Hibino Masahiro, Tsuchiya Hiroshi
Department of Applied Science, Muroran Institute of Technology , 27-1 Mizumoto-cho, Muroran 050-8585, Japan.
Langmuir. 2014 Jun 17;30(23):6852-7. doi: 10.1021/la500944t. Epub 2014 Jun 6.
The molecular arrangements of self-assembled monolayers (SAMs) of cholesterol, cholesteryl laurate, and cholesteryl stearate adsorbed on a graphite surface were studied using scanning tunneling microscopy (STM) at the liquid-solid interface. The STM images of the SAMs showed two-dimensional periodic arrays of bright regions that corresponded to the sterol rings. However, individual sterol rings could not be observed in the bright regions in the STM images of the cholesterol monolayers. Nevertheless, by comparing the STM images and the crystallographic data, it is concluded that the cholesterol molecules are arranged in pairs oriented head-to-head owing to the hydrogen bonds between the hydroxyl groups. These dimers, in turn, are oriented parallel to each other, owing to the interactions between the sterol rings. The STM images of cholesteryl ester monolayers had molecular resolution and showed pairs of cholesteryl ester molecules oriented in an antiparallel manner, with their fatty acid chains located in the central regions. Furthermore, the fatty acid chains of cholesteryl stearate were observed to be oriented in the (1120) zigzag direction of the graphite lattice, whereas those of cholesteryl laurate were oriented in the (1010) armchair direction. These observations reveal that the interactions between the fatty acid chains affect the structure of the SAMs. The molecular arrangements also depend on the lengths of the fatty acid chains of the cholesterol esters and hence on the interactions between the alkyl chains and the graphite surface. The self-assembly at the liquid-solid interface is therefore controlled by the interactions between sterol rings, between alkyl chains, and between alkyl chains and the substrate.
利用扫描隧道显微镜（STM）在液固界面研究了吸附在石墨表面的胆固醇、月桂酸胆固醇酯和硬脂酸胆固醇酯自组装单分子层（SAMs）的分子排列。SAMs的STM图像显示出对应于甾醇环的二维周期性亮区阵列。然而，在胆固醇单分子层的STM图像中，在亮区无法观察到单个甾醇环。尽管如此，通过比较STM图像和晶体学数据，可以得出结论，由于羟基之间的氢键作用，胆固醇分子以头对头的方式成对排列。这些二聚体又由于甾醇环之间的相互作用而彼此平行排列。胆固醇酯单分子层的STM图像具有分子分辨率，显示出胆固醇酯分子对以反平行方式排列，其脂肪酸链位于中心区域。此外，观察到硬脂酸胆固醇酯的脂肪酸链沿石墨晶格的（1120）之字形方向排列，而月桂酸胆固醇酯的脂肪酸链沿（1010）扶手椅方向排列。这些观察结果表明，脂肪酸链之间的相互作用会影响SAMs的结构。分子排列还取决于胆固醇酯脂肪酸链的长度，因此取决于烷基链与石墨表面之间的相互作用。因此，液固界面的自组装受甾醇环之间、烷基链之间以及烷基链与底物之间相互作用的控制。
