Vesicles to Concentrate Iron in Low‐Iron Media: An Attempt to Mimic Marine Siderophores

Abstract

Amphiphilic catechol-type iron chelators were studied with the aim of mimicking the properties of marine bacterial siderophores. The Fe(III) complexation constants and aqueous solution speciation of L(S10), a sulfonated catechol unit that has a C(10) lipophilic carbon chain connected by an amide linkage, were determined by spectrophotometric titration. The calculated value of pFe3+ is 18.1 at pH 7.4. Cryogenic transmission electron microscopy showed that the tris(catecholate) ferric complex formed at physiological pH initially assembles into micelles, in which the catecholate-iron units stay on the exterior of the micelle. The average diameter of these micelles was estimated to be 4.2 nm. The micelles then slowly rearrange into clusters of different sizes, which leads to the formation of unilamellar and bilamellar vesicles. The reorganization processes are comparable to those observed by Butler et al. for the marinobactin siderophores produced by marine bacteria, but in contrast to the marinobactins, vesicles of the Fe3+-L(S10) complex form without an excess of iron relative to ligand concentration. The time-dependent micelle-to-vesicle transition is discussed herein.

From Molecular to Nanostructured Iron Complexes of Amphiphilic Chelators Based on 8‐Hydroxyquinoline Subunits – Evidence of Self‐Assembled Edifices Mimicking Siderophores from Marine Bacteria

“…This phenomenon was already observed for L S10 and its iron complex and attributed to the higher hydrophobicity of the complex that contains three ligands and, therefore, three hydrocarbon tails. [7] One can note that the CMC of [Fe(Q S10 ) 3 ] 3-is significantly lower than the 0.45 m value obtained for [Fe(L S10 ) 3 ] 6-but close to the one determined for Marinobactin E (≈75 µ) [5] , which is the natural amphiphilic siderophore of Marinobacter sp. DS40M6.…”

“…This behavior is different from previous observations concerning [Fe(L S10 ) 3 ] 6-, where two size distributions were observed (around 4 and 50 nm). [7] Moreover, these results have to be compared to data reported by Butler and coworkers on Marinobactin E showing the presence of spherical particles of 140 to 180 nm in diameter. [2] …”

“…An upper limit would be a diameter of 3-4 nm, which is in agreement with the average size of [Fe(Q S10 ) 3 ] 3-and close to the results obtained for [Fe(L S10 ) 3 ] 6-. [7] Loose networks with a different morphology were sometimes observed, such as that shown in Figure 6b. It is still composed of electron-dense nanoparticles, but they seem to be organized into irregular semicapsular aggregates.…”

“…Moreover, the heterogeneous distribution of iron-rich regions in the [Fe(Q S10 ) 3 ] 3-aggregate was unexpected in comparison to the results obtained for [Fe(L S10 ) 3 ] 6-, where the contrast of the resulting particles was homogeneous. [7] This might be explained by the differences in charge and bulkiness (catechol vs. quinoline scaffolds) of the ferric complexes. However, our results are in good agreement with those obtained by Butler and coworkers on ferric complexes of Marinobactin D showing cryo-TEM pictures of spherical vesicules that are 50 to 200 nm in diameter.…”

“…The amphiphilic surfaceactive nature of these siderophores leads to the formation of self-assembled structures. [2][3][4][5] In previous papers, we described the self-assembling properties of amphiphilic iron complexes involving catechol subunits [6,7] (Scheme 1). These ligands appeared to be good models for the physicochemical properties of siderophores from marine bacteria.…”