>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...
