Jacob
Yes, discussions have taken place but perhaps more private than on the
CCP4 bulleting board! So I am glad someone has raised the issue.

The present approximation that people make works very well in the
majority of cases. The approximation is implied during the integration
of a spot and is that the crystal consists of an infinite number of
identical unit cells (same contents and lattice repeats). I have
jokingly "blamed" Bragg for this approximation but James Holton has
pointed out that he also derived the "minimum wavelength principle" so
was fully aware of continuous transforms etc.

With significant different conformations in the unit cells, different
lattice constants, lattice couplings, strain, misaligned domains etc.
both complex spot profiles and diffuse scatter will result. By modeling
this properly (e.g. a proper model of crystal plus its contents and the
effects on the diffraction pattern) one should be able to get more
information. Measurements of the 3D shape of the Bragg peaks (ideally in
the near and far fields!) plus the diffuse scatter near and far from the
Bragg peaks would be ideal. A parallel beam and high resolution
detectors would be required to do this properly i.e. a similar set up to
that used for topographic studies (e.g. Lovelace and Borgstahl work plus
of course many other investigators). I think much of this was undertaken
as a method to monitor improvements in crystal growth rather than get
better diffraction data but the model of the crystal should be
applicable to obtaining better data for structure determination.  At the
moment this is too ambitious but, a lower resolution model of the
crystal could still be obtained with standard set ups and used in a
variety of ways.

Coincidently, I am involved at the moment on a beamline with a project
to obtain high resolution images of diffraction spots using a coherent
beam. Small molecule crystals at the moment but my motivation is to
extend to protein crystals. Don't know whether will ever get there
though.

So yes, plenty to discuss but doing something is a lot harder! 

I have only mentioned using such data. Making it available is another
issue, as others have said.

Regards
  Colin


 

> -----Original Message-----
> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of 
> Jacob Keller
> Sent: 20 January 2010 17:47
> To: [log in to unmask]
> Subject: [ccp4bb] Refining against images instead of only reflections
> 
> Dear Crystallographers,
> 
> One can see from many posts on this listserve that in any given x-ray 
> diffraction experiment, there are more data than merely the 
> diffraction spots. Given that we now have vastly increased 
> computational power and data storage capability, does it make sense to

> think about changing the paradigm for model refinements? Do we need to

> "reduce" data anymore? One could imagine applying various functions to

> model the intensity observed at every single pixel on the detector.
> This might be unneccesary in many cases, but in some cases, in which 
> there is a lot of diffuse scattering or other phenomena, perhaps 
> modelling all of the pixels would really be more true to the 
> underlying phenomena? Further, it might be that the gap in R values 
> between high- and low-resolution structures would be narrowed 
> significantly, because we would be able to model the data, i.e., 
> reproduce the images from the models, equally well for all cases. More

> information about the nature of the underlying macromolecules might 
> really be gleaned this way. Has this been discussed yet?
> 
> Regards,
> 
> 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: [log in to unmask]
> *******************************************
> 

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