If I understand the idea correctly, I would still expect to see good
Bragg spots, but the amplitudes would represent the rotationally
averaged protein. This is like the hexagonal water lattice (Ih):
there is "disorder" in how the water molecules are oriented at each
lattice point (not really disorder, but more than one choice for
orientation), but the structure is solvable and the resulting density
is a spatial average where hydrogens appear to be nearly overlapping.
I agree that the lattice itself has to be distorted or imperfect for
the Bragg spots to go away.
It would be interesting to see how much lattice distortion can occur
before the spots are gone. Actually I'd like to be able to simulate
stuff like this for several reasons. Not sure how to do it other than
brute-force building a massive lattice of proteins and applying FFT
directly. Maybe separate treatment of structure factor and form
factor would be easier. Surely this has been done in the solid state/
small molecule/diffuse scattering literature ... Ideally a system
where you can tweak a parameter to go from crystal lattice to
solution scattering continuously.
Richard
I would think that a "perfect HCP lattice," no matter the disorder
in the organization of the molecules, would lead to Bragg
diffraction, albeit of low resolution. The "ghost crystals"
probably consist of very imperfect lattice(s) which fluctuate in
their dimensions and kind over space and time.
Jacob Keller
*******************************************
Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
Dallos Laboratory
F. Searle 1-240
2240 Campus Drive
Evanston IL 60208
lab: 847.491.2438
cel: 773.608.9185
email: j-kell...@northwestern.edu
*******************************************
----- Original Message -----
From: George DeTitta
To: CCP4BB@JISCMAIL.AC.UK
Sent: Tuesday, June 23, 2009 12:37 PM
Subject: [ccp4bb] Phantom Crystals - a recap
Thanks to all who replied regarding experiences with phantom
crystals (objects with crystal-like morphologies but NO
diffraction). The answers were more fascinating than the original
poorly worded inquiry deserved. Here is a recap.
The observation of phantoms may be rare but not so rare: a number
of people replied with first hand experience. Classes of compounds
that may lead to these bad actors: membrane-associated proteins
and RNAs. NO diffraction may be interpreted as no OBSERVABLE Bragg
diffraction, but beware of behind-the-beamstop diffraction; i.e. a
few Bragg peaks that are not typically observed unless care is
taken to insure a small beamstop.
I think of a mental image as follows. Say proteins are spherically
shaped and present as cats’ eyes marbles. You might be able to lay
them down in a perfect HCP lattice but rotationally the eyes might
point in all directions. The object at macroscopic dimensions
would look like a crystal but at atomic dimensions there would be
no buildup of scattering from cooperative effect of many atoms at
the same lattice spacing.
Thanks to all.
George
George T. DeTitta, Ph.D.
Principal Research Scientist
Hauptman-Woodward Institute
Professor and Chairman
Department of Structural Biology
SUNY at Buffalo
700 Ellicott Street Buffalo NY 14203-1102 USA
(716) 898-8600 (voice)
(716) 898-8660 (fax)
www.hwi.buffalo.edu
