(a) The peptide ring of surfactin (carbons: grey, nitrogens: green, oxygens: red) which contains seven residues closed by two functional groups of a b-hydroxy-fatty acid. The two charged groups are drawn larger in pink.
(b) A structure of surfactin with the hydrocarbon chain of the fatty acid projecting away from the peptide ring, the presumption being that the ring is hydrophilic.
(c) The structure of surfactin most consistent with the neutron reflectivity data.
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Understanding the biological function of surfactin
Surfactin is a naturally occurring surface active molecule produced from bacteria. Like many other biosurfactants, its biological function is not understood but it is extremely surface active at very low concentrations and is a powerful disrupter of phospholipid membranes. The chemical structure of surfactin, from NMR data, does not reveal how it possesses such surface activity. The charged groups are the only units capable of conferring water solubility on surfactin and yet are so buried in the generally hydrophobic surroundings that this part of the molecule bears little resemblance to a normal amphiphile's typical hydrophilic group. Neutron reflectivity measurements have allowed determination of the distribution of the three component fragments (hydrocarbon chain, leucines, and remaining part of the ring) in the structure of surfactin at the air/water interface. The structure accentuates the hydrophobic ball-like nature of the surfactin whose solubility in water is only maintained by the double charge, leading to its high surface active at such low concentrations, without self-assembly.
HH Shen, RK Thomas, CY Chen, R Darton (University of Oxford)
Research date: December 2006
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