Dear Colleagues, Re the CCP4bb most interesting discussion thread initiated by Richard Gillilan at CHESS I summarise below some relevant references alluded to in my last email of about 3 weeks ago.
In the acoustic scattering discussion re :- I.D. Glover, G.W. Harris, J.R. Helliwell and D.S. Moss 'The variety of X-ray diffuse scattering from macromolecular crystals and its respective components' Acta Cryst. (1991) B47, 960-968. and page 966 in particular See also:- Re acoustic halo contributions to Bragg intensities, and their removal, for protein structure refinement:- Grigorieff, N., and Henderson, R. (1995): Diffuse scattering in electron diffraction data from protein crystals. Ultramicroscopy 60, 295–309. Re an inelastic contribution to the diffraction background see:- A. Gonzalez<http://scripts.iucr.org/cgi-bin/citedin?search_on=name&author_name=Gonzalez,%20A.>, R. Denny<http://scripts.iucr.org/cgi-bin/citedin?search_on=name&author_name=Denny,%20R.>and C. Nave<http://scripts.iucr.org/cgi-bin/citedin?search_on=name&author_name=Nave,%20C.>Data collection at short wavelengths in protein crystallography Acta Cryst. (1994). D50, 276-282. a paper which mentions 'background caused by inelastic scattering' (notably by Compton scattering). Re the approach of focussing the beam at the detector or the sample see:- J.R. Helliwell, T.J. Greenhough, P. Carr, S.A. Rule, P.R. Moore, A.W. Thompson and J.S. Worgan 'Central data collection facility for protein crystallography, small angle diffraction and scattering at the Daresbury SRS' (1982) J. Phys.E. *15*, 1363-1372. For the focussing the beam at the detector or the sample but a more general discussion than Helliwell et al 1982 ie covering SAXS, high angle fibre diffraction as well as crystal diffraction see G Rosenbaum and K C Holmes in ‘Synchrotron Radiation Research’ Edited by H Winick and S Doniach. [This book I had disappear from my shelves some years back and I successfully got a copy via Amazon 10 days ago; it was this that caused my delay to provide these references as I wanted to check this first for relevance to the current CCP4bb discussion thread]. Best wishes to all for 2010, John Prof John R Helliwell DSc On Wed, Dec 2, 2009 at 3:45 PM, Richard Gillilan <r...@cornell.edu> wrote: > Hi John, thanks for your input. I would certainly like to get a copy of > your references list on this subject when convenient. > > Best > > Richard > > > > On Dec 2, 2009, at 4:46 AM, John R Helliwell wrote: > > >> Dear Colleagues, >> Further discussions of this most interesting topic have continued between >> Colin and myself off-line. Not least we have basically ended up assembling a >> references list for further reading! One such I need to check and my copy of >> the relevant book has required me to get a replacement as my copy has >> disappeared. I certainly did not wish to raise 'anxieties' either about the >> nature of inelastic background (not acoustic ie rather Compton) nor about >> the success or otherwise of ever smaller crystals or ever larger complexes >> and associated S/N of the diffraction data. On this aspect Colin and I >> diverge between his optimism and my own (not like me!) caution. However the >> spectrum of structural information between a polypeptide chain trace on the >> one hand and the detailed structural chemistry of eg protonation states of >> ionisable groups on the other will no doubt embrace these differing views. >> No doubt more later..... >> Seasons' Greetings, >> John >> Prof John R Helliwell DSc >> On Fri, Nov 27, 2009 at 9:48 AM, John R Helliwell <jrhelliw...@gmail.com> >> wrote: >> >> Dear Richard, >> A most interesting discussion has ensued! >> >> The balance of elastic versus inelastic scattering becomes the core point >> re benefit of moving back the detector as mentioned by Ian. It should be >> easier now ie with much more beamtime available to measure this as a >> function of wavelength. Colin I believe has made a start in this direction. >> >> The acoustic scattering discussion needs to recall from:- >> >> I.D. Glover, G.W. Harris, J.R. Helliwell and D.S. Moss 'The variety of >> X-ray diffuse scattering from macromolecular crystals and its respective >> components' Acta Cryst. (1991) B47, 960-968. >> and page 966 in particular >> >> that moving the detector back was not the setting required but a small >> collimator (0.2mm) and slitting down the divergence to control the spot size >> versus the broader halo of acoustic scattering. These days much more readily >> accomplished with an undulator. >> >> These are both important points then for the growing categories of >> microcrystals, which I know you have been very usefully surveying, and ever >> larger molecular weight complexes ie both of which are challenged by S/N for >> the Bragg spots notably at higher resolution. >> >> Best wishes, >> John >> Professor John R Helliwell DSc >> beam divergence. >> >> On Mon, Nov 23, 2009 at 10:54 PM, Richard Gillilan <r...@cornell.edu> >> wrote: >> It seems to be widely known and observed that diffuse background >> scattering decreases more rapidly with increasing detector-to-sample >> distance than Bragg reflections. For example, Jim Pflugrath, in his 1999 >> paper (Acta Cryst 1999 D55 1718-1725) says "Since the X-ray background falls >> off as the square of the distance, the expectation is that a larger >> crystal-to-detector distance is better for reduction of the x-ray >> background. ..." >> >> Does anyone know of a more rigorous discussion of why background scatter >> fades while Bragg reflections remain collimated with distance? >> >> >> Richard Gillilan >> MacCHESS >> >> >> >> -- >> >> >> >> >> >> -- >> Professor John R Helliwell DSc >> >> > -- Professor John R Helliwell DSc
