Page 6 - Microsoft Word - Lund BioinorgChem 08.doc

6

Hexaaquairon(III) ions are cationic Brønstedt acids:
2+
3+
[Fe(H 2O) 6] + H 2O ' [Fe(H 2O) 5OH] + H 3O + pK S1 = 2.2
+
2+
[Fe(H 2O) 5(OH)] + H 2O ' [Fe(H 2O) 4(OH) 2] + H 3O + pK S2 = 3.5
+
[Fe(H 2O) 4(OH) 2] + H 2O ' [Fe(H 2O) 3(OH) 3] (= Fe(OH) 3·aq) + H 3O + pK S3 = 6.0

The formation of ferric hydroxide Fe(OH) 3·aq hence already starts in weakly acidic media. The
protolytic reactions are accompanied by condensation reactions, leading to hydroxido- and
oxido-bridged aggregates and finally to colloids and hardly soluble ferric oxide hydrates. The
5
colloids can be spheroids of molecular mass M ca. 1.5⋅10 Da and ca. 70 Å diameter, or
6
needles/rods (M = 1.9·10 , length up to 500 Å). The composition of these ironoxide hydrates
correspond to that of the minerals FeO(OH) (goethite) and 5Fe 2O 3·9H 2O (ferrihydrite).

OH 2 H OH 2 4+ OH 2 OH 2 2+
-H 2O, -2H + H 2O O OH 2 -2H + H 2O O OH 2
2[Fe(H 2O) 6] 3+ Fe Fe Fe Fe
H 2O O OH 2 H 2O O
H OH 2
OH 2 OH 2 OH 2 OH 2

Fe
H O
O
Fe Fe
O
Kolloide
Colloids H O
Fe
3+
Mobilisation of Fe by siderophores

-39
The extremely low solubility of Fe(OH) 3 [solubility product L = 2·10 , solubility (pH 7) l =
-1
-18
10 mol·l ], and thus the unavailability of iron in aqueous media under oxic conditions, has
forced many groups of organisms to develop suitable systems for the mobilisation of iron.
These systems, so-called siderophores (Greek for iron transporter), excreted by the organisms,
3+
are multidentate anionic ligands which form extremely stable complexes with Fe (complex
-1
50
formation constants up to 10 M ). The functional groups of these ligands are, in many cases,
catecholates (o-
hydroxyquinolates), as in
the case of enterobactin, or O - O O - 3-
hydroxamates Catecholate -O HN O NH O -
(ferrioxamines and O O O O O
ferriochromes). The O - O O O O Fe O
complexes are more or less R O - O O
globular, with the outer O NH
sphere furnished with Enterobaktin O - - Fe 3+ -Enterobaktin-Komplex
hydrophilic groups (amide O
and ester groups), allowing O NH O NH O
for the water solubility and Hydroxamate NH NH NH NH
easy transport in the aquatic O O O O
medium. Internalisation of R-C N O - (C H 2)3 O - (C H 2)3 - (C H 2)3 O -
N
N
N
O
the iron-loaded siderophore R' C O C O C O
by the organism typically Ferrichrom CH 3 CH 3 CH 3
3+
takes place by endocytosis; the cytosolic remobilisation of the iron either by reduction of Fe
2+
to Fe and recycling of the siderophore, or by oxidative destruction of the siderophore.