Ae. Witter et al., Determination of conditional stability constants and kinetic constants forstrong model Fe-binding ligands in seawater, MAR CHEM, 69(1-2), 2000, pp. 1-17
Conditional stability constants and the rates of formation and dissociation
for Fe3+ complexation with nine model ligands were measured in chelexed, p
hoto-oxidized seawater. The ligands were chosen to represent Fe-binding org
anic functional groups that are present in seawater as a result of sideroph
ore production by marine prokaryotes, or as a result of release during cell
lysis or grazing. Four Fe-chelating moieties were studied including: tetra
pyrrole ligands (i.e., phaeophytin and protoporphyrin IX land its dimethyl
ester); a terrestrial catecholate siderophore (i.e., enterobactin); terrest
rial hydroxamate siderophores (i.e., ferrichrome and desferrioxamine) and m
arine siderophores containing a mixed functional moiety: beta-hydroxyaspart
ate/catecholate (i.e., Alterobactin A) and the bis-catecholate siderophore
(i.e., Alterobactin B). Also considered were the Fe storage protein apoferr
itin, and the Fe-complexing ligand inositol hexaphosphate (phytic acid). Th
e competitive ligand 1-nitroso-2-naphthol (1N2N) was used with cathodic str
ipping voltammetry (CLE-CSV) to determine conditional stability constants f
or these FeL complexes. Conditional stability constants (log KFe3+L) for th
e nine ligands ranged from log KFe3+L= 21.6 to greater than 24.0, remarkabl
y close to the values that have been reported for natural ligands in seawat
er. 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 rat
e 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) (ente
robactin). Kinetic measurements showed log KFe3+L values ranging between 20
.8 and 22.9. Results suggest that the CLE-CSV method cannot distinguish bet
ween different organic moieties that may be present in seawater, because th
e measured conditional stability constants do not vary in a systematic mann
er with Fe-binding ligand structure. The dissociation rate constant does pr
ovide structural information on the organic compounds binding Fe3+ in seawa
ter, and its variation for model ligands appears to correlate with changes
in ligand structure. (C) 2000 Elsevier Science B.V. All rights reserved.