Dear James, let's spin the thought a bit further. What if there is a new program (Phaser-II) some day, for which the coordinates in your "new crystal form" are all of a sudden within the radius of convergence again? Does this bring your "new crystal form" back to the old crystal form again?
I'd say it is neither cell dimensions nor space group that define a new crystal form. It is the packing of the molecules. It happens quite often in dehydration experiments that molecules in a lattice move a bit and symmetry elements are lost or new symmetry elements are created. The space group changes, as well as the unit cell dimensions. But the packing remains essentially the same. You can usually infer what happens, but you still need molecular replacement to actually solve the "new" form. Best, Manfred Am 09.12.2020 um 01:56 schrieb James Holton:
I have a semantics question, and I know how much this forum loves discussing semantics. We've all experienced non-isomorphism, where two crystals, perhaps even grown from the same drop, yield different data. Different enough so that merging them makes your overall data quality worse. I'd say that is a fairly reasonable definition of non-isomorphism? Most of the time unit cell changes are telling, but in the end it is the I/sigma and resolution limit that we care about the most. Now, of course, even for non-isomorphous data sets you can usually "solve" the non-isomorphous data without actually doing molecular replacement. All you usually need to do is run pointless using the PDB file from the first crystal as a reference, and it will re-index the data to match the model. Then you just do a few cycles of rigid body and you're off and running. A nice side-effect of this is that all your PDB files will line up when you load them into coot. No worries about indexing ambiguities, space group assignment, or origin choice. Phaser is a great program, but you don't have to run it on everything. My question is: what about when you DO have to run Phaser to solve that other crystal from the same drop? What if the space group is the same, the unit cell is kinda-sorta the same, but the coordinates have moved enough so as to be outside the radius of convergence of rigid-body refinement? Does that qualify as a different "crystal form" or different "crystal habit"? Or is it the same form, and just really non-isomorphous? Opinions? -James Holton MAD Scientist ######################################################################## To unsubscribe from the CCP4BB list, click the following link: https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 This message was issued to members of www.jiscmail.ac.uk/CCP4BB, a mailing list hosted by www.jiscmail.ac.uk, terms & conditions are available at https://www.jiscmail.ac.uk/policyandsecurity/
-- Dr. Manfred S. Weiss Macromolecular Crystallography Helmholtz-Zentrum Berlin Albert-Einstein-Str. 15 D-12489 Berlin Germany ________________________________ Helmholtz-Zentrum Berlin für Materialien und Energie GmbH Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V. Aufsichtsrat: Vorsitzender Dr. Volkmar Dietz, stv. Vorsitzende Dr. Jutta Koch-Unterseher Geschäftsführung: Prof. Dr. Bernd Rech (Sprecher), Prof. Dr. Jan Lüning, Thomas Frederking Sitz Berlin, AG Charlottenburg, 89 HRB 5583 Postadresse: Hahn-Meitner-Platz 1 D-14109 Berlin ######################################################################## To unsubscribe from the CCP4BB list, click the following link: https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 This message was issued to members of www.jiscmail.ac.uk/CCP4BB, a mailing list hosted by www.jiscmail.ac.uk, terms & conditions are available at https://www.jiscmail.ac.uk/policyandsecurity/
