Predictions of Pattern Formation in Amino Acid Adlayers at the In Vacuo Graphene Interface: Influence of Termination State

Self-assembly of amphiphilic amino acid derivatives for biomedical applications

Amino acids are one of the simplest biomolecules and they play an essential role in many biological processes. They have been extensively used as building blocks for the synthesis of functional nanomaterials, thanks to their self-assembly capacity. In particular, amphiphilic amino acid derivatives can be designed to enrich the diversity of amino acid-based building blocks, endowing them with specific properties and/or promoting self-assembly through hydrophobic interactions, hydrogen bonding, and/or π-stacking. In this review, we focus on the design of various amphiphilic amino acid derivatives able to self-assemble into different types of nanostructures that were exploited for biomedical applications, thanks to their excellent biocompatibility and biodegradability.

Side-chain effects on the co-existence of emergent nanopatterns in amino acid adlayers on graphene

The spontaneous tendency of amino acid adlayers to self-assemble into ordered patterns on non-reactive surfaces is thought to be chiefly influenced by amino acid termination state. Experiments have shown that different side chains can produce different patterns, with a distinction drawn between side chains that can support hydrogen bonds or electrostatic interactions, and those that are hydrophobic. However, as is demonstrated in this work, this distinction is not clear cut, implying that there is currently no way to predict in advance what type of pattern will be formed. Here, we use molecular dynamics simulations of amino acid adlayers in neutral, zwitterion, and neutral-zwitterion states for two types of amino acids, either histidine or alanine, adsorbed at the in-vacuo graphene interface. In contrast to earlier studies on adlayers of tryptophan and methionine on graphene that reveal the presence of only a single type of pattern motif, the canonical dimer row, here we find that emergent patterns of histidine and alanine adlayers supported the co-existence of several different types of motifs, influenced by the different side-chain characteristics. For alanine, the compact side-chain does not support hydrogen bonding and engages weakly with the surface, leading to the emergence of a new dimer row configuration in addition to the canonical dimer row motif. On the contrary, for histidine, the side-chain supports hydrogen bonding, leading to the emergence of a dimer row motif different from the canonical dimer row, co-existing with several different monomer row motifs. On this basis, we propose that emergent canonical dimer row patterns are more likely for amino acids with side-chains that are non-compact and that also lack extensive hydrogen bonding capacity, and that engage strongly with the underlying substrate. These findings provide a fundamental basis to rationally guide the design of desired self-assembled nanostructures on planar surfaces.