Anisotropy in the diffraction pattern could simply be due to the shape of the crystals. The intensity of diffraction is a function of the volume of diffracting matter that is hit by the X-ray beam. Think for example of a thin plate crystal, which you rotate in the X-ray beam. When the plate is perpendicular to the X-ray beam, the volume of matter hit by the X-rays is much smaller than when the plate is parallel to the X-ray beam.
When processing data using XDS, at the integration level (INTEGRATE) there is for each frame a single scale factor that is reported (I cannot tell you what the INTEGRATE.LP file says exactly - I am sitting at a conference where I cannot access my data files at home), but you can follow the increase/decrease in intensity of diffraction (and thus the volume of diffracting matter in the X-ray beam) by following the variation of these scale factors. Otherwise, sometimes (once the structure is solved) it is possible, 'a posteriori', to give a plausible explanation to such an anisotropy. For example, by noticing anisotropic crystal contacts (e.g. multiple contacts along two directions, and very few crystal forming contacts with plenty of solvent in the third direction). HTH, Fred. > Message du 09/06/10 14:33 > De : "Marie Lacroix" > A : CCP4BB@JISCMAIL.AC.UK > Copie à : > Objet : [ccp4bb] Anisotropic data and an extremely long c axis > > > > > > Hi, > > I also have a question concerning anisotropic data. Collected a data set and > the best crystal gave highly anisotropic diffraction patterns ( 3.7 A - 5.8 > A). So my first question is how to handle these data. I got only experience > with "normal data" using the ccp4 suite. Are there any program specially for > these kind of data? There are? > The second question is how anisotropic data occur? The protein I work with > has a tetragonal sg with a=b= 86.0 and an extremely long c axis of 651 A. > Secondary Structure prediction suggest a lot of beta strands. How can I > explain the anisotropy (for my own interest and my thesis)? > Thank you very much. > > Marie > > >
