Structure and Dynamics of Surfactin Studied by NMR in Micellar Media

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Abstract
The NMR structure of the cyclic lipopeptide surfactin from Bacillus subtilis was determined in sodium dodecyl sulfate (SDS) micellar solution. The two negatively charged side chains of surfactin form a polar head opposite to most hydrophobic side chains, accounting for its amphiphilic nature and its strong surfactant properties. Disorder was observed around the fatty acid chain, and 15N relaxation studies were performed to investigate whether it originates from a dynamic phenomenon. A very large exchange contribution to transverse relaxation rate R2 was effectively observed in this region, indicating slow conformational exchange. Temperature variation and Carr−Purcell−Meiboom−Gill (CPMG) delay variation relaxation studies provided an estimation of the apparent activation energy around 35−43 kJ·mol-1 and an exchange rate of about 200 ms-1 for this conformational exchange. 15N relaxation parameters were also recorded in dodecylphosphocholine (DPC) micelles and DMSO. Similar chemical exchange around the fatty acid was found in DPC but not in DMSO, which demonstrates that this phenomenon only occurs in micellar media. Consequently, it may either reflect the disorder observed in our structures determined in SDS or originate from an interaction of the lipopeptide with the detergent, which would be qualitatively similar with an anionic (SDS) or a zwitterionic (DPC) detergent. These structural and dynamics results on surfactin are the first NMR characterization of a lipopeptide incorporated in micelles. Moreover, they provide a model of surfactin determined in a more biomimetic environment than an organic solvent, which could be useful for understanding the molecular mechanism of its biological activity.