(a) Molecular structure for HOPO. (b) Computed DFT structure of Bk(IV)-HOPO and (c) complex structure of Es(III)-HOPO. The octadentate chelator of HOPO typically compose four 1-hydroxy-pyridin-2-one metal binding unites attached to a spermine scaffold (marked in blue in panel c). Eight metal-binding oxygen atoms (red) fulfill the first coordination sphere around metal center on ligand deprotonation and metal complexation. Panel (b) was adapted from Ref [125] with permission.