My understanding of coherence is a constant phase relation between
waves. Of course, this breaks down for inelastic scattering, but
(in)coherence can also be described without any change in wavelength.
Best regards,
Dirk.
Am 12.01.12 11:27, schrieb Bernhard Rupp (Hofkristallrat a.D.):
Does out of phase imply incoherent scattering? I though it means inelastic
Compton scattering?
-----Original Message-----
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Dirk
Kostrewa
Sent: Thursday, January 12, 2012 1:58 AM
To: CCP4BB@JISCMAIL.AC.UK
Subject: Re: [ccp4bb] NMR review
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 ?exible in the crystal do not provide
coherent X-ray scattering and hence do not contribute to the ?nal
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?
--
*******************************************************
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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