James Some comments! >The diffuse scatter does NOT tell you about the > deviations from this average; it tells you how the deviations > are correlated from unit cell to unit cell. Does this just refer to the scatter near the Bragg peaks? If there is no correlation from unit cell to unit cell then the diffuse scatter (between rather than near the Bragg peaks) should provide structural information about the disordered entity (e.g. whole protein displacements or just a side chain). This could of course be static disorder rather than motion.
> trying to do this). But I think what WOULD be a major step > forward is handling overlaps more > intelligently: Yes, but having a realistic model of the crystal imperfections would assist in this > Refining against pixels or not, a program that could be used > equally well for powder, single crystal or clusters of > crystals, would definitely be nice! I agree! > coming out the other end is hkl and F. The "R factor Gap" > between Rmerge and Rcryst/Rfree does not come from > measurement errors, it comes from our inability to accurately > model the average electron density in a unit cell. Probably true. However if there were two (or more) distinctly different but highly ordered crystals (e.g. different unit cell dimensions) in the specimen then one would like to pick these out and refine the two structures independently. Regards Colin > -----Original Message----- > From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On > Behalf Of James Holton > Sent: 21 January 2010 08:39 > To: CCP4BB@JISCMAIL.AC.UK > Subject: Re: [ccp4bb] Refining against images instead of only > reflections > > It is interesting and relevant here I think that if you > measure background-subtracted spot intensities you actually > are measuring the AVERAGE electron density. Yes, the > arithmetic average of all the unit cells in the crystal. It > does not matter how any of the vibrations are "correlated", > it is still just the average (as long as you subtract the > background). The diffuse scatter does NOT tell you about the > deviations from this average; it tells you how the deviations > are correlated from unit cell to unit cell. > > So, in order to use diffuse scattering information you will > have to refine a model in not just one unit cell, but many, > and the only information you have is correlations of motions, > not the motion itself. > It would still be nice to have some software (the kind you > can download and install) to do diffuse scattering stuff, but > I fear the drop in Rfree you would get by incorporating > diffuse scatter into model refinement would be slim. > > There may be some advantages to measuring weak spots if we > had better models to fit to them than the "learned profiles" > Rossmann suggested in the '70s (the EVAL programs seem to be > trying to do this). But I think what WOULD be a major step > forward is handling overlaps more > intelligently: bridging the gap between powder diffraction (Riteveld > stuff) and single-crystal diffraction processing. I think > the world is full of single-crystal crystallographers who > would love to have been able to process that one really good > crystal that was ruined by split or otherwise messy or > overlapping spots. What if you had a big unit cell and a > mosaic spread of 10 degrees? I've seen a lot of crystals > like this! It would be great if we didn't have to throw them > away. Believe it or not, there are also a lot of powder > people who screen through hundreds of preps cursing those > "spotty rings", because Riteveld only works for nice, even > powder rings (mosaic spread of 180 degrees). > Refining against pixels or not, a program that could be used > equally well for powder, single crystal or clusters of > crystals, would definitely be nice! But I think all we need > coming out the other end is hkl and F. The "R factor Gap" > between Rmerge and Rcryst/Rfree does not come from > measurement errors, it comes from our inability to accurately > model the average electron density in a unit cell. > > -James Holton > MAD Scientist > > Harry wrote: > > Hi > > > > I, too, was struck by the potential similarity with Rietveld > > refinement, then started to think about the differences. > The biggest > > difference, of course, is refining against "many" two-dimensional > > images compared to refining against a linear plot - so I'd guess in > > principle you'd get a big benefit in increasing the > data:parameter ratio. > > > > Of course, there's a hint above in the "refinement" part of the > > process; you need the structure and a model for the causes of the > > diffuse scatter etc to have something to refine against, so it may > > still be worthwhile extracting the Bragg intensities first to solve > > your structure. > > > > (I've never done Rietveld refinement, but read a good book on the > > topic a few years ago - "The Riteveld Method" ed R.A. > Young. It might > > be a little out of date now...) > > > > > > On 20 Jan 2010, at 21:14, Klaus Fütterer wrote: > > > >> A long time ago I did a bit of Rietveld refinement and I see some > >> similarities between this approach and what people have been > >> proposing in this thread. Refining against the profile of the 1-d > >> powder diffraction pattern rather than extracting integrated > >> intensities helped to improve the quality of the refined > structures > >> significantly. Finding the correct (or best) profile function, > >> however, took a while, at least for the X-ray case. > >> > >> Klaus > >> > >> > ===================================================================== > >> == > >> > >> Klaus Fütterer, Ph.D. > >> Reader in Structural Biology > >> > >> School of Biosciences P: +44-(0)-121-414 5895 > >> University of Birmingham F: +44-(0)-121-414 5925 > >> Edgbaston E: k.futte...@bham.ac.uk > >> Birmingham, B15 2TT, UK W: > www.biochemistry.bham.ac.uk/klaus/ > >> > ===================================================================== > >> == > >> > >> > >> > >> > >> > >> On 20 Jan 2010, at 20:29, Edward Snell wrote: > >> > >>> Hi Paul, > >>> > >>> I'll probably open myself up to criticism (welcomed) but > I think I'd > >>> disagree with this somewhat. While crystallography from the Bragg > >>> reflections provides a nice static picture of the > structure, looking > >>> at the diffuse scatter in more detail may give more > knowledge about > >>> mechanism - i.e. if there are any characteristic modes associated > >>> with significant motion etc. Higher resolution is not > always good, > >>> one of my enlightening experiences came from paying attention to > >>> collecting very complete, very low resolution data. > Similarly, after > >>> collecting 0.8A data from a large protein I leant a lot > about data > >>> processing but even more about how to not tell anyone, move the > >>> detector back, and then attenuate the beam :) The > high-res provided > >>> a lot more work and didn't provide any more useful structural > >>> knowledge than a 1.2A data set collected in a fraction of > the time. > >>> However, it did provide a window into how X-rays can perturb the > >>> structure - being greedy is not always good. > >>> > >>> Diffuse scattering has been neglected in the field (for > good reason) > >>> but I think we have the processing power to take advantage of it > >>> now. To misquote Richard Feynman, "there is plenty of room at the > >>> bottom", make sure you get the low resolution information > as well as > >>> the high. > >>> > >>> I do agree that we may have to rethink image storage somewhat. > >>> Looking over a paper not too long ago that had over 30,000 images > >>> involved in the analysis made me remember the days when the tape > >>> drives were slower writing data than the detectors producing it. > >>> That mad scramble to start backup before starting collection ;) > >>> Realtime readout, continuous rotation etc., may need to > redefine our > >>> thoughts of images. > >>> > >>> Cheers, > >>> > >>> Eddie > >>> > >>> Edward Snell Ph.D. > >>> Assistant Prof. Department of Structural Biology, SUNY Buffalo, > >>> Hauptman-Woodward Medical Research Institute 700 Ellicott Street, > >>> Buffalo, NY 14203-1102 > >>> Phone: (716) 898 8631 Fax: (716) 898 8660 > >>> Skype: eddie.snell Email: > esn...@hwi.buffalo.edu > >>> Telepathy: 42.2 GHz > >>> > >>> Heisenberg was probably here! > >>> > >>> > >>> -----Original Message----- > >>> From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] > On Behalf > >>> Of Paul Smith > >>> Sent: Wednesday, January 20, 2010 3:00 PM > >>> To: CCP4BB@JISCMAIL.AC.UK > >>> Subject: Re: [ccp4bb] Refining against images instead of only > >>> reflections > >>> > >>> Hi Jacob, > >>> > >>> I see you're still in the crystallography business. > >>> > >>> While you have an interesting idea, I doubt refining structures > >>> against entire images would be of any use in obtaining higher > >>> quality macromolecular structures. Much of what you see on the > >>> screen is a function of parameters completely unrelated or > >>> irrelevant to the structure being studied. Diffuse > scattering can > >>> come from the cryo liquid surrounding the crystal as well as the > >>> fibers of the mounting loop itself. Background scattering is > >>> related to beam collimation. Spot size/shape is a function of > >>> crystal morphology among other things. In addition, > every detector > >>> has its own peculiarities that make the intensities > observed apart > >>> from diffraction spots particular to that detector. > Also, you would > >>> have to take into account other physical properties such > as ambient > >>> temperature, detector dark current fluctuations, > variations in air > >>> absorption, etc. > >>> > >>> So, you could conceivably fit all of these various > parameters to the > >>> images on hand, but none of them give you any actual information > >>> about your structure. As always, if you want more > information about > >>> your structure, get higher resolution data. > >>> > >>> Nonetheless, I do think some thought could be put in to > exactly how > >>> data are reduced. Perhaps the impending era of real time > detector > >>> readout will help us rethink about spot profiles and intensity > >>> integration in a more sophisticated way. We may see a return to > >>> thinking about ccd readouts like an area detector which makes the > >>> process of analyzing images moot. > >>> > >>> --Paul > >>> > >>> --- On Wed, 1/20/10, Jacob Keller <j-kell...@md.northwestern.edu> > >>> wrote: > >>> > >>>> From: Jacob Keller <j-kell...@md.northwestern.edu> > >>>> Subject: [ccp4bb] Refining against images instead of only > >>>> reflections > >>>> To: CCP4BB@JISCMAIL.AC.UK > >>>> Date: Wednesday, January 20, 2010, 12:47 PM 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 > >>>> ******************************************* > >>>> > > > > Harry > > -- > > Dr Harry Powell, MRC Laboratory of Molecular Biology, MRC Centre, > > Hills Road, Cambridge, CB2 0QH > -- This e-mail and any attachments may contain confidential, copyright and or privileged material, and are for the use of the intended addressee only. 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