Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115, USA.
Phys Chem Chem Phys. 2020 Sep 14;22(34):19089-19099. doi: 10.1039/d0cp01834a. Epub 2020 Aug 18.
The electron density profile of bilayers of DPPC/cholesterol mixtures supported on semiconductor grade silicon substrates were studied with the objective of determining how the proximity of a solid interface modifies the phase diagram of mixed bilayers. The bilayers were studied in situ immersed in water via synchrotron X-ray reflectivity (XRR). Measurements were performed as a function of temperature through the main phase transition and cholesterol mole fractions up to 40%. Analysis of XRR yields the bilayer thickness, roughness and leaflet asymmetry. We find that the structure of the pure DPPC bilayers in the gel phase is in agreement with previous X-ray measurements of supported bilayers deposited via vesicle fusion and multilamellar vesicles but show more clearly defined features than measurements made on films formed using Langmuir-Blodget Langmuir-Shaffer (LB) deposition. Examination of bilayer thickness vs. temperature shows that the melting temperature for supported bilayers is shifted upwards by approximately 4 °C relative to multilamellar vesicles and that the melting temperature decreases with increasing cholesterol content up to 20%. For pure DPPC bilayers the leaflets melt in two stages with the distal leaflet melting first. For cholesterol concentrations of 10% and 20% there is no clear indication of separate melting. For 33% and 40% cholesterol content no clear transition is seen in the bilayer thickness, but an abrupt change in roughness indicates possible microdomain formation in the 40% cholesterol sample.
我们研究了 DPPC/胆固醇混合物双层在半导体级硅衬底上的电子密度分布，目的是确定固体界面的临近如何改变混合双层的相图。通过同步辐射 X 射线反射率（XRR）原位浸入水中研究双层。通过主相变和胆固醇摩尔分数高达 40%的温度进行测量。XRR 的分析产生了双层厚度、粗糙度和叶瓣不对称性。我们发现，纯 DPPC 双层在凝胶相中结构与之前通过囊泡融合和多层囊泡沉积的支撑双层的 X 射线测量结果一致，但比使用 Langmuir-Blodget Langmuir-Shaffer（LB）沉积形成的薄膜测量结果具有更清晰的特征。双层厚度与温度的关系表明，与多层囊泡相比，支撑双层的熔点向上移动了约 4°C，并且随着胆固醇含量的增加，熔点降低至 20%。对于纯 DPPC 双层，叶瓣分两个阶段熔化，首先是远端叶瓣熔化。对于 10%和 20%的胆固醇浓度，没有明显的单独熔化迹象。对于 33%和 40%的胆固醇含量，在双层厚度中没有明显的转变，但粗糙度的突然变化表明 40%胆固醇样品中可能形成了微域。
