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Dear Phenix Users,<br><br>I'm currently using version:<br> Version: 1.4<br> Release tag: 3<br> cctbx tag: 2008_12_07_1353<br> Platform: mac-intel-osx osx-10.5.6<br><br>I am working on characterising substrate/inhibitor interactions of a protein, the structure has been solved many times so its unit cell dimensions, contents (1 molecule per ASU) and space group (P3121) are well characterized.<br><br>However, I have recently encountered several crystals that appear exhibit merohedral twinning, and am having trouble converging the R-factors to reasonable levels. The details of one such data set is described below:<br><br>Diffraction to ~1.9A was recorded using a high and low resolution pass of 180� each. When processed with HKL2000 separately or together, Scalepack wanted to reject huge amounts of my data (~20%). I was forced to increase the error scale factor to 2 and adjust my error model to the very high R-factors of the low-resolution shells. The summary table from my final round of scalepack is:<br>Shell Lower Upper Average Average Norm. Linear Square<br> limit Angstrom I error stat. Chi**2 R-fac R-fac<br> 50.00 4.09 7648.3 1498.7 182.7 1.030 0.376 0.424<br> 4.09 3.25 3885.1 568.9 64.1 1.410 0.416 0.480<br> 3.25 2.84 1609.7 225.0 26.1 1.227 0.451 0.517<br> 2.84 2.58 807.9 97.1 25.3 1.958 0.439 0.561<br> 2.58 2.39 496.6 59.7 22.0 1.060 0.257 0.268<br> 2.39 2.25 335.5 53.0 18.1 0.717 0.275 0.238<br> 2.25 2.14 221.7 19.0 17.4 2.389 0.354 0.298<br> 2.14 2.05 143.1 19.5 17.5 1.537 0.459 0.351<br> 2.05 1.97 87.0 20.2 17.1 1.079 0.663 0.480<br> 1.97 1.90 54.6 20.3 16.7 1.073 0.990 0.696<br> All reflections 1588.3 269.2 41.9 1.350 0.400 0.444<br><br>In addition, many systematic absences appear to be present:<br><br> Intensities of systematic absences<br> h k l Intensity Sigma I/Sigma<br><br> 0 0 5 177.7 56.7 3.1<br> 0 0 7 255.5 70.6 3.6<br> 0 0 8 264.8 73.2 3.6<br> 0 0 10 595.0 164.2 3.6<br> 0 0 11 2343.5 1289.9 1.8<br> 0 0 13 321.3 125.4 2.6<br> 0 0 14 66.5 21.8 3.1<br> 0 0 16 238.0 76.3 3.1<br> 0 0 17 394.3 109.1 3.6<br> 0 0 19 13.0 7.1 1.8<br> 0 0 20 650.1 179.5 3.6<br> 0 0 22 199.6 55.6 3.6<br> 0 0 23 953.9 372.0 2.6<br> 0 0 25 192.2 62.2 3.1<br> 0 0 26 1038.7 405.0 2.6<br> 0 0 28 96.3 28.9 3.3<br> 0 0 29 2303.0 897.0 2.6<br> 0 0 31 12.0 11.5 1.0<br> 0 0 32 1622.4 516.5 3.1<br> 0 0 34 11.5 13.8 0.8<br> 0 0 35 22.8 14.8 1.5<br> 0 0 37 27.9 18.0 1.5<br> 0 0 38 154.3 45.6 3.4<br> 0 0 40 55.8 27.8 2.0<br> 0 0 41 35.5 23.7 1.5<br> 0 0 43 -25.3 34.4 -0.7<br> 0 0 44 67.7 32.1 2.1<br><br><br><br>The data has an overall completeness of 99.7%.<br><br>When I run the data trough xtriage, it tells me:<br><br>Statistics depending on twin laws<br>-----------------------------------------------------------------<br>| Operator | type | R obs. | Britton alpha | H alpha | ML alpha |<br>-----------------------------------------------------------------<br>| -h,-k,l | M | 0.307 | 0.301 | 0.256 | 0.022 |<br>-----------------------------------------------------------------<br><br>Patterson analyses<br> - Largest peak height : 5.288<br> (corresponding p value : 0.86552)<br><br><br>The largest off-origin peak in the Patterson function is 5.29% of the <br>height of the origin peak. No significant pseudotranslation is detected.<br><br>The results of the L-test indicate that the intensity statistics<br>are significantly different than is expected from good to reasonable,<br>untwinned data.<br>As there are twin laws possible given the crystal symmetry, twinning could<br>be the reason for the departure of the intensity statistics from normality.<br>It might be worthwhile carrying out refinement with a twin specific target function.<br><br>I also estimated the twin fraction with the following server:<br><br>http://nihserver.mbi.ucla.edu/Twinning/<br><br>And found it to about 20% (using data from 6-3A).<br><br>After phasing the data with rigid body refinement in Refmac with intensity based twin refinement (brings both R-factors to ~36%), I have tried doing a twin refinement with Phenix:<br><br>%phenix.refine model.pdb data.mtz twin_law="-h,-k,l" strategy=rigid_body+individual_sites+group_adp<br><br><br>The R_factors consistently diverge to the following values:<br><br>Final R-work = 0.3392, R-free = 0.4360<br><br>Simulated annealing/TLS/ordered solvent/messing with wxc/wxu weights does not seem to improve the situation. I have also tried this with and without prior detwinning with the CCP4 program "Detwin", with similar results.<br><br>Reindexing to -h,-k,l gives similar R-factors after rigid body refinement but after refinement with Phenix, things get even worse:<br>Final R-work = 0.3919, R-free = 0.5059<br><br>I was surprised to see that the fit to the map using both indexing conventions are comparable even though this should effectively reverse the hand of the structure. I assume this is because twin refinement is basically doing the job of "partial reindexing" on those parts of the lattice which are twinned?<br><br>I have been working on this for about a week now and can't for the life of me find a way forward. If anyone can offer any suggestions It would greatly be appreciated. Im slowly beginning to lose my sanity....<br><br>BTW I don't think my problems stem from misindexing, although the beam was wondering all over the place during this block of data collection, I have previously solved another structure at the same d and xbeam ybeam values without any trouble.<br><br>Sorry about the v.long post. Thought it would be better to give too much information rather than not enough =)<br><br>Many thanks for all your suggestions in advance and for everything I
have learned from this community in the past. Cheers,<br><br>Joe<br>PhD student. <br><br><br><br><br><br><br><br><br><br><br><br> <br><br><br><br><br><br><br>                                            <br /><hr />Download a free gift for your PC. <a href='http://experience.windows.com' target='_new'>Get personal with Windows.</a></body>
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