Re: [sdpd] Dubious structure

[pstephens...@notes.cc.sunysb.edu]

One classic example of a powder structure with vastly higher quality than 
a corresponding single crystal structure is C60 at low temperature.  The 
single crystal structure (Liu et al., Science 254, 408 (1991)) has thermal 
ellipsoids that never should have gotten past a reviewer, and should have 
told the authors that they have something seriously wrong.  Whereas the 
neutron powder experiment (David et al., Nature 353, 147 (1991)) has the 
complete story of statistical occupancy of two different orientations. 
(Note also that the single crystal structure is actually only a refinement 
of starting coordinates obtained from -- an earlier powder experiment!)

As for the question of only publishing "pure" structures that do not have 
restraints, that would eliminate most macromolecular crystallography even 
from single crystals.  Modern practice depends heavily on imposing prior 
knowledge of the configurations of amino acids, and indeed the protein 
sequence of the target protein..

^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~
Peter W. Stephens
Professor, Department of Physics and Astronomy
Stony Brook University
Stony Brook, NY 11794-3800
fax 631-632-8176


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