Binding to Isolated Carboxylate Groups

Search the datafile for crystal structures containing metal ions and carboxyl groups. Select data for a metal ion of interest. 

Eliminate entries with more than one interaction with the metal ion or other undesirable features. 

Lay each carboxyl group in a constant defined orientation and mark where the metal ion lies. This gives a scatterplot. 

The contoured scatterplots showed that the most likely arrangements of metal cations are those designated syn, anti, and direct. The percentages of directionalities of metal liganding for a total of 1558 metal-carboxylate interactions are 62.9% syn, 22.7% anti, and 14.4% direct. Minimum metal ion-ligand distances are given in Table IX(a). syn is generally preferred, except when the distances are short 

The conclusions from this analysis of carboxylate—metal binding in small structures studied to high resolution are (1) The metal cation generally lies in the plane of the carboxyl group. There are some notable exceptions. Alkali metals and some alkaline earth metals, which ionize readily and form strong bases, appear to have less specific locations of binding. (2) If the distance lies in the range 2.3-2.6 A, it is 

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