Hi, Consulting with people around the table at the Phenix workshop, we have heard rules of thumb like the cell dimensions should differ by less than 0.5-1%. (I think there's some discussion of this in Blundell & Johnson, but I don't have my copy handy.) If the SAD data come from a crystal without a lot of extra scattering, then you could consider the crystals to be reasonably isomorphous if, say, the R-factor between the data sets is less than 15%. In any case, differences of 2% in a and b probably mean that these crystals are not sufficiently isomorphous to use them for SIRAS phasing. So what you are probably looking at is either using the SAD data alone (if the resolution is similar to the native data) or doing cross-crystal averaging. I would start by getting the MR solution for the SAD data, either by rigid body refinement of the native solution or starting from scratch. Then use the MR+SAD option in AutoSol to find the anomalous scatterers. The cross-crystal averaging operator can be obtained by superimposing one MR solution on the other. The MR model will give you a possible starting point for an averaging mask, but if it's very incomplete then you'll have to use the MR+SAD density to define the full mask. Good luck! Randy Read On 7 Mar 2010, at 13:14, Manoj saxena wrote:
Hi, I have partial solution from MR on a crystal with dimensions 52.729 60.364 142.908 90.00 90.00 90.00 P 21 21 21. I have SAD data from another crystal with dimension 53.860 61.470 142.492 90.00 90.00 90.00 P 21 21 21.
My question is can I treat both as isomorphous and combine data from both of them and use my MR as model with Autosol. What is the maximum difference in unit cell dimensions allowed to consider two unit cell isomorphous?
If you can point to some references/examples that will be highly appreciated. Thanks a lot.
Marusa
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------ Randy J. Read Department of Haematology, University of Cambridge Cambridge Institute for Medical Research Tel: + 44 1223 336500 Wellcome Trust/MRC Building Fax: + 44 1223 336827 Hills Road E-mail: [email protected] Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk