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Hi Zhen,
Does your crystal diffract to better than 1.7A, in other words is I/SigI
still considerably larger than 2 in your highest resolution shell. If so
then you are most likely looking at Fourier truncation effects.
Basically, many Fourier terms try to come together to build a peak at
your selenium positions, but since the Fourier terms are cosine waves,
when many cosine maxima come together many cosine minima will be nearby
causing an artificial dip in the electron density, especially near
strong scatterers like your selenium. It's not a real problem and to
solve it the best thing is to collect a proper native data set that
includes all the weak high resolution data.
Bart
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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
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Bart Hazes (Assistant Professor)
Dept. of Medical Microbiology & Immunology
University of Alberta
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Canada, T6G 2H7
phone: 1-780-492-0042
fax: 1-780-492-7521
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