On Wednesday 13 May 2009 09:30:06 Jacob Keller wrote: > > The reason is that you've missed out one important term: the atomic > > displacement parameters (B-factors), which describe a combination of > > thermal motion and positional disorder between unit cells. > > A somewhat niggling point: isn't it true that the thermal motion is > insignificant at 100K?
No. True thermal motion doesn't bottom out until 0 Kelvin. But that is kind of irrelevant, since "motion" in the sense of "things moving in the crystal while we measured the data" is only one contribution to the overall ADP (B factor). > Does anybody know of a paper which systematically > measures B-factors as a function of temperature? The asymptote of the > resulting curve would represent all of the non-thermal elements, right? The theory for this is well laid out in Bürgi, H.B., and Förtsch, M. (1999). Dynamic processes and disorder in crystal structures as seen by temperature-dependent diffraction experiments. J. Molecular Structure 486, 457-463. But to the best of my knowledge a full analysis based on temperature-dependent diffraction experiments has never been done for a protein structure. I had a preliminary go at it some years back, but collecting comparable data sets over a range of temperatures spanning liquid He to room temperature is technically challenging. The analysis is also non-trivial. -- Ethan A Merritt Biomolecular Structure Center University of Washington, Seattle 98195-7742
