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

• Published in: Marine chemistry Vol. 69; no. 1; pp. 1 - 17
• Main Authors: Witter, Amy E., Hutchins, David A., Butler, Alison, Luther, George W.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-03-2000; Elsevier Science
• Subjects: conditional stability constant; Earth sciences; Earth, ocean, space; Exact sciences and technology; External geophysics; Fe-binding ligands; Geochemistry; kinetic constants; Marine; Mineralogy; Physical and chemical properties of sea water; Physics of the oceans; seawater; Silicates; Water geochemistry; Fe-binding ligands; conditional stability constant; kinetic constants; seawater; sea water; ligands; geochemical cycle; world ocean; ferric iron; complexing; speciation; biogenic origin; prokaryotes; hydrochemistry; dissociation; iron; dissolved materials; biogenic effects; residence time; kinetics; stability
• ISSN: 0304-4203; 1872-7581
• DOI: 10.1016/S0304-4203(99)00087-0

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 (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
K
Fe
3+L
) for the nine ligands ranged from log
K
Fe
3+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 0.93×10
5 M
−1 s
−1 (apoferritin) to 19.6×10
5 (desferrioxamine). Dissociation rate constants,
k
d, 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
K
Fe
3+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.