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The argument that the absolute change of the unit cell dimensions is indicative of non-isomorphism depends on the assumption that all the content of the unit cell behaves as a rigid body. Sometimes this may happen, but most of the time the unit cell contains multiple rigid bodies that get rearranged during the unit cell change. If it were pure contraction/expansion, the non-isomorphism would be much smaller than based on the logic stated in the previous post. In practice, the rearrangement of the unit cell will involve rotations and other (non-expansion) translations of the rigid bodies within. The magnitude of these changes has only some correlation with the unit cell dimensional change. For this reason, the argument about non-isomorphism being correlated with relative change in the unit cell and not just the absolute change, is not merit-less, but these correlations need to be investigated. In particular, crystal packing that builds a unit cell with a long unit-cell parameter resulting from the formation of a multi-step helix structure, is a special case in non-isomorphism considerations. In such a case, non-isomorphism comes from small changes occurring at each step of the helix. If such a change is a pure translation along the unit cell axis, the non-isomorphism will be related to the size of the step rather than the long unit cell dimension change. For these reasons, in the case presented here, 2A change may represent quite a range of possibilities in terms of non-isomorphism. Merging-based assessment is really the only way to know if the amount of non-isomorphism is acceptable, and the unit cell dimensional change is a rather poor proxy to make this decision. Zbyszek Otwinowski > Actually, the thing that matters for non-isomorphism is the absolute value > of the change in cell dimension compared to the resolution of the data. A > change of 2A is half dmin in this case, so there will be a significant > impact on isomorphism in the highest resolution shells (which should also > be seen in statistics from aimless). > > To give credit appropriately, I first heard the argument for the absolute > change being important in a lecture from Kevin Cowtan, where he was > turning it around and saying that the size of cell dimension change > determines whether you will get any phase information from multi-crystal > averaging (where you *don't* want the crystals to be isomorphous). > However, I think the same point was made in some of the earlier literature > on MIR, before people (certainly not just Harry — probably more like the > majority!) got stuck on the idea that the percentage change was the way to > look at it. > > Best wishes, > > Randy Read > >> On 30 Sep 2015, at 10:49, Harry Powell <[log in to unmask]> wrote: >> >> Hi >> >>>> >>>> I have a few datasets from multiple crystals with unit cells of about >>>> 440, 440, 80 A (90, 90, 120 degrees; point group P 622). While the >>>> best 3 clusters together well in BLEND, there is one that does not as >>>> its a and b axes are 2 A longer. For the purpose of merging them, can >>>> they be considered isomorphic to each other? >> >> Possibly - 0.5% difference in cell edges is not *too* much. I’d try it >> and see… >> >> Harry >> -- >> Dr Harry Powell, MRC Laboratory of Molecular Biology, Francis Crick >> Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH >> Chairman of International Union of Crystallography Commission on >> Crystallographic Computing >> Chairman of European Crystallographic Association SIG9 (Crystallographic >> Computing) >> >>>> On 30 Sep 2015, at 08:59, Mohamed Noor <[log in to unmask]> >>>> wrote: >>>> >>>> Dear all >>>> >>>> I have a few datasets from multiple crystals with unit cells of about >>>> 440, 440, 80 A (90, 90, 120 degrees; point group P 622). While the >>>> best 3 clusters together well in BLEND, there is one that does not as >>>> its a and b axes are 2 A longer. For the purpose of merging them, can >>>> they be considered isomorphic to each other? >>>> >>>> There is an extensive radiation damage (initial resolution from the >>>> first 20 degrees is about 3.8 A), so I am hoping to get a good >>>> complete dataset. >>>> >>>> Thanks. >>>> Mohamed > > ------ > 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: [log in to unmask] > Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk > Zbyszek Otwinowski UT Southwestern Medical Center at Dallas 5323 Harry Hines Blvd. Dallas, TX 75390-8816 Tel. 214-645-6385 Fax. 214-645-6353