Colin,

Your point:
"I think the point here (probably the one you are making) is that if
crystallographers produce a pseudo rigid body motion (or static
disorder) model described by TLS parameters then it would make specific
predictions of diffuse scatter. These predictions could be used to test
the model. In other words data is already there to validate the TLS
model and this data is being ignored."
is well taken.

This is essentially what was done by Caspar, Clarage et al in their work on 
insulin and lysozyme crystals, publshed in Nature and Proteins.
Nature 332 659-662 1988
Proteins 12 145-157 1992

Diffuse scatter patterns were compared to simulations of correlated motions 
that were long-range  (say, the size of the whole molecule) and short range 
(say, the size of an amino acid). The TDS data were best fit when the 
majority of the motion was put in the short-coupled range component with a 
much more minor component in the whole-molecule component. i.e. it does 
not seem that TLS models as used to represent most of the B-factor by rigid 
protein motions are consistent with this TDS data. 

For that matter, pretty much every type of experimental measurement I know 
of protein dynamics, including those which probe the normal mode spectrum 
directly (inelastic neutron scattering work by Cusack) also put most of the 
motion into high frequency components and deemphasize the low order modes 
(the TLS components). A review of all this is
Prog.Biophys.Molec.Biol. 63 251-276, 1995.

From this perspective it is quite surprising that crystallographers are so keen 
on using TLS models for fitting displacement amplitudes. 

John Badger

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