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
Dave,f" = 1.3 versus 2.8 sounds like quite a difference ... what is the anom peak height of the sulfurs in your structure? The f" of sulfur at Cu Ka wavelength is 0.55 . So I'd expect the ratio of peakheights of your unknown metal divided by the average peakheight of sulfurs (of roughly 18-30 A**2 B-factor) to give you an idea of what you have. Of course this is no proof ...
Are there any other anom scatterers in your structure? best, Kay -- Kay Diederichs http://strucbio.biologie.uni-konstanz.de email: [EMAIL PROTECTED] Tel +49 7531 88 4049 Fax 3183 Fachbereich Biologie, Universität Konstanz, Box M647, D-78457 Konstanz
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