Fig. 6.

(Bottom) Carboxylate asymmetric stretching frequencies and oxygen ion coordination radii derived from DFT calculations of EDTA bound to a series of metal ions. Coordination radii increase, and carboxylate asymmetric stretching frequencies decrease, as a function of ionic radius. A comparison between experimental and calculated frequencies is provided in Fig. 7. (Top) The six-coordinate ionic radii of the species used are included for comparison. Ca²⁺ in the CaM binding site is seven-coordinate, but we use six-coordinate radii for comparison because seven-coordinate ionic radii have not been accurately measured for some of the ions used in this study.