For both DFO and 4HMS, we study the case of deprotonated molecule where all hydrogens bound to the oxygen atoms of the hydroxamic acid are removed. From experimental acidity constants, we know that DFO in water at neutral pH has completely protonated hydroxamic groups and the NH + 3 terminal is also protonated. Highly charged Zr 4+ has a high affinity for hard Lewis bases and complexes stepwise with DFO under deprotonation of the hydroxyl of the hydroxamate group already starting at pH 2. The DFO molecule, with a positively charged ammonium group, has thus a total charge of -2, whereas the deprotonated 4HMS molecule has a total charge of -4, as it has no -NH 2 terminus.