High-resolution atomic force microscopy study of hexaglycylamide epitaxial structures on graphite.

Area of Science:

• Surface Science
• Supramolecular Chemistry
• Materials Science

Background:

• Hexaglycylamide (HGA) is an oligopeptide with potential applications in surface modification.
• Understanding the self-assembly of HGA on hydrophobic surfaces like highly oriented pyrolytic graphite (HOPG) is crucial for designing functional materials.

Purpose of the Study:

• To investigate the epitaxial lamellar structures of HGA formed on HOPG surfaces.
• To elucidate the structural characteristics and growth mechanisms of different HGA lamellar phases.
• To propose molecular models explaining the observed AFM data.

Main Methods:

• High-resolution atomic force microscopy (AFM) was employed to image HGA lamellar structures on HOPG.
• Molecular modeling was used to predict and analyze possible epitaxial packing arrangements of HGA on graphite.

Main Results:

• Two distinct types of HGA lamellar structures (L1 and L2) were observed, differing in growth direction, morphology, and ordering.
• L1 lamellae, from dilute solutions, are close-packed with a period of ~5.2 nm, resembling alkane lamellae.
• L2 lamellae, from higher concentrations, show variable width and interlamellar spacing (~7.5-8.0 nm), with a tendency to merge.
• AFM revealed sixfold rotational symmetry for both lamellae types, indicating epitaxy with graphite, but with a 30° misalignment between L1 and L2 orientations.
• Structural models propose L1 as parallel β-sheets and L2 as antiparallel β-sheets, with distinct molecular orientations and hydrogen bonding patterns.

Conclusions:

• HGA forms well-ordered, dense monolayers on hydrophobic graphite surfaces.
• The study provides detailed structural models for HGA lamellae, explaining their formation and properties.
• The findings demonstrate the feasibility of covering hydrophobic surfaces with hydrophilic oligopeptides, opening avenues for surface functionalization.