicates P43 is better so > most likely P43(also the systematic absence indicate P43 is it) > Matthew coeffient indicates: P43 3mol/a.u(2.58,52%) 2mol/a.u(3.87, 68%) > > MR: Both Molerep and Phaser find out 2mol/au initially. Refinement reaches > Rfree 30% , R 27% finally. WE think, comparing with 1.8 resolution, The R and > Rfree are too high. Also considering the 1.8A resolution, there should be 3 > molecules in total. So we engaged into the effort to find out the 3rd > molecule. > > Phased MR by MOLREP did find out the 3rd, so did Phaser(fix two, find out the > 3rd). Both solutions gave relatively poorer statistics but packing is perfect. > The 3rd molecule, solved by phased MR of MOLREP and PHaser respectively, can > not superpose showing slightly different orientations, but position is > similar. Refinement indicates PHased MOLREP MR gives lower Rfree. > > Using totally 3 molecule, When I restrain or constrain NCS during > refinement(used TLS),R free goes way up (R~30, Rfree > 36%). If I refine > without NCS,R factors slip right away to 27/30 but this strange thing > happens: two copies of the original solution refines very well - low B > factors,very good looking map. The 3rd copy, however, B factors are high and > the map is bad, no good density match with the model.And the 3rd molecule did > no help to Rfree or R. > > Story ends with my questions: > 1,It seems the 3rd copy non-exist or globally disordered, under this > circumstance, can I say 'because of the 3rd molecule is globally disordered so > Rfree and R stay high (2mol/a.u with Rfree 30% R 27%, 1.8A data)? > 2,Anything I should worry about in the process finding out the 3rd? > 3,Are there suggestions to further lower down Rfree (the reasonable Rfree > should be around 22%)? > > THANKS > Yanming >
