Regulating the Topology of Polypeptide Nanotoroids through Solvent-Triggered Nanofiber Twisting

Nanotoroids have attracted increasing attention recently; however, rounded nanotoroids are typically obtained in most cases. It is desirable to develop a new preparation method for nanotoroids to enrich their topological diversity. In this study, we report solvent-triggered polypeptide nanofiber twisting into nanotoroids with diverse topologies, which is governed by the content, addition rate, and type of inducer solvents. The nanofibers are prepared in an aqueous solution using dimethylformamide as the initial solvent, and the inducer solvent includes tetrahydrofuran, ethyl acetate, 1,4-dioxane, and dimethyl sulfoxide. The inducer solvents exhibit lower solubility for the polypeptides than dimethylformamide. The addition of the inducer solvents induces a transition in the arrangements of the pendant phenyl groups of polypeptides from highly extended to less extended, which generates internal stress that serves as the driving force for nanofiber twisting. Simultaneously, the polypeptide chains in the nanofibers gradually relax, reducing the contact area with the surrounding solvent and dissipating internal stress. Therefore, at a high content and a rapid addition rate of the inducer solvents, the effective driving force for nanofiber twisting is sufficient to yield multitwisted nanotoroids, whereas low content and slow addition favor rounded nanotoroids. This work not only presents a method for controlling the topology of polypeptide nanotoroids but also deepens our understanding of the role of solvents in affecting self-assembled nanostructures.