Stability of YREE complexes with the trihydroxamate siderophore desferrioxamine B at seawater ionic strength

Organic complexation of yttrium and the rare earth elements (YREEs), although generally believed to be important, is an understudied aspect of YREE solution speciation in the open ocean. We report the first series of stability constants for complexes of YREEs (except Ce and Pm) with the trihydroxamate siderophore desferrioxamine B (DFOB), representing a class of small organic ligands that have an extraordinary selectivity for Fe(III) and are found in surface seawater at low-picomolar concentrations. Constants were measured by potentiometric titration of DFOB (pH 3-10) in the presence of single YREEs, in simple media at seawater ionic strength (NaClO₄ or NaCl, I = 0.7 M). Under these circumstances, the terminal amine of DFOB does not deprotonate. The four acid dissociation constants of the siderophore were determined separately by potentiometric titration of DFOB alone.Values for the bidentate (log β₁), tetradentate (log β₂), and hexadentate (log β₃) complexes of La-Lu range from 4.88 to 6.53, 7.70 to 11.27, and 10.09 to 15.19, respectively, while Y falls between Gd and Tb in each case. Linear free-energy relations of the three stability constants with the first YREE hydrolysis constant, log , yield regression coefficients of >0.97. On the other hand, plots of the constants vs. the radius of the inner hydration sphere display an increasing deviation from linearity for the lightest REEs (La > Pr > Nd). This may signify steric constraints in DFOB folding around bulkier cations, a larger mismatch in coordination number, or a substantial degree of covalence in the YREE-hydroxamate bond.Complexes of the YREEs with DFOB are many orders of magnitude more stable than those with carbonate, the dominant inorganic YREE ligand in seawater. Speciation modeling with MINEQL indicates that, for an average seawater composition, the hexadentate complex could constitute as much as 28% of dissolved Lu at free DFOB concentrations as low as 10⁻¹³ M. Such conditions might occur when DFOB or other siderophores are present in excess over metals for which they have high affinity, like Fe(III) and Co(III), for example during plankton blooms. Even if it turns out that trihydroxamate siderophores are not the dominant organic YREE ligand in seawater, our results establish a benchmark for producing effects on YREE solution speciation comparable to that of DFOB: the free concentration of any weaker organic ligand L must exceed that of DFOB by a factor β₃/_Lβ₁, assuming its first order complex is formed in greatest abundance.