β-谷甾醇、豆甾醇与DPPC相互作用的单分子膜研究

A Monolayer Study on the Interfacial Behavior of DPPC with β-sitosterol and Stigmasterol

植物甾醇对生物膜具有重要的调节作用,甾醇尾部的饱和程度对生物膜性质的影响一直是个热点问题.通过LB技术和原子力显微镜分别研究了尾部是单键的β-谷甾醇(β-sito)和尾部是双键的豆甾醇(stig)与二棕榈酰磷脂酰胆碱(DPPC)的相互作用,深入分析了β-谷甾醇、豆甾醇对DPPC单分子膜液态扩张-液态聚集(LE-LC)相变过程的影响.实验结果表明:β-谷甾醇、豆甾醇添加到DPPC单分子膜中,膜的压缩性以及分子的排列都发生了较大变化.当植物甾醇含量Xsterols=0.8时,β-谷甾醇、豆甾醇与DPPC相互作用具有相同的规律,过量分子面积和过量吉布斯自由能均为负值,说明分子间的吸引力比排斥力更强;在低浓度下,Xsterols=0.2,0.4时,两种系统的过量分子面积和过量吉布斯自由能呈现出明显的区别,尾部是单键的β-谷甾醇和DPPC混合单分子膜的排斥力更强烈,而尾部是双键的豆甾醇和DPPC混合单分子膜的吸引力更强烈,说明尾部是双键的豆甾醇比尾部是单键的β-谷甾醇更容易和DPPC发生凝聚,AFM数据进一步证实了这些结果.

It is known that sterols are key components of cellular membranes, which can modulate membrane properties and induce influence on the organization of membrane phospholipids. Howerer, the influence of these sterols on membrane properties sees to be unclear. It is worth studying. The interactions between β-sitosterol and stigmasterol and dipalmitoyl phosphatidycholine （DPPC） in mixed monolayers were studied by langmuir technique and atomic force microscopy （AFM）. The influence of β-sitosterol and stigmasterol effects on properties of DPPC membrane on phase transition from the liquid expanded （LE） to liquid condensed （LC） state were investigated and the condensing and ordering potency of binary monolayers containing β-sitosterol and stigmasterol were thoroughly analyzed. Based on the analysis of mixing values as well as AFM images it was evidenced that the effect of the plant sterol depends on the composition of model membrane. The results show that the addition of the plant sterols into model membrane modifies the condensation, ordering and interactions in the system. Moreover, at their higher concentration Xsterols=0.8, the values of the excess molecular area and excess Gibbs enerqy displayed negative values, which indicate that between molecules in the mixed monolayer exist stronger attraction than in the respective one-component films and suggest β-sitosterol and stigmasterol in the mixed system have the similar effects on its properties. Examination of the excess free energy of mixing reveals an enhanced stability of binary monolayers. At lower content, Xsterols=0.2, 0.4, the values of the excess molecular area and excess Gibbs energy suggested stronger attractive interaction between stigmasterol and DPPC, while an intermolecular stronger repulsion interaction between β-sitosterol and DPPC. It shows that stigmasterol was found to modify the properties of model membrane more strongly than β-sitosterol. It reflects that side-chain can reduce packing effectiveness, increased tilt of the molecules with respect to the monolayer plane, and the presence of the double bond reduces the hydrophobicity of the branched hydrocarbon tail of stigmasterol, which could cause strols to have long axis parallel to the interface. The result of AFM supported the theoretic analysis.