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[ccp4bb] Commensurate superlattices in protein crystallography?
 

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CCP4bb <-- 2007 <-- March 2007 <-- 15 March 2007
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Date: 2007-03-15


Subject: Commensurate superlattices in protein crystallography?
From: John R Helliwell jrhelliwell {- at -} GMAIL {- dot -} COM
Date: 2007-03-15
Dear Colleagues,
I am currently engaged analysing a protein crystal structure which has two
unusual aspects about it, at least to me (hence this consultation).

The first oddity is the unit cell and space group:-

Triclinic P1 64.43 70.81 79.26 89.83 110.95 116.59

ie note the value of alpha very near 90 degrees. Autoindexing does offer a
C2 space group solution but, on inspection, is clearly too far from a proper
prediction. A complication that follows from the triclinic alpha near 90
degrees is the setting of the crystal and the indexing within P1 can need
altering (Phil Evans' initiative with 'pointless' solved that; thanks to
Eleanor for a pre-release alert of this software). Taking the analysis
through to a 2 derivative electron density map, with DM, was apparently fine
but the electron density connectivity was too poor to develop. I thus went
back to the diffraction patterns, image by image. This led to the second
oddity, now I believe truly odd:-


Second oddity:- some diffraction images showed a superlattice effect. These
can be neatly predicted through a doubling of the 'a'' unit cell value to
128Angstrom. This is an example of a commensurate lattice. Some of this
subset of images show a quadrupling superlattice ie where spots are
explained on a prediction of 256Angstrom for the 'a' value. Maps based on
'2a' might be ok to develop but still look 'difficult' to me re electron
density connectivity. An analysis based on '4a' is confounded by the
prediction showing nearly all spots as overlaps ie detector too close and
rotation angle too wide for a 256 Angstrom axis.


My checking of the IUCr Journals literature yields an example of an
incommensurate superlattice:-(J*. Appl. Cryst.* (2004). *37*, 327-330 [
doi:10.1107/S0021889804001773
] Imaging modulated
reflections from a semi-crystalline state of
profilin:actin crystals J. J.
Lovelace,
K. Narayan,
J. K. Chik,
H. D. Bellamy,
E. H. Snell,
U. Lindberg,
C. E. Schuttand
G.
E. O. Borgstahl.
I am making contact with these authors.



*Do colleagues on CCP4bb have similar examples and did they manage to
progress to a structure? *

Best wishes and thankyou,

John



--
John R Helliwell
Professor of Structural Chemistry, The University of Manchester;
ProteinCrystallography.org: Copyright 2006-2008 by Quid United Ltd