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Updated: Aug 6, 2026
07:19
Structural Studies of Macromolecules in Solution using Small Angle X-Ray Scattering
Published on: November 5, 2018
Calcium complexation in aqueous solution: An x-ray and ab initio approach
Xiao You1,2, Amity Andersen3, Macon Abernathy1
1Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
The Journal of Chemical Physics
|July 23, 2026
Summary
Calcium coordination in water involves seven water molecules. Even with ethylenediaminetetraacetic acid (EDTA), calcium maintains sevenfold coordination, revealing its tendency for low-symmetry complexes in aqueous solutions.
Area of Science:
- Inorganic Chemistry
- Aqueous Solution Chemistry
- Spectroscopy
Background:
- Understanding cation aqueous chemistry is vital for ion transport and reactivity.
- Calcium's role in biological and environmental systems is critical, yet its coordination is debated.
- Limited studies exist on calcium ion (Ca2+) coordination with ligands other than water.
Purpose of the Study:
- Investigate the coordination environment of aqueous Ca2+.
- Determine the number of water molecules in Ca2+'s first solvation shell.
- Examine Ca2+ coordination in ethylenediaminetetraacetic acid (EDTA) complexes.
Main Methods:
- Ca K-edge X-ray Absorption Near-Edge Structure (XANES) spectroscopy.
- Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy.
- Ab initio molecular dynamics and time-dependent density functional theory.
Main Results:
- Confirmed an average of seven water molecules coordinating Ca2+ in dilute aqueous solution.
- Determined that Ca2+ in EDTA complexes is seven-coordinate, including one water molecule and four oxygen and two nitrogen atoms from EDTA.
- Observed that Ca2+ favors low-symmetry, seven-coordinate complexes even with strong chelators.
Conclusions:
- Ca2+ exhibits a preference for sevenfold coordination in aqueous solutions.
- This coordination preference persists even in the presence of hexadentate chelators like EDTA.
- The findings provide foundational knowledge for understanding Ca coordination in diverse systems.
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