Dear Eva and Harry and others, I am not sure that an overall B-factor is the best solution for a 3.2 Å structure. In general, the true B-factors will vary a lot for different parts of the protein and poor diffracting proteins often have parts which are partially or completely disordered. An overall B-factor is a very poor estimate of such a true B-factor distribution and together with the low-resolution data will create a lot of model bias i.e. very nice density for side chains which are, in fact, disordered. I have struggled a long time with a 3.2 Å antithrombin data set and as luck would have it, the most interesting and unknown part of the structure was a loop with extremely high temperature factors. All standard protocols I tried produced very nice density for the model I put in, but did not tell me anything of how to build the unknown parts. A side remark by Randy Read finally helped me to solve this problem, which was to reduce model bias by using very tight geometric restraints (e.g. not allow the xyz positions produce bias) and individual temperature factors. What I oberved was that wrongly fitted residues would be pushed out of the density due to the tight geometric restraints, the artificially good electron density of some flexible parts would get weaker and most importantly: electron density appeared were I could fit the important reactive site loop. It worked in this case, it may not work in other cases and at 3.2 Å one should always proceed very cautiously and try to use whatever information (biochemical, mutations) is available to validate the model. This was my experience and I have never used an overall B-factor afterwards. Herman Schreuder
________________________________ Von: CCP4 bulletin board [mailto:[EMAIL PROTECTED] Im Auftrag von Harry M. Greenblatt Gesendet: Mittwoch, 18. April 2007 14:38 An: CCP4BB@JISCMAIL.AC.UK Betreff: Re: [ccp4bb] Stop Refmac from refining B factors? BS"D Dear Eva, You should be refining an overall temperature factor at that resolution. It's one of the choices in the list, instead of "isotropic". Hi, I have a little problem with B-factor refinement. I'm using the CCP4i interface, Refmac 5.2.0019, a resolution of 30-3.2 A (I tried 8-3.2 A as well, it doesn't make a big difference for this problem), and a current Rfree of 30.4%. Refmac refines the B-factors so that they are nearly the same for main chain and side chain, and I don't like that (or could it make sense in any way?). Moreover, my structure is a protein complex, and Refmac is mainly doing this for one component of the complex. If I take the B-factors from the original uncomplexed protein (around 18, 1.75 A) and add 44 to them with moleman to get them in the range they are in the complex, Refmac "flattens" them remarkably in only 5 cycles of restricted refinement. Does anyone have an explanation for this? I am pretty sure that the complex components are in the right place, I see beautiful density and everything I should see at this resolution. Here is what I tried further: * I de-selected "Refine isotropic temperature factors" in the Refmac interface. There was no REFI BREF ISOT any more in the com file. But there was also no difference in the B-factors compared to when there _was_ REFI BREF ISOT in the com file... So does Refmac just _ignore_ my wish not to refine B-factors? (The REFI keywords were as follows: type REST - resi MLKF - meth CGMAT - is there any B-factor-thing hidden in this?) * I played around with the geometric parameters. If I select the B-factor values there (the keywords are TEMP|BFAC <wbskal><sigb1><sigb2><sigb3><sigb4>), it does not make _any_ difference, what values I fill in there, the resulting B-factors are always the same (but different from when I don't use the TEMP keyword, and even "flatter"). Default for WBSCAL is 1.0, I tried 10, 1.0, 0.1, 0.01, and the equivalent numbers for the sigbs. Thanks for any thoughts on this, Eva ------------------------------------------------------------------------- Harry M. Greenblatt Staff Scientist Dept of Structural Biology [EMAIL PROTECTED] Weizmann Institute of Science Phone: 972-8-934-3625 Rehovot, 76100 Facsimile: 972-8-934-4159 Israel