Influence of Peptoid Sequence on the Mechanisms and Kinetics of 2D Assembly

Abstract

Two-dimensional (2D) materials have attracted intense interest due to their potential for applications in fields ranging from chemical sensing to catalysis, energy storage, and biomedicine. Recently, peptoids, a class of biomimetic sequence-defined polymers, have been found to self-assemble into 2D crystalline sheets that exhibit unusual properties, such as high chemical stability and the ability to self-repair. The structure of a peptoid is close to that of a peptide except that the side chains are appended to the amide nitrogen rather than the α carbon. In this study, we investigated the effect of peptoid sequence on the mechanism and kinetics of 2D assembly on mica surfaces using in situ AFM and time-resolved X-ray scattering. We explored three distinct peptoid sequences that are amphiphilic in nature with hydrophobic and hydrophilic blocks and are known to self-assemble into 2D sheets. The results show that their assembly on mica starts with deposition of aggregates that spread to establish 2D islands, which then grow by attachment of peptoids, either monomers or unresolvable small oligomers, following well-known laws of crystal step advancement. Extraction of the solubility and kinetic coefficient from the dependence of the growth rate on peptoid concentration reveals striking differences between the sequences. The sequence with the slowest growth rate in bulk and with the highest solubility shows almost no detachment; i.e., once a growth unit attaches to the island edge, there is almost no probability of detaching. Furthermore, a peptoid sequence with a hydrophobic tail conjugated to the final carboxyl residue in the hydrophilic block has enhanced hydrophobic interactions and exhibits rapid assembly both in the bulk and on mica. These assembly outcomes suggest that, while the π−π interactions between adjacent hydrophobic blocks play a major role in peptoid assembly, sequence details, particularly the location of charged groups, as well as interaction with the underlying substrate can significantly alter the thermodynamic stability and assembly kinetics.

Two-Dimensional Molecular Architectonics through Controlled Noncovalent Interactions: Structure, Functional Properties, and Applications

“…Structural analyses revealed that the nanosheet surfaces were sheltered by the hydrophilic heads, whereas the hydrophobic tails were shown to be at the interior of the nanosheets. A recent study outlined the successful formation of 2D sheet structures involving amphiphilic peptoid sequences ( 79 – 81 ) at solution–solid interfaces via hydrophobic, π–π stacking, and hydrogen bonding interactions . However, all three sequences displayed similar growth patterns, and the rate of surface coverage was observed to be the highest for sequence 79 , followed by 80 and then 81 at similar concentrations (Figure e–g).…”

“…Copyright 2020 Royal Society of Chemistry. Panels (e–g) were adapted from ref . Copyright 2024 American Chemical Society.…”