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Dear Zhen:
If you are anywhere near the inflection point, there will be a significant
f' contribution, which is negative. This means your Se sites will appear
to have lower occupancy or fewer scattering electrons than you (and the
refinement libraries) think they should have. The imaginary component
(f") corresponds to absorption, and the real component (f', which always
has an absolute negative value) corresponds to the
effect you are seeing (which by virtue of the Kramers-Kronig transform is
really the same thing physically, just like ORD and CD).
I rediscovered the obvious when I asked the same question regarding Mn++ a
few weeks ago. I teach the formalism, but sometimes it is too easy to
become disconnected from the practical reality...
All the best,
Bill
On Tue, 26 Sep 2006 [EMAIL PROTECTED] wrote:
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Deal all,
I recently solved a structure at 1.7A by MAD. However, there is
always strong negative density arround all 8 selenium in the
protein during CNS refinement. The Selenium-methionines fit 2fo-fc
density well. I tried to use peak, edge or remote dataset with or
without scale anomalous flag on and all did not matter. Does
anybody have a solution for this matter?
Thanks.
Zhen
