Naturally engineered glycolipid biosurfactants leading to distinctive self-assembled structures
Naturally engineered glycolipid biosurfactants leading to distinctive self-assembled structures
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DOI：
10.1002/chem.200501199
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发表时间：
2006-03-08
影响因子：
4.3
通讯作者：
Kitamoto, D
中科院分区：
文献类型：
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作者：
Imura, T;Ohta, N;Kitamoto, D
利用荧光探针法、动态光散射（DLS）分析、冷冻断裂透射电镜（FF-TEM）和同步辐射小/广角X射线散射（SAXS/WAXS）分析等方法研究了酵母菌大量生产的“天然”糖脂生物表面活性剂甘露糖基-甘露糖脂A和B（MEL-A、MEL-B）的自组装特性。MEL-A和MEL-B在极低浓度下都表现出优异的自组装特性，它们在略高于其临界聚集浓度（CAC）时自组装成大的单层囊泡（LUV）。MEL-A的CAC（I）值为4.0 × 10（-6）M，MEL-B的CAC（I）值为6.0 × 10（-6）M。此外，发现在CAC（II）值为2.0 × 10（-5）M以上的MEL-A的自组装结构急剧转变为海绵结构（L-3），其由通常从复杂的多组分“合成”表面活性剂体系获得的随机连接的双层网络组成。有趣的是，海绵结构的平均水通道直径为100 nm。这与从“合成”表面活性剂体系获得的那些相比是相对大的。此外，MEL-B在甘露糖上的C-4'位置具有羟基而不是乙酰基，仅产生一个CAC; MEL-B的自组装结构似乎逐渐从LUV移动到晶格常数为4.4 nm的多层囊泡（MLV），这取决于浓度。此外，在高浓度下的溶致液晶相观察表明，MEL-A形成了倒置的六方相（H-2），MEL-B形成了层状相（L-alpha），这表明MEL-A和MEL-B分子在组装体的自发曲率方面存在差异。这些结果清楚地表明，由头基上的单个乙酰基引起的自发曲率的差异可能决定了糖脂生物表面活性剂的自组装方向。糖脂生物表面活性剂具有独特而复杂的分子结构，具有由微生物分子工程改造的多个手性中心，这必然导致了糖脂生物表面活性剂复杂的自组装特性。
Self-assembling properties of "natural" glycolipid biosurfactants, mannosyl-erythritol lipids A and B (MEL-A, MEL-B), which are abundantly produced from yeast strains, were investigated by using the fluorescence-probe method, dynamic light-scattering (DLS) analysis, freeze-fracture transmission electron microscopy (FF-TEM), and synchrotron small/wide-angle X-ray scattering (SAXS/WAXS) analysis, among other methods. Both MEL-A and MEL-B exhibit excellent self-assembly properties at extremely low concentrations', they self-assemble into large unilamellar vesicles (LUV) just above their critical-aggregation concentration (CAC). The CAC(I) value was found to be 4.0x10(-6)M for MEL-A and 6.0x10(-6)M for MEL-B. Moreover, the self-assembled structure of MEL-A above a CAC(II) value of 2.0x10(-5) M was found to drastically change into sponge structures (L-3) composed of a network of randomly connected bilayers that are usually obtained from a complicated multicomponent "synthetic" surfactant system. Interestingly, the average water-channel diameter of the sponge structure was 100 nm. This is relatively large compared with those obtained from "synthetic" surfactant systems. In addition, MEL-B, which has a hydroxyl group at the C-4' position on mannose instead of an acetyl group, gives only one CAC; the self-assembled structure of MEL-B seems to gradually move from LUV to multilamellar vesicles (MLV) with lattice constants of 4.4 nm, depending on the concentration. Furthermore, the lyotropic-liquid-crystal-phase observation at high concentrations demonstrates the formation of an inverted hexagonal phase (H-2) for MEL-A, together with a lamella phase (L-alpha) for MEL-B, indicating a difference between MEL-A and MEL-B molecules in the spontaneous curvature of the assemblies. These results clearly show that the difference in spontaneous curvature caused by the single acetyl group on the head group probably decides the direction of self-assembly of glycolipid biosurfactants. The unique and complex molecular structures with several chiral centers that are molecularly engineered by microorganisms must have led to the sophisticated self-assembling properties of the glycolipid biosurfactants.