AW: [ccp4bb] truncate ignoranceI have had a general crystallogrphy question for quite a while, upon which perhaps the CCP4BB members can shed some light: Are small shifts in cell parameters real?
I know that datasets at cryo vs. rt, for example, are often the same spacegroup, but have smaller cell dimensions by (several?) angstroms, with accompanying lower B-factors, so the reality of changes seems pretty plausible, but I find it difficult to imagine how all of the units cells composing a crystal can shrink in harmony, without cracking the crystal or otherwise. The *mechanism* of cell changes is hard to imagine, even in the case of two similar crystals, perhaps even from the same drop. Do all of the unit cells have to change at exactly the same time, making one macro-macromolecular motion? And that is only the case of translations, but what about the observed phenomenon of molecules rotating slightly in unit cells throughout a crystal? Must that also be concerted? What would the intermediate state look like? Is packing really not very specific? But if so, how is it that we get such nice diffraction spots, as it should be the case that some molecules should be in one orientation, others in another? Again, we fit cell parameters in indexing, but perhaps for crystals from exactly the same conditions, this parameter should be fixed at some best value, and other experimental parameters massaged to make up the difference? It seems to me that detector distance et al would be more likely to be difference between two crystals than that there should be two solutions to the crystal packing problem under identical conditions. Also, shouldn't unit cell parameters perhaps be refined during the model building process, or is the experimental error so small that it would make no difference? The bottom line questions are: 1. given that there really are cases of cell shifts, and that there are also probably experimental artifactual changes, how is one to decide what to do? 2. Can there be (or what is?) a plausible mechanism for these shifts? Jacob Keller ******************************************* Jacob Pearson Keller Northwestern University Medical Scientist Training Program Dallos Laboratory F. Searle 1-240 2240 Campus Drive Evanston IL 60208 lab: 847.491.2438 cel: 773.608.9185 email: [EMAIL PROTECTED] *******************************************
