Dear All, We have a nice round blob of density that looks to be located between four sp3 oxygen atoms at long hydrogen-bond distances (>3.0 Angs and up to 3.2 Angs) and two axial -NH from TRPs. If this blob is set to a NH4+ it refines to similar B-factors to the surrounding atoms and the same happens if we model a half-occupied Na+ cation. However, we cannot explain the hydrogen-bond pattern has its unexpected to have hydrogen donors as the -NH groups coordinating a positive species.
We have this blob in different crystal structures where crystallization conditions are slightly different (some were exposed to NaOH others to NH4+ but not Na+). We do not see what else could explain this density. Maybe a disordered water molecule? Comments & suggestions welcome, Regards, David
