Dear Jacob, Ah yes, I see. Your wording is perfectly clear. Sorry for my misunderstanding. Best wishes, John
Prof John R Helliwell DSc On 7 Mar 2014, at 15:18, "Keller, Jacob" <kell...@janelia.hhmi.org> wrote: >> You indicate that oxygen anomalous scattering could be used; whilst this is >> applicable to chirality determination in small molecule organic >> crystallography the oxygen anomalous signal is very small and to my >> knowledge not used thus far in protein crystallography. > > Perhaps I should have been clearer--I meant that anomalous scattering could > be used to distinguish between Cl- and H20, since Cl- does have a small but > measurable anomalous signal at the usual wavelengths, whereas water does not, > as you point out. Parenthetically, I have found that, in line with Randy > Read's suggestion to me, the LLG maps in Phaser are dramatically better than > regular adf's for finding such small signals. > > JPK > > ================== > > I was curious whether there has been a rigorous evaluation of ion binding > sites in the structures in the pdb, by PDB-REDO or otherwise. I imagine that > there is a considerably broad spectrum of habits and rigor in assigning > solute blobs to ion X or water, and in fact it would be difficult in many > cases to determine which ion a given blob really is, but there should be at > least some fraction of ions/waters which can be shown from the x-ray data and > known geometry to be X and not Y. This could be by small anomalous signals > (Cl and H2O for example), geometric considerations, or something else. Maybe > this does not even matter in most cases, but it might be important in > others...
