Re: crystal

zaw wrote:

> Why do we refine the data after x-ray diffraction method?

During data refinement you use additional knowledge about your system
to increase the quality of the final structure.
If you have for example a crystal that scatters at 1.5 \AA resolution,
which is already pretty good for large molecules like proteins,
naively speaking you can't say much about atom positions below 1.5
\AA (which is pretty bad if you want to understand how an enzyme
works).

The solution is 'data refinement'. If you know for example that two
atoms are bonded covalently, it takes an enormous amount of energy to
stretch such a bond. It's most likely that these atoms will be close
to their minimum bond length (which are known quite often).
That way, by starting with a blueprint of your molecule, you can come
up with an energetically reasonable structure containing more
information than is inherent in the raw electron density.
(As a rule of thumb, with the above resolution one can trace back
structural changes in relative positions of atoms down to 0.3 \AA.
But i'm not sure about that number. You have to ask a crystallographer
for more details)

Hope it helps,
Torsten


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Torsten Becker Structural Biology/Bioinformatics
University of Bayreuth
+49-921-55-3547 (office) Universitaetsstr. 30, BGI
+49-921-7273406 (home) D-95447 Bayreuth; Germany

torsten.becker@xxxxxxxxxxxxxxx
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