Hi Paul, I'm not sure that I'd expect a "zinc-water" ligand in a bio macro-molecule to act like zinc in an aqueous solution. The zinc will really be bound to that one "water" in the molecule, which is pretty different than a zinc ion in solution, where exchange of water will happen very frequently (although perhaps not so much at 20K, the low temperature might just emphasize the point that an aqueous solution isn't that good a model). Given that the "zinc-water" bond is somewhat different than a "zinc-water" in solution, and that zinc also binds to cysteine, I wouldn't be surprised by a very different sigma2. For example, static disorder (from bond-to-bond variations in bond length) is probably very small for these molecules. It would be interesting to find out how transferable metal-"water" bond lengths and sigma2 are from solutions to bio-molecules. I'd expect that a literature search would be enough to tell you. --Matt