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

Conditional stability constants and the rates of formation and dissociation for Fe(3+) 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 land 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: beta-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 K(Fe3+L)) for the nine ligands ranged from log K(Fe3+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, k(f), for inorganic Fe' complexation by these Fe-binding ligands varied by a factor of 21 and ranged from 0.93 x 10(5) M(-1)s(-1) (apoferritin) to 19.6 x 10(5) (desferrioxamine). Dissociation rate constants, k(d), of the model FeL complexes varied by a factor of 316 and ranged from 0.05 x 10(-6) s(-1) (ferrichrome) to 15.8 x 10(-6) s(-1) (enterobactin). Kinetic measurements showed log K(Fe3+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 Fe(3+) in seawater, and its variation for model ligands appears to correlate with changes in ligand structure. (C) 2000 Elsevier Science B.V. All rights reserved.