The Zr 4+ aquo species has been experimentally determined in acidic aqueous solution by means of extended X-ray absorption fine structure (EXAFS) and shown to be coordinated with eight water molecules in the first coordination shell, with the bound oxygen atoms located at an average distance of ≃2.2 Å. Upon binding of the chelate, the number of water molecules changes depending on the number of hydroxamate groups that become coordinated with the ion. To understand the magnitude of this change, we conducted a 200ns MD simulation of an isolated Zr 4+ in bulk water. The number of water molecules coordinating the Zr 4+ was then compared to that obtained from the free energy minimum of the chelator complex. In Fig. , the coordination numbers of Zr 4+ , as a function of the distance of oxygen atoms from the ion, are shown. These plots show regions where there are sharp changes in the coordination followed by plateaus which essentially correspond to different solvation shells. In the case of Zr 4+ aquo species (yellow line), the number of coordinating water molecules grows to ∼8 between 2.0 Å to 2.5 Å, in agreement with experimental reports 52 (see a representative snapshot from simulations in the bottom left panel of Fig. ). As expected, these numbers change for Zr 4+ -DFO and Zr 4+ -4HMS simulations. In the case of the Zr 4+ -DFO complex, two oxygen atoms from the DFO hydroxamate groups and six oxygen atoms from water are coordinated to Zr 4+ in the range from 2.2 Å to 3 Å. Thus, the Zr 4+ remains almost fully solvated when complexed with DFO (see a representative snapshot from simulations bottom middle panel of Fig. ).