[phenixbb] Rfree and a low resolution data set

Randy Read rjr27 at cam.ac.uk
Wed Apr 4 11:36:14 PDT 2018

Dear Toon & Vincent,

Please note that, even if you tick the box to append the original data, the Staraniso server will still discard any reflections that it considers to be too weak to be observed.  So you still need to go back to the data prior to Staraniso to keep all the original data for deposition in the PDB.

The elimination of weak data can be very useful for programs that do not account well for the effect of measurement error, but it can in fact be a serious problem for Phaser, which assumes implicitly that no systematic truncation has been carried out.  Phaser does its own analysis of anisotropy, which can be misled by throwing away all the weak reflections that indicate which direction of diffraction is weakest.  Phaser also has its own method to decide (after the anisotropy and, if relevant, tNCS analysis) which reflections will not contribute significantly to the likelihood calculations; in the cases I've looked at, Staraniso discards a substantial number of reflections that Phaser's criteria label as being at least marginally useful.

As time goes on, one hopes that more and more programs will be adapted to make better use of the data and its estimated errors, and it would be a great pity if data they could use were left out of the PDB.

Best wishes,

Randy Read

> On 4 Apr 2018, at 11:58, vincent Chaptal <vincent.chaptal at ibcp.fr> wrote:
> Dear Toon, 
> if you have data up to 2.3A resolution, you should keep it, it is so valuable. The isotropic statistics that you are looking at and output by Aimless are I'm sure awful, because you simply don't have data (lack of completeness) in the high resolution shells, and the statistics are produced out of a mixture of real data (weak) and noise. 
> Staraniso has done a lot of work on outputting statistics more meaningful in term of Table 1 and anisotropy, so it could be worth a try to submit your data and see what comes out of it. 
> In any case, I would use the data to the resolution limit suggested by Aimless, and judge your electron density maps. I'm sure you will have a big improvement if you include the 2.3A limit, even though it is not going to look like an isotropic 2.3A. 
> If you use the data output by staraniso, please ask the server to also output your original data so you can deposit the original intensities in the PDB and investigators can look at it untouched in the future. There is a check box on the server to tick. 
> Best of luck with your data
> Vincent
> On 04/04/2018 11:47, Toon Van Thillo wrote:
>> Hi all,
>> Currently I am refining a data set which showed anisotropic diffraction. Aimless suggested cutoffs at 2.3, 2.6 and 3.6 angstrom for the h,k and l axis.
>> I chose a general 3.6 cutoff to obtain satisfactory statistics for Rmeas, I/sd(I) and CC1/2. At this resolution the data set consists of approximately 2800 reflections.
>> Generally 5% of the set is set aside as the Rfree test set and I found that a minimum of 500 reflections in total is used to produce a reliable Rfree. However, 5% only amounts to 140 reflections in this case. I am hesitant to include more reflections as I would have to go up to 20% of the reflections to obtain more than 500 reflections for the test set. In a discussion on the CCP4 message boards some time ago it was suggested to do multiple refinements with different test sets:
>> https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind1411&L=ccp4bb&F=&S=&P=125570 <https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind1411&L=ccp4bb&F=&S=&P=125570>
>> In the thread it was also discussed that a least squares approach is prefered when using a small test set. However, when using a LS target, the resulting Rfree is very high (10% higher than when using the automatic option) and phenix.refine​ produces awful geometry (24% ramachandran outliers, 105 clashcore...). It seems that the refinement is performed without restraints? Optimize X-ray/stereochemistry weight does not result in improved stereochemistry. My question is if the LS approach is still relevant and if so, is there an explanation (and solution) for the bad statistics?
>> Kind regards,
>> Toon Van Thillo
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> -- 
> Vincent Chaptal, PhD
> Institut de Biologie et Chimie des Protéines
> Drug Resistance and Membrane Proteins Laboratory
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Randy J. Read
Department of Haematology, University of Cambridge
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