Another option would be to collect data at the Mn K edge (1.89A) - at this wavelength Mg has a weak anomalous signal that you should still be able to detect. I've used Mn K edge to successfully distinguish between Mg, Mn and Ca ions as well as identify them as ions and not waters by looking at peaks in the anomalous maps since peak intensity relates to anomalous signal arising from different ions at different wavelengths.
For details, see: http://scripts.iucr.org/cgi-bin/paper?S0907444904026800 http://www.cell.com/structure/retrieve/pii/S0969212604000243 On 21 December 2010 15:04, John Bruning <jbrun...@gmail.com> wrote: > Hi, > > I'm not sure if it has been mentioned yet, but you could take an > experimental approach if you still have crystals. You could soak them with > EDTA/EGTA and if the density disappears that is good evidence it was Mg (or > other divalent cation), if not then putatively water. You could also soak > with Mn (assuming the Mn will bind the Mg site) and look for anomolous > signal. > > > John > >
