Different binding modes of Cu and Pb vs. Cd, Ni, and Zn with the trihydroxamate siderophore desferrioxamine B at seawater ionic strength

Stabilities of the hexadentate Ni-DFOB, Zn-DFOB, and the tetradentate Pb-DFOB complex are nearly equal, yet about 2 orders of magnitude higher and 4 orders of magnitude lower than those of the hexadentate Cd-DFOB and tetradentate Cu-DFOB complex, respectively. Linear free-energy relations defined by the rare earth elements are able to predict stabilities of the Cd, Zn, and one of the Pb complexes, but underestimate those of the Ni and Cu complexes. The comparison with metal-specific organic ligands detected in seawater yields fair agreement for three of the five metals, implying that they could be siderophore-like. The Cd- and Ni-specific ligands are much stronger and may contain quite different functional groups. Calculations with MINEQL incorporating our new stability constants indicate that very high DFOB concentrations would be required to match the extent of metal-organic complexation observed in seawater, however DFOB may well represent a much broader class of structurally related ligands.