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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