Shya, Did phaser complain that the asymmetric unit was too full? How do the self rotation maps look? Are the crystallographic peaks exact or off by a few degrees (your resolution data may make it difficult to see this)? How do the N(z) cumulative intensity distributions look (make sure to calculate this with thin resolution bins, i.e. increase BINS in Scala I think)? Does your data look sigmoidal on this plot? Perfect twinning or an NCS that's close to a crystallographic axis is difficult to diagnose from merged intensity statistics and even more difficult with resolution worse than 2.5. I recommend Dauter Acta Cryst (2003) D59 2004-2016 for a good discussion of this. Your space group might be too high. See the subgroups of P422 at http://cci.lbl.gov/~phzwart/p422_2.png . Reintegrate and merge the data in each space group, MR a single copy of your model (let phaser complete the ASU) and compare the Rpim's (from scaling/merging) and the Rwork/Rfree from a rigid body refine without NCS, with NCS, with appropriate twin laws, and with twin laws + NCS. No need to do a full refinement just yet. Allow phenix.refine to create the Rfree flags. Choose the space group which gives the best statistics. I recently had a case (Hardin, Reyes, Batey J. Biol. Chem., Vol. 284, Issue 22, 15317-15324, May 29, 2009) of a protein that merged into P422 but was difficult to refine in that space group. I brought it back to P4 and refined with NCS+twin to give more reasonable Rwork/ Rfree (5-7% difference from the P422 to P4). HTH, FR On Jul 23, 2009, at 3:54 PM, [email protected] wrote:
Hi Francis, Thanks for your response. The matthews coefficient suggests two molecules in the AU. Phaser also finds two molecules. I ran the dataset through phenix xtriage it did not indicate twinning though. The molecule also exists in nature as a monomer. Shya
Twinning? What's your matthews coefficient say? Do you know if your structure is a multimer (biochemistry, etc)? Does it agree with the matthews coefficient?
If the unit cell is not big enough to hold all of the contents,then this is an indicator for twinning .
FR
On Jul 23, 2009, at 3:09 PM, [email protected] wrote:
Hi all,
I was trying to solve a structure by molecular replacement. I scaled the data in P4222 space group (resolution 2.7A) with two molecules in the assymmetric unit (molecule A and B) I ran phaser with my model and got a Zscore of 5.1. When I look at the map that I got from phaser I could easily see good electron density for both molecules, However upon inspection of the electron density map there were considerable interaction or clashes with molecule B and a symmetry atom. Molecule A had no clashes however with the symmetry atoms. I was wondering if anyone knows how to resolve this. Could it be a problem of space group. The statistics are good for space group P4222 and the I/sigI was good till 2.7A. Any advice is appreciated, Shya
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