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Dear Jacob, for NH4+, you would expect a (partial) tetrahedral coordination with typical H-bond distances of ~2.9 A. For Na+, you would expect a (partial) octahedral coordination with Metal-to-ligand distances of ~2.4 A (see Harding, Acta Cryst., D62, 678-682 (2006); Harding, Acta Cryst., D58, 872-874 (2002); Glusker, Advances in Protein Chemistry, 42, 1-76 (1991)). But depending on your data resolution and quality, and on the completeness of the coordination sphere, it might be difficult to distinguish between them. Best regards, Dirk. Am 16.11.11 19:20, schrieb Jacob Keller: > Dear Crystallographers, > > I have crystals containing 666mM NH4 and 540mM Na, and there appears > to be a "water" which is only about 2.2 Ang from some polar atoms. It > is currently reasonably happy as a Na, but is there any reasonable way > to decide which cation is there? > > JPK > -- ******************************************************* Dirk Kostrewa Gene Center Munich, A5.07 Department of Biochemistry Ludwig-Maximilians-Universität München Feodor-Lynen-Str. 25 D-81377 Munich Germany Phone: +49-89-2180-76845 Fax: +49-89-2180-76999 E-mail: [log in to unmask] WWW: www.genzentrum.lmu.de *******************************************************