My apologies my second paragraph was badly wrong. What Ian Tickle said in 2008 (and to save him writing it all again ;-) is:
"In general crystallographic usage 'isomorphous' refers to the similarity of crystal structures, i.e. same arrangement of atoms in the a.u. and the same symmetry. So strictly speaking, isomorphous crystals may have different cell parameters, so that NaCl and KCl are isomorphous (both being cubic with the same atomic arrangment), but the cell parameter of KCl is significantly greater than that of NaCl because the K ion is bigger. In MX 'isomorphous' usually implies both that the crystal form and structure are very similar and that the cell parameters are equal. It's perfectly possible that two crystals of completely different structures in the same space group accidentally have the same cell parameters, but clearly they would not be isomorphous, since the primary criterion is that the structures are similar. So it's not true to say by either definition that crystals are isomorphous if they have the same cell and symmetry, nor is it true that under this definition of isomorphism chemical similarity is a pre-requisite." "Rather the primary criterion of isomorphism is that the components that the structures have in common are not just similar but essentially identical (so the structures may have additional components that are not in common), and equality of symmetry and lattice parameters follow from this." On Wednesday, 20 December 2023 at 22:21, Jon Cooper <0000488a26d62010-dmarc-requ...@jiscmail.ac.uk> wrote: > Unless you have a degree in maths, the IUCr's "Little Dictionary of > Crystallography" by A. Authier and G. Chapuis (2014) defies comprehension on > this matter (it's all to do with set / group theory, I think, and there are > many more morphisms covered in about 6 pages: homo, epi, mono, endo, auto). > > Having discussed this with Ian Tickle, about 10 or 12 years ago, the formal > (?) definition of isomorphous simply means that the unit cells of two or more > crystals are the same, but the structure/molecule/compound/mineral, etc, does > not even have to be the same. A sensible definition for dumb biologists might > be to say that A and B are isomorphous, but C isn't. > > Best wishes, Jon Cooper. jon.b.coo...@protonmail.com > > Sent from Proton Mail mobile > > -------- Original Message -------- > On 20 Dec 2023, 20:15, Hekstra, Doeke Romke < doeke_heks...@harvard.edu> > wrote: > >> Dear colleagues, >> >> Something to muse over during the holidays: >> >> Let’s say we have three crystal forms of the same protein, for example >> crystallized with different ligands. Crystal forms A and B have the same >> crystal packing, except that one unit cell dimension differs by, for >> example, 3%. Crystal form C has a different crystal packing arrangement >> altogether. What is the right nomenclature to describe the relationship >> between these crystal forms? >> >> If A and B are sufficiently different that their phases are essentially >> uncorrelated, what do we call them? Near-isomorphous? Non-isomorphous? >> >> Do we need a different term to distinguish them from C or do we call all >> three datasets non-isomorphous? >> >> Thanks for helping us resolve our semantic tangle. >> >> Happy holidays! >> >> Doeke >> >> ===== >> >> Doeke Hekstra >> >> Assistant Professor of Molecular & Cellular Biology, and of Applied Physics >> (SEAS), >> >> Director of Undergraduate Studies, Chemical and Physical Biology >> >> Center for Systems Biology, Harvard University >> >> 52 Oxford Street, NW311 >> >> Cambridge, MA 02138 >> >> Office: 617-496-4740 >> >> Admin: 617-495-5651 (Lin Song) >> >> --------------------------------------------------------------- >> >> To unsubscribe from the CCP4BB list, click the following link: >> https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 >> >> --------------------------------------------------------------- >> >> To unsubscribe from the CCP4BB list, click the following link: >> https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 ######################################################################## 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/
