So what is the approximate percent contribution of the
*temperature-dependent* b-factor at 100K, for an average crystal, or how to
determine such? In other words, if I have a crystal with an avg B of 20,
when I go from 100K to 0K, how much lower will it drop? I recall seeing
papers exploring liquid helium temperatures, which I believe concluded that
there was not much gain in lowering the temp, implying that the B's did not
go down much after 100K.
I had thought that the reason for calling it a temperature factor was more
because it represented the many states of the atoms caught *in flagrante
vibratio* by the liquid nitrogen plunge upon freezing the crystal, but not
actual motions in the crystal. Room temperature is of course different.
Jacob
*******************************************
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: j-kell...@northwestern.edu
*******************************************
----- Original Message -----
From: "Ethan Merritt" <merr...@u.washington.edu>
To: <CCP4BB@JISCMAIL.AC.UK>
Sent: Wednesday, May 13, 2009 12:12 PM
Subject: Re: [ccp4bb] phasing with se-met at low resolution
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
