High-Resolution Atomic Force Microscopy Study of Hexaglycylamide Epitaxial Structures on GraphiteстатьяИсследовательская статья
Статья опубликована в высокорейтинговом журнале
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Аннотация: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.