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Re: [ccp4bb] indexing multiple lattices |
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CCP4bb navigationCCP4bb <-- 2008 <-- February 2008 <-- 18 February 2008Subject: Re: indexing multiple lattices From: Clemens Vonrhein vonrhein {- at -} GLOBALPHASING {- dot -} COM Date: 2008-02-18 > I have a crystal that grows with multiple lattices despite the fact > that the crystals look more or less like single crystals under the > microscope. > > With a lot of guesswork I am able to index with mosflm by carefully > selecting spots that appear to be in the same lattice. > Now I would like to use the known cell and symmetry to index more > datasets that have the same "twinning" problem. This is real fun to do - you can even index/integrate these several lattices: gives you higher multiplicity etc. But (there's always a but): be careful in using the data. It is very likely that a very large proportion of reflexions overlap - either completely or partially. There are ways of classifying these reflections, but a better way is to use a program that can process these twinned crystals (and no: neither XDS nor MOSFLM can ... at least not without hacking the source code or doing some other weird stuff). > Is there any way to do a search of crystal orientation matrices > with a known cell to find the best fit to the diffraction pattern? > The data were collected on the Pilatus6M detector so I am limited to > mosflm and XDS for processing. Both packages always trying to index > the crystal without using the known cell and symmetry to get the > orientation matrix. Not quite true: in both packages you can give the cell and symmetry. I'm not sure how it is handled internally exactly by those programs - but it seems to me that MOSFLM will use this information only after autoindexing to pick a solution closests to the user-given cell/symmetry. I _think_ XDS might use that info a bit earlier during indexing. But I could be wrong on both accounts. > I thought of writing a script to do a full search > (testing all orientations sequentially) in mosflm but I thought that > I should submit to the experts before I try anything. If you want some more detailed 'recipe' for processing non-merohedrally twinned crystals with XDS, please let me know directly. However, if you're planning to use that data for experimental phasing on a medium to small signal: don't get your hopes up. It might be enough for molecular replacement (not that MR wouldn't be helped with good data as well ...). Cheers Clemens -- *************************************************************** * Clemens Vonrhein, Ph.D. vonrhein AT GlobalPhasing DOT com * * Global Phasing Ltd. * Sheraton House, Castle Park * Cambridge CB3 0AX, UK *-------------------------------------------------------------- * BUSTER Development Group (http://www.globalphasing.com) *************************************************************** CCP4bb navigationCCP4bb <-- 2008 <-- February 2008 <-- 18 February 2008 |
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