XAS study of a metal-induced phase transition by a microbial surfactant

The metal-induced micelle-to-vesicle phase change that the ferric complex of the microbially produced amphiphile, marinobactin E (M(E)), undergoes has been investigated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Marinobactin E is one member of the suite of siderophores, marinobactins A-E, that are used by the source bacterium to facilitate iron acquisition. Fe(III)-M(E) undergoes a micelle-to-multilamellar vesicle transition in the presence of Cd(II) and Zn(II). XRD measurements indicate the interlamellar repeat distance of the Cd(II)- and Zn(II)-induced multilamellar vesicles is approximately 5.3 nm. XAS spectra of the sedimented Cd(II)- and Zn(II)-induced multilamellar vesicles suggests hexadentate coordination of Cd(II) and Zn(II) consisting of two monodentate carboxylate ligands and four water ligands. This coordination environment supports the hypothesis that Cd(II) and Zn(II) bridge the terminal carboxylate moiety of two Fe(III)-M(E) headgroups, pulling the headgroups together in an arrangement that favors vesicle formation over the formation of micelles. XAS spectra of the Fe(III) center in the sedimented Cd(II)- and Zn(II)-induced vesicles confirm the anticipated six-coordinate geometry of Fe(III) by the M(E) headgroup via the two hydroxamate groups and the alpha-hydroxy amide moiety.