Le Mardi 12 Mars 2019 19:55 CET, Dale Tronrud <[log in to unmask]> a écrit:
Dale
Good to have the opportunity of going back to the crystallography of the fifties in these post-modern times...
There is an essential argumentation that should be recalled. The only reason for the fact that one ignores low-resolution data in a Wilson plot is that a Wilson plot is based precisely upon Wilson statistics, which assumes that the atoms are cast randomly in the unit cell.
This assumption obviously does not hold at low resolution and there is no reason to obtain a straight line that stems from the latter assumption.
Therefore, I do not think one may say that a Wilson plot tends to ignore atoms with high B values.
Consequence: if one has data at rather low resolution, a Wilson plot is inherently inaccurate, but if one has data at very high resolution, the Wilson plot should give a very decent estimate of the average B and any significant discrepancy should ring the bell.
Philippe Dumas
> The numeric average of the B factors of the atoms in your model only
> roughly corresponds to the calculation of the Wilson B. While I always
> expect the average B to be larger than the Wilson B, how much larger
> depends on many factors, making it a fairly useless criteria for judging
> the correctness of a model.
>
> While it is pretty easy to understand the average of the B factors in
> your model, the Wilson B is more difficult. Since it is calculated by
> drawing a line though the (Log of) the intensity of your structure
> factors as a function of the square of sin theta over lambda, it is
> rather removed from the atomic B factors. When drawing the line the low
> resolution data are ignored because those data don't fall on a straight
> line, and this means that the large B factor atoms in your model are
> ignored in the Wilson B calculation.
>
> The Wilson B is (sort of) a weighted average of the B factors of your
> model, with the smallest B's given the largest weight. The actually
> weighting factor is a little obscure so I don't know how to simulate it
> to adjust the averaging of atomic B's to come out a match. The easiest
> way to compare your model to the Wilson B is to calculate structure
> factors from it and calculate the Calculated Wilson B. No one does this
> because it will always come out as a match. If your calculated Wilson B
> doesn't match the observed Wilson B your R values are guaranteed to be
> unacceptable and your refinement program will have to be malfunctioning
> to create such a model.
>
> If all the B factors in your model are equal to each other, your
> refined model will have an average B that matches the Wilson B, because
> weighting doesn't matter in that situation. If you allow the B's to
> vary, the difference between the average and the Wilson B will depend on
> how high of an individual B factor you are willing to tolerate. If you
> are a person who likes to build chain into weak loops of density, or
> build side chains where there is little to no density, then your average
> B will be much larger than the Wilson B. This does not mean there is an
> error, it is simply a reflection of the Wilson B's insensitivity to
> atoms with large B.
>
> I do not believe comparing the average B to the Wilson B has any
> utility at all.
>
> Dale Tronrud
>
> On 3/12/2019 11:34 AM, Eze Chivi wrote:
> > Dear CCP4bb community,
> >
> >
> > The average B-factor (calculated from model) of my protein is 65,
> > whereas the Wilson B is 52. I have read in this BB that "it is expected
> > that average B does not deviate strongly from the Wilson B". How I can
> > evaluate if the difference calculated for my data is razonable or not?
> >
> >
> > Thank you in advance
> >
> >
> > Ezequiel
> >
> >
> > ------------------------------------------------------------------------
> >
> > To unsubscribe from the CCP4BB list, click the following link:
> > https://www.jiscmail.ac.uk/cgi-bin/webadmin?SUBED1=CCP4BB&A=1
> >
>
> ########################################################################
>
> To unsubscribe from the CCP4BB list, click the following link:
> https://www.jiscmail.ac.uk/cgi-bin/webadmin?SUBED1=CCP4BB&A=1
########################################################################
To unsubscribe from the CCP4BB list, click the following link:
https://www.jiscmail.ac.uk/cgi-bin/webadmin?SUBED1=CCP4BB&A=1
|
