Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, UK.
Langmuir. 2010 May 18;26(10):7334-42. doi: 10.1021/la904212x.
The lipopeptide surfactin from Bacillus subtilis strains exhibits strong surface and biological activity, the latter probably because of its interaction with biological membranes. We have investigated the interaction of aqueous solutions of surfactin with supported bilayers of diphosphatidylcholine (DPPC) on silica using neutron reflectometry. We have also used small-angle neutron scattering (SANS) to study the solubilized aggregates formed as a result of the destruction of the supported membrane by surfactin. Although surfactin on its own does not attach to the silica supporting surface, it is taken up from solution by the membrane, confirming that there is an attractive interaction between DPPC and surfactin. The surfactin concentration in the layer can reach up to about 20 mol % relative to DPPC. The membrane is stable provided that the surfactin concentration is below its critical micelle concentration (cmc, 5 x 10(-5) M). Above the cmc, however, the membrane is solubilized and removed from the surface, though not always completely, over a period of hours. There are signs that there is an induction period while the surfactin concentration builds up in the membrane. This would be consistent with the need for a threshold concentration of surfactin in the bilayer. The presence of a surfactin correlation peak in the SANS showed that in the bulk solution, at the same concentrations as used for the deposition, surfactin forms aggregates that must be localized in the DPPC multilamellar vesicles at a separation of about 160 A. The structure could be fitted with an approximate model where the surfactin has an aggregation number of 50 +/- 10 with a radius of about 27 A. Given the very small water thicknesses in the DPPC lamellar aggregates, the surfactin must exist as aggregates in the phospholipid bilayer, and these structures are responsible for solubilizing the DPPC.
枯草芽孢杆菌来源的脂肽表面活性剂具有很强的表面活性和生物活性，后者可能是由于其与生物膜相互作用的结果。我们使用中子反射法研究了表面活性剂在二氧化硅上的二磷酸二酯（DPPC）支撑双层膜中的水溶液相互作用。我们还使用小角中子散射（SANS）研究了由于表面活性剂破坏支撑膜而形成的溶解聚集体。尽管表面活性剂本身不会附着在二氧化硅支撑表面上，但它会被膜从溶液中吸收，这证实了 DPPC 和表面活性剂之间存在吸引力。在膜中，表面活性剂的浓度可以达到 DPPC 的约 20 mol%。只要表面活性剂的浓度低于其临界胶束浓度（cmc，5 x 10(-5) M），膜就是稳定的。然而，超过 cmc 后，膜会被溶解并从表面去除，尽管在数小时内并不总是完全去除。有迹象表明，在膜中表面活性剂浓度增加时存在诱导期。这将与双层中需要有一个表面活性剂的阈值浓度一致。在 SANS 中存在表面活性剂相关峰表明，在与沉积相同浓度的本体溶液中，表面活性剂形成的聚集体必须定位于 DPPC 多层囊泡中，间隔约 160 A。该结构可以用一个近似模型拟合，其中表面活性剂的聚集数为 50 +/- 10，半径约为 27 A。考虑到 DPPC 层状聚集体中的水层非常薄，表面活性剂必须以聚集体的形式存在于磷脂双层中，这些结构负责溶解 DPPC。
