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Amphiphilic properties of a physiological mixture of marine siderophores . Iron containing proteins are essential for electron transport during respiration and photosynthesis as well as in nitrogen fixation among other important biological processes. Siderophores are low molecular weight organic iron coordinating compounds produced by certain bacteria and fungi. These compounds facilitate iron uptake in a variety of environments from terrestrial soils to the ocean surface. Iron acquisition in marine environments is especially challenging. The low solubility [pico to nanomolar (1)] of Fe(III) in neutral aqueous solutions leads to a limited iron supply. Low total iron levels in marine environments are significant enough to limit growth in certain areas of the world's oceans. Siderophores represent one important class of molecules that solublize iron in the marine environment. Understanding the coordination environment and uptake processes of the limiting nutrient is very important for understanding the effects on primary production and the global carbon cycle. The Butler group is currently studying Marinobactins; unique siderophores produced by a Marinobacter species. The distinguishing structural feature of these siderophores is an additional fatty acid "tail" attached to the common peptidic iron chelating head group. In general, amphiphilic compounds have both a polar and nonpolar region within one molecule. Opposing interactions between solvents and the separate domains of an amphiphilic molecule often force such molecules to self-assemble into ordered aggregates in solution. The molecules can form micelles, bilayers, or vesicles among other geometrical motifs depending upon the structure of the amphiphilic molecule and solution chemistry. Geometry and other aggregate properties also depend on the abundances of the individual amphiphilic species when heterogeneous aggregates are present. Recently, the Butler group has shown that the marinobactins exhibit unique amphiphilic properties (2). The marinobactins are produced in biological mixtures of varying fatty acid chain length and saturation (fig. 1). Marinobactin D showed a phase transition from micelles to vesicles upon addition of Fe(III). This novel and surprising transition from micelles to vesicles is effectively an iron switch. 
Whether this so called "iron switch" is actually an important step for iron acquisition in this Marinobacter species is an interesting question. In the natural marine environment a physiological mixture of marinobactins of varying hydrophobicities is produced. The physiological mixture is of more interest than an isolated component when attempting to understand marinobactins actual behavior in the ocean. Using light scattering the so called "iron switch" was investigated for the physiological mixture of marinobactins. While the results from light scattering experiments can provide information about the existence and size of vesicles, it cannot provide information as to the composition of those vesicles. In an attempt to assess the degree to which each component of the physiological mixture participates in vesicle formation, High Performance Liquid Chromatography (HPLC) was used. The composition of the vesicles was assessed by comparing the relative ratios of the physiological mixture components in a complete "vesicle phase" preparation to a preparation of marinobactins in which the vesicles had been removed. In the experiment centrifuge filtration through a 0.22 µm filter was used as a new technique to remove vesicles from a solution. 1. M. Geldhill, C. M. G. van der Berg, Mar. Chem. 47, 41 (1994).2. J. S. Martinez, G. P. Zhang, P. D. Holt, H.-T. Jung, C. J. Carrano, M. G. Haygood, A. Butler, Science 287, 1245 (2000) Return to the RISE 2000 project list |
