Sharpening of Fs

Clemens Vonrhein vonrhein at bio5.chemie.uni-freiburg.de
Fri Jun 4 07:30:34 EST 1993


Dear xtallographers!

I'm using the program SQUASH for histogram-matching and solvent levelling.
The documentation of this program as well as the papers about the method
recommend to use sharpened Fs for the calculations. My questions are:

-what are sharpened Fs exactly?

-what is the formula to sharpen measured Fs ? something like:

                    N
                   sum (Zi)
                   i=1
 F(point) = ------------------------------------------------- * F(real)
                                                     N
               exp( -B * (sin(theta)/lambda)**2 ) * sum (fi)
                                                    i=1

             N=atoms in unit cell; 
             f=scattering factors=f(sin(theta)/lambda);
             B=temperature facture
             Z=atomic number

           ( I took this formula from Stout & Jensen, "Xray structure
             determination, A practical guide, p273 (11.11))

-what is the effect of sharpened Fs on refinement of MIR-parameters,
 solvent-leveling, histogram-matching...?

-do I have to sharpen my Fs before/after scaling, before/after heavy atom
 parameter refinement?

-do I have to calculate the sums only over the atoms of my protein or over
 the aproximated content of solvent molecules too?

-are there any programs that are sharpening?

-any experiences?


These are quite a lot of questions, but noone in our lab has any
experience. And the more or less theoretical articles are interesting, but
not very helpful in practical view.

Thanks for any comment!

Clemens Vonrhein

vonrhein at bio5.chemie.uni-freiburg.de

Albert-Ludwigs-Universitaet Freiburg
Institut fuer Organische Chemie und Biochemie
Albertstr.21
7800 Freiburg i. Br.
Germany

 




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