In cases like this I use the S atoms to calibrate the peak height.
Of course it isnt definitive a) it is near the noise level, and b) peak
height is very dependent on B factor..
But the ratio might distinguish between an atom with an f" of 1.3 or f"=2.8
Eleanor
David Briggs wrote:
Dear all.
I have recently solved a structure in-house, 2.8A, CuKa.
I have a metal ion bound very obvious hepta-valent co-ordination,
which would suggest either Ca or Mn.
Neither was present in the crystallisation setup, but there was some
Mg around, which has contaminants of both Ca & Mn.
At 2.8A, I don't really think I can reliably discriminate between
2.15A & 2.36A distances to coordinating atoms
(http://tanna.bch.ed.ac.uk/newtargs_06.html
<http://tanna.bch.ed.ac.uk/newtargs_06.html>).
The B factors for refined Ca are 18, and Mn 30. The B-factors of
coordinating atoms vary from... 18 > 30 - so no help there.
I have a nice clear 6sigma anomalous difference peak, but then,
according to http://skuld.bmsc.washington.edu/scatter/ both Ca (f"
~1.3) and Mn (f" ~2.8) scatter anomalously at that wavelength.
The obvious solution is go to a synchrotron and scan around the Mn
edge and see what happens, however, whilst waiting for beam time, is
there any way I could... oh I don't know, use the peak in my anomalous
difference Fourier to figure out what anomalous signal would be
required to generate a peak of that size - a sort of back-transform???
Is this do-able, and if so, how would one go about it?
Cheers,
Dave
--
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David Briggs, PhD.
Father & Crystallographer
www.dbriggs.talktalk.net <http://www.dbriggs.talktalk.net>
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