Angewandte
Chemie
electron microscopy (cryo-TEM) images of 1.4 ꢀ 10À3 m sol-
utions of the FeIII complexes showed polydisperse spherical
particles with diameters ranging from 100 to 250 nm.[12] An
example is given in Figure 1b for the [FeIII(LT)] system. The
the outer membrane, which releases iron in the periplasm and
then Yfe internalizes iron (Yfe is not TonB-dependent).
Further experiments are needed to reach more definite
conclusions about the precise role of the self-assembly
process in iron acquisition. This process, which is evidenced
in marine bacteria, may allow new perspectives for applica-
tions in iron nutrition and for iron chelation therapy.
Received: December 7, 2004
Published online: March 16, 2005
Keywords: amphiphiles · iron · self-assembly · siderophores ·
.
vesicles
[1] J. B. Neilands, J. Biol. Chem. 1995, 270, 26723.
[2] A. Stinzi, C. Barnes, J. Xu, K. N. Raymond, Proc. Natl. Acad. Sci.
USA 2000, 97, 10691.
[3] a) J. S. Martinez, G. P. Zhang, P. D. Holt, H.-T. Jung, C. J.
Carrano, M. G. Haygood, A. Butler, Science 2000, 287, 1245;
b) K. Barbeau, E. L. Rue, K. W. Bruland, A. Butler, Nature 2001,
413, 409; c) G. Xu, J. S. Martinez, J. T. Groves, A. Butler, J. Am.
Chem. Soc. 2002, 124, 13408; d) J. S. Martinez, J. N. Carter-
Franklin, E. Mann, J. D. Martin, M. G. Haygood, A. Butler,
Proc. Natl. Acad. Sci. USA 2003, 100, 3754.
[4] a) D. Imbert, P. Baret, D. Gaude, I. Gautier-Luneau, G. Gellon,
F. Thomas, G. Serratrice, J.-L. Pierre, Chem. Eur. J. 2002, 8, 1091;
b) L. Yun-Ming, M. J. Miller, U. Mꢁllmann, Biometals 2001, 14,
153; c) J. I. Wirgrau, A. L. Crumbliss, Inorg. Chem. 2003, 42,
5762.
Figure 1. Cryo-TEM images of [FeIII(Ln)] complexes (10À3 m) in water/
methanol (95/5, v/v) solution at pH 7.4 (MOPS buffer) embedded in
vitreous ice;[10] a) ligand La 3 h after addition of 1% octanol and
b) ligand LT.
[5] D. Imbert, D. Thomas, P. Baret, G. Serratrice, D. Gaude, J.-L.
Pierre, J.-P. Laulhꢂre, New J. Chem. 2000, 24, 281.
micrographs depicted in Figure 1 compare well with those of
FeIII-marinobactin vesicles.[3a] In the experiment depicted in
Figure 1a, octanol (1 vol%) was added to the [FeIII(La)]
solution to favor structuration of the particles. Successive
cryoelectron micrographs allowed visualization of the evolu-
tion of the aggregates with time: vesicles of octanol first
appeared at the surface of the aggregates (see Supporting
Information) and after several hours perfectly spherical
particles were observed. The contrast in the images of the
particles (clear external surface and dark interior) suggests
that they are filled. Furthermore, this observation indicates an
aggregation of the molecules of the complexes, in agreement
with an external hydrophilic surface as schematically depicted
in reference [3a].
[6] The ligands and their ferric complexes have low solubilities in
aqueous solution, therefore the potentiometric titrations were
carried out in water/methanol (50/50, v/v; ligand concentration
0.5 mm) and the spectrophotometric titrations in water/meth-
anol (95/5, v/v; ligand concentration 0.1 mm). The methods have
been previously described.[4a] Data analysis of the potentiomet-
ric and spectrophotometric titrations was performed using the
Hyperquad[7] and Letagrop[8] programs. The spectrophotometric
titration of a mixture of FeIII and ligands La or Lb in a 1:3 molar
ratio was carried out over the pH range 5–12.
[7] P. Gans, A. Sabatini, A. Vacca, Talanta 1996, 43, 1739.
[8] a) L. G. Sillen, Acta Chem. Scand. 1964, 18, 1085; b) L. G. Sillen,
B. Warnqvist, Ark. Kemi 1968, 31, 377.
[9] T. M. Garrett, P. W. Miller, K. N. Raymond, Inorg. Chem. 1989,
28, 128.
The ligands studied herein are good models for the
physicochemical properties of the siderophores from marine
bacteria. For a better understanding of the biological
relevance of the self-assembly process (in correlation with
the interesting discussion in reference [3a]), studies of bacte-
rial iron nutrition were performed with E. chrysanthemi.[13–15]
Preliminary results show that: 1) the iron aggregates allow
nutrition; 2) the best results are obtained with the iron
complex of ligand La; 3) the natural siderophore way is used
when available (wild-type bacteria); and 4) nutrition is
allowed in mutants without the siderophore way. Moreover,
we showed that 70–75% of iron transport involves the TonB-
dependent receptors Fct and/or 88 Da with Cbu for the inner
membrane (see ref. [14] for a description of the iron transport
system in E. chrysanthemi). For the remaining 25–30% of iron
transport, it can be suggested that the aggregate merges with
[10] Surface tension measurements were carried out with a drop
tensiometer (Tracker; I.T. Concept, Longessagne, France) at
258C, which allowed the determination of the interfacial tension
by analyzing the axial symmetric shape (Laplacian profile) of the
rising air drop in solution. The CMC values were estimated from
the intersection of the extrapolated linear portions of the plot of
surface tension versus ligand or complex concentration.
[11] The size of the particles was measured by quasielastic light
scattering. The movement of particles in suspension is charac-
terized by a diffusion coefficient, and the particles undergo
Brownian motion. A Malvern zetasizer 300HsA instrument was
used at a scattering angle of 908. This apparatus includes a 5-mW
He–Ne laser that operates at a wavelength of 633 nm. The
detector is an avalanche photodiode and the numerical corre-
lator (type 7132CN with 256 channels) is controlled by the
AutoSizer software version XY. The sample is contained in a
flow-through cell kept at 238C. The data were analyzed by the
CONTIN method. The translational diffusion coefficient is
Angew. Chem. Int. Ed. 2005, 44, 2580 –2582
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