Determination of conditional stability constants and kinetic constants for strong model Fe-binding ligands in seawater

Abstract

Abstract Conditional stability constants and the rates of formation and dissociation for Fe3+ complexation with nine model ligands were measured in chelexed, photo-oxidized seawater. The ligands were chosen to represent Fe-binding organic functional groups that are present in seawater as a result of siderophore production by marine prokaryotes, or as a result of release during cell lysis or grazing. Four Fe-chelating moieties were studied including: tetrapyrrole ligands (i.e., phaeophytin and protoporphyrin IX (and its dimethyl ester); a terrestrial catecholate siderophore (i.e., enterobactin); terrestrial hydroxamate siderophores (i.e., ferrichrome and desferrioxamine) and marine siderophores containing a mixed functional moiety: β-hydroxyaspartate/catecholate (i.e., Alterobactin A) and the bis-catecholate siderophore (i.e., Alterobactin B). Also considered were the Fe storage protein apoferritin, and the Fe-complexing ligand inositol hexaphosphate (phytic acid). The competitive ligand 1-nitroso-2-naphthol (1N2N) was used with cathodic stripping voltammetry (CLE-CSV) to determine conditional stability constants for these FeL complexes. Conditional stability constants (log KFe3+L) for the nine ligands ranged from log KFe3+L=21.6 to greater than 24.0, remarkably close to the values that have been reported for natural ligands in seawater. Formation rate constants, kf, for inorganic Fe′ complexation by these Fe-binding ligands varied by a factor of 21 and ranged from 0.93×105 M−1 s−1 (apoferritin) to 19.6×105 (desferrioxamine). Dissociation rate constants, kd, of the model FeL complexes varied by a factor of 316 and ranged from 0.05×10−6 s−1 (ferrichrome) to 15.8×10−6 s−1 (enterobactin). Kinetic measurements showed log KFe3+L values ranging between 20.8 and 22.9. Results suggest that the CLE-CSV method cannot distinguish between different organic moieties that may be present in seawater, because the measured conditional stability constants do not vary in a systematic manner with Fe-binding ligand structure. The dissociation rate constant does provide structural information on the organic compounds binding Fe3+ in seawater, and its variation for model ligands appears to correlate with changes in ligand structure.