High-Resolution Atomic Force Microscopy Study of Hexaglycylamide Epitaxial Structures on Graphiteстатья Исследовательская статья

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[1] High-resolution atomic force microscopy study of hexaglycylamide epitaxial structures on graphite / V. V. Prokhorov, D. V. Klinov, A. A. Chinarev et al. // Langmuir : the ACS journal of surfaces and colloids. — 2011. — Vol. 27, no. 10. — P. 5879–5890. 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 frommore 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; themost frequently observed value is∼7.5-8.0 nmcorresponding to the tripleHGA 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 structuralmodels 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 Æ1120æ and Æ1010æ, (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. [ DOI ]

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