M. M. Shemyakin & Yu. A. Ovchinnikov Institute of Bioorganic Chemistry, Miklukho-Maklaya str., 16/10, 117997 GSP Moscow V-437, Russia.
Langmuir. 2011 May 17;27(10):5879-90. doi: 10.1021/la103051w. Epub 2011 Feb 25.
Two types of hexaglycylamide (HGA) epitaxial lamellar structures coexisting on the surface of highly oriented pyrolytic graphite (HOPG) exposed to water solutions were studied by high-resolution atomic force microscopy (AFM). Lamellae are distinguished by growth direction and by morphology. The lamellae of the first type (L1) produced by depositions from more dilute solutions are close-packed with a period of ∼5.2 nm, twice the HGA molecular length, and form highly ordered domains morphologically similar to the lamellar domains of alkanes. The less-ordered lamellae of the second type (L2) appear at intermediate and large HGA concentrations and demonstrate variable lamellar width, morphological diversity, and a tendency to merge. The interlamellar separation in the domains of close-packed L2 lamellae varies with the discrete increment ∼2.5 nm; the most frequently observed value is ∼7.5-8.0 nm corresponding to the triple HGA molecular length. The growth directions of lamellae of each type have sixfold rotational symmetry indicating epitaxy with graphite; however, the rosettes of L1 and L2 lamellae orientations are misaligned by 30°. The molecular modeling of possible HGA epitaxial packing arrangements on graphite and their classification have been conducted, and the energetically preferable structures are selected. On this basis, the structural models of the L1 and L2 lamellae are proposed explaining the experimentally observed peculiarities as follows: (1) the L1 and L2 lamellae are respectively parallel and antiparallel β-sheets with two HGA molecules in the unit cell oriented normally to the lamellae boundaries, (2) HGA molecules in L1 and L2 lamellae have different orientations with respect to the graphite lattice, respectively along the directions <1120> and <1010>, (3) L1 lamella is the assembly of two hydrogen-bonded parallel β-sheets oriented head-to-head, (4) L2 lamellae are assemblies of several molecular rows (antiparallel β-sheets) cross-linked by hydrogen bonds. The AFM observations indicate that the covering of the hydrophobic graphite by the dense, closely packed, well-ordered monolayers of hydrophilic oligopeptide is possible.
两种六甘氨酰胺（HGA）外延层状结构共存于高度取向的热解石墨（HOPG）表面，暴露于水溶液中，通过高分辨率原子力显微镜（AFM）进行了研究。层状结构通过生长方向和形态来区分。第一种类型（L1）的层片是由更稀溶液沉积产生的，它们紧密堆积，周期约为 5.2nm，是 HGA 分子长度的两倍，并形成与烷烃层状结构相似的高度有序的畴。第二种类型（L2）的层片在中等和较大的 HGA 浓度下出现，表现出可变的层片宽度、形态多样性和合并的趋势。紧密堆积的 L2 层片中的层间间隔随离散增量约 2.5nm 而变化；最常观察到的值约为 7.5-8.0nm，对应于三倍 HGA 分子长度。每种类型的层片的生长方向具有六重旋转对称性，表明与石墨的外延生长；然而，L1 和 L2 层片取向的玫瑰花结以 30°的角度错开。对 HGA 在石墨上可能的外延堆积排列进行了分子建模和分类，并选择了能量上有利的结构。在此基础上，提出了 L1 和 L2 层片的结构模型，解释了实验观察到的如下特点：（1）L1 和 L2 层片分别是平行和反平行的β-片层，每个单元中含有两个 HGA 分子，沿层片边界垂直取向，（2）L1 和 L2 层片中的 HGA 分子相对于石墨晶格的取向不同，分别沿方向 <1120> 和 <1010>，（3）L1 层片是由两个氢键平行的β-片层头对头组装而成，（4）L2 层片是由几个分子行（反平行的β-片层）通过氢键交联而成的组装体。AFM 观察表明，疏水性石墨被密集、紧密堆积、有序的亲水性寡肽单层覆盖是可能的。
