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:ccp...@jiscmail.ac.uk] On Behalf Of > Jacob Keller > Sent: 20 January 2010 17:47 > To: CCP4BB@JISCMAIL.AC.UK > 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: j-kell...@northwestern.edu > ******************************************* > -- This e-mail and any attachments may contain confidential, copyright and or privileged material, and are for the use of the intended addressee only. If you are not the intended addressee or an authorised recipient of the addressee please notify us of receipt by returning the e-mail and do not use, copy, retain, distribute or disclose the information in or attached to the e-mail. Any opinions expressed within this e-mail are those of the individual and not necessarily of Diamond Light Source Ltd. Diamond Light Source Ltd. cannot guarantee that this e-mail or any attachments are free from viruses and we cannot accept liability for any damage which you may sustain as a result of software viruses which may be transmitted in or with the message. Diamond Light Source Limited (company no. 4375679). Registered in England and Wales with its registered office at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom
