Micelle-to-vesicle transition of an iron-chelating microbial surfactant, marinobactin E

Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) techniques have been applied to study the self-assembly processes of a microbially produced siderophore, marinobactin E (ME). ME is one of a series of marinobactins A-E that facilitate Fe(III) acquisition by the source bacterium through coordination of Fe(III) by the marinobactin headgroup. ME is a six-amino-acid peptide amphiphile appended by palmitic acid (C16), and differs only in the nature of the fatty acid moiety from the other marinobactins. Apo-ME (uncoordinated ME) assembles to form micelles with an average diameter of 4.0 nm. Upon coordination of one equivalent of Fe(III), the mean micellar diameter of Fe(III)-ME shrinks to approximately 2.8 nm. However, in the presence of excess Fe(III), Fe(III)-ME undergoes a micelle-to-vesicle transition (MVT). At a small excess of Fe(III) over Fe(III)-ME (i.e., <1.2 Fe(III)/ME), a fraction of the Fe(III)-ME micelles rearrange into approximately 200 nm diameter unilamellar vesicles. At even greater Fe(III)/ME ratios (e.g., 2-3) multilamellar aggregates begin to emerge, consistent with either multilamellar vesicles or lamellar stacks. The MVT exhibited by ME may represent a unique mechanism by which marine bacteria may detect and sequester iron required for growth.