Dear Yafang,
If radiation damage is not a major problem, MAD should give you more
phase information than SAD, i.e. better maps, especially
at low resolution. If SAD works but MAD doesn't, there are several
possible explanations:
1. (most likely) your datasets are inconsistently indexed. This can
hapen in various ways depending on the Laue group and the unit-cell. If
you are using hkl2map the plots of the anomalous CC between the
different datasets are a good quick check. Some programs (e.g. the
current shelxc) will try to detect this and correct it automatically.
2. You have mixed up the wavelengths or labels of the datasets and so
the dispersive difference comes out with the wrong sign.
3. You have significant radiation damage and the RIP and dispersive
differences have opposite signs. Always measure the inflection
dataset last so that they reinforce each other rather than canceling.
4. You have severe radiation damage and only the first (peak) dataset is
usable.
Best wishes, George
On 08/20/2013 11:05 PM, Yafang Chen wrote:
Hi All,
I have three datasets of SeMet-incorporated protein at peak, infl and
high wavelength respectively. SAD with peak dataset works well to
solve the phase problem. However, MAD with all three datasets didn't
work at all. The completeness of all three datasets are more than 99%.
So I think radiation damage should not be a problem. Does anyone have
any idea about the possible reasons that MAD didn't work in this case?
Thank you so much for any of your help!
Best,
Yafang
--
Yafang Chen
Graduate Research Assistant
Mesecar Lab
Department of Biological Sciences
Purdue University
Hockmeyer Hall of Structural Biology
240 S. Martin Jischke Drive
West Lafayette, IN 47907
--
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-33021 or -33068
Fax. +49-551-39-22582
