Dear Bernhard,
Am 12.01.12 10:30, schrieb Bernhard Rupp (Hofkristallrat a.D.):
Dear All,
I read an interesting statement in an NMR review:
".... regions of a protein or
DNA ⁄ RNA molecule that are flexible in the crystal do
not provide coherent X-ray scattering and hence do
not contribute to the final electron density map. Thus,
for all intents and purposes, they can effectively be
ignored."
Besides that I was not aware that disorder across molecules implies incoherence
in scattering, I think this is quite some strong tobacco coming from what is
primarily a crystallization screening tool ;-)
That doesn't sound wrong to me: the flexible parts are at different
relative positions in the unit cells and thus their "partial-structure
scattering waves" do not have a constant phase relation to each other,
i.e., they don't give a coherent contribution to the total scattering.
But I don't agree to their conclusion, since disorder doesn't
necessarily mean, that there won't be any interpretable electron density
left. The floppy parts could still be interpreted at an effective lower
resolution and thus will not be ignored.
Maybe the authors were annoyed by a vanishing NMR signal because the
macromolecule crystallized in the NMR test tube ;-)
Best regards,
Dirk.
Cheers, BR
PS: I am grappling with the meaning of resolution in NMR. I can see that it
could be related to comparable data/parameter ratios, although I am even
less clear about the weights of NMR restraint weights than in the case of MX...
some cross-trained person out there who can explain?
--
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Dirk Kostrewa
Gene Center Munich
Department of Biochemistry
Ludwig-Maximilians-Universität München
Feodor-Lynen-Str. 25
D-81377 Munich
Germany
Phone: +49-89-2180-76845
Fax: +49-89-2180-76999
E-mail: kostr...@genzentrum.lmu.de
WWW: www.genzentrum.lmu.de
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