In a structure I'm refining, there are a couple of oblong blobs associated with carboxylates. (screenshots at http://sb20.lbl.gov/berry/ccp4/azide/) If I modeled with two waters, they refine too close together for normal H-bond, 2.3 to 2.5 A; and their density is connected.
I considered one water with alternate locations, but the distal position wouldn't make much sense if the proximal water wasn't there. The density is the right size for azide, which was present in the medium, but I expect a chemist would find it unreasonable to have anionic azide (pKa of hydrazoic acid ~4.6) associating with a carboxylate. Would that make sense? or does anyone have other suggestions? (resolution is 2.2A, contour 0.25 e/A^3 or about 1.3 sigma) Thanks, Ed
