Structure-Rheology Relationship in Nanosheet-Forming Peptoid Monolayers

Peptoid nanosheets are novel protein-mimetic materials that form from the supramolecular assembly of sequence-defined peptoid polymers. The component polymer chains organize themselves via a unique mechanism at the air-water interface, in which the collapse of a compressed peptoid monolayer results in free-floating, bilayer nanosheets. To impart functionality into these bilayer materials, structural engineering of the nanosheet-forming peptoid strand is necessary. We previously synthesized a series of peptoid analogues with modifications to the hydrophobic core in order to probe the nanosheet tolerance to different packing interactions. Although many substitutions were well-tolerated, routine surface pressure measurements and monolayer collapse isotherms were insufficient to explain which molecular processes contributed to the ability or inability of these peptoid analogues to form nanosheets. Here, we show that surface dilational rheology measurements of assembled peptoid monolayers at the air-water interface provide great insight into their nanosheet-forming ability. We find that a key property required for nanosheet formation is the ability to assemble into a solidlike monolayer in which the residence time of the peptoid within the monolayer is very long and does not exchange rapidly with the subphase. These collapse-competent monolayers typically have a characteristic time of diffusion-exchange values, τ_D, of >5000 s. Thus, rheological measurements provide an efficient method for assessing the nanosheet-forming ability of peptoid analogues. Results from these studies can be used to guide the rational design of peptoids for assembly into functional nanosheets.