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On Thursday, 19 January, 2017 20:35:14 you wrote: > A PhD student asked me what causes diffraction anisotropy. Quoting from the Diffraction Anisotropy Server webpage that it is caused by whole-body anisotropic vibration of unit cells. He asked whether a colder cyrostream could improve anisotropy. My answer would be yes, as colder temperatures would lower the vibrations. > > My two questions are; (1) am I right? and (2) if so, has it ever been done before in practice? I do not know if there is past work and literature that answers your question with specific regard to whole-body anisotropic vibration of unit cells. However with regard to anisotropy in general you must consider two components. (1) Vibration that is still present in the crystal, so that atoms or larger groups are moving while the diffraction is measured. The vibrational amplitude will be temperature dependent, but the anisotropy may remain the same since it depends on the ratio of vibration amplitude in different directions rather than the magnitude in any one direction. (2) Vibrational displacement of a group in one unit cell relative to copies of the same group in other unit cells that was "locked in" when the crystal was frozen. The frozen crystal captures a sampling of states that were present at room temperature. The diffraction experiment sees a positional average over space that is equivalent to a single-copy average over time. This component is not temperature dependent so long as the crystal stays frozen. Diffraction measurements at a single temperature do not distinguish between these two components. In principle a series of diffraction measurements from the same crystal at different temperatures would allow partitioning the observed vibrational into the two components. [Burgi (2000) Rev. Phys. Chem. 51:275] So far as I know this has been confirmed for small molecule crystals but is too difficult experimentally to be worth the trouble for protein crystals (and I've tried :-) This equivalence of states sampled from a single copy over time to multiple copies in a frozen crystal is the basis for TLSMD analysis. In the special case of a single molecule per unit cell I suppose a one-group TLS treatment reduces to what you originally asked about - vibration of whole unit cells - but in general it does not. cheers, Ethan -- Ethan A Merritt Biomolecular Structure Center, K-428 Health Sciences Bldg MS 357742, University of Washington, Seattle 98195-7742