Institute of High Performance Computing, Agency for Science, Technology and Research, Connexis, Singapore.
Langmuir. 2012 Sep 11;28(36):13008-17. doi: 10.1021/la302161x. Epub 2012 Aug 27.
The epicuticle is the outermost layer of the human hair, and consists of a monolayer of fatty acids that is predominantly 18-methyleicosanoic acid (18-MEA) covalently bound to a protein matrix. Surprisingly, despite the clear scientific and industrial importance, the detailed molecular structure of this fatty acid layer is still poorly understood. In this work, we aim to gain insight into the structure of this so-called F-layer by performing molecular dynamics simulations on a simplified hair surface model consisting of a monolayer of 18-MEA covalently attached to graphene sheets at various separation distances. The relative free energy of the fatty acid layer was calculated as a function of separation distance in order to obtain the optimal packing density of the fatty acids. Conformational properties such as the thickness, tilt angle, and order parameter of the fatty acid layers were also calculated to characterize the structure of the F-layer. Simulations of the structurally similar eicosanoic acid (EA) were also performed as a comparison and to investigate the role of the anteiso-methyl side chain at the 18th position of 18-MEA. The degree of water penetration into the fatty acid layer at the various separation distances was also investigated. Our simulations suggest that the optimal spacing for the fatty acids is between 0.492 and 0.651 nm, in contrast to the generally accepted literature value of around 0.9-1.0 nm. This results in a packing density of between 0.21 and 0.37 nm(2) per fatty acid molecule and a thickness of around 2.01-2.64 nm. We also show that, at larger separation distances, the 18-MEA fatty acid provides a slightly better hydrophobic layer than the EA fatty acid, suggesting that the 18-MEA fatty acid may have been naturally selected to provide better protection for the hair when it loses some of the fatty acids due to daily wear and tear. To our knowledge, this is the first attempt to systematically investigate the hair surface structure and properties with molecular simulations.
表皮层是人类头发的最外层，由单层脂肪酸组成，主要是共价结合到蛋白质基质上的 18-甲基二十烷酸（18-MEA）。令人惊讶的是，尽管具有明显的科学和工业重要性，但这种脂肪酸层的详细分子结构仍知之甚少。在这项工作中，我们旨在通过对由共价附着在不同分离距离的石墨烯片上单层 18-MEA 组成的简化头发表面模型进行分子动力学模拟，深入了解所谓的 F-层的结构。为了获得脂肪酸的最佳堆积密度，计算了脂肪酸层的相对自由能作为分离距离的函数。还计算了脂肪酸层的构象性质，如厚度、倾斜角和有序参数，以表征 F-层的结构。还进行了结构相似的二十烷酸（EA）的模拟作为比较，并研究了 18-MEA 中第 18 位的 anteiso-甲基侧链的作用。还研究了在不同分离距离下水进入脂肪酸层的程度。我们的模拟表明，脂肪酸的最佳间隔在 0.492 和 0.651nm 之间，与普遍接受的约 0.9-1.0nm 的文献值形成对比。这导致每个脂肪酸分子的堆积密度在 0.21 和 0.37nm(2) 之间，厚度约为 2.01-2.64nm。我们还表明，在较大的分离距离下，18-MEA 脂肪酸提供了稍微更好的疏水性层，这表明 18-MEA 脂肪酸可能已经被自然选择，以在头发由于日常磨损而失去一些脂肪酸时为其提供更好的保护。据我们所知，这是首次尝试使用分子模拟系统地研究头发表面结构和性质。
