Amphiphilic molecules with aromatic amide backbones (“aramid amphiphiles”, or AAs) have been shown to self-assemble in water, forming ultra-robust nanoribbons with undetectable molecular exchange. It was assumed that the unusual mechanical stability of AA nanoribbons is due to strong intermolecular interactions between aramid domains. Correspondingly, we expected crystalline solid-like rigidity within the nanoribbon aramid backbone, while the aliphatic surface hydrophilic domains should remain flexible. However, characterization of conformational dynamics with sub-nanometer resolution remained necessary to experimentally validate these expectations. In this study, we installed nitroxide spin labels at different depths in an AA nanoribbon and measured electron paramagnetic resonance (EPR) spectra to investigate local conformational dynamics. We used an in-house spectral fitting program to quantify the rotational diffusion rates at each spin label site. By constructing Arrhenius plots through measurement of EPR spectra with variable temperature, we extracted site-specific rotational activation energy barriers. Room-temperature rotational diffusion rates and activation energies are consistent with a dynamically diverse nanostructure in which the nanoribbon interior is solid-like while the surface is fluid and thus kinetically labile. This study demonstrates the utility of conformational Arrhenius analysis as a tool in local phase determination in polyphasic supra- and macromolecular materials.

Electron paramagnetic resonance spectroscopy reveals polyphasic behavior in self-assembled small molecule nanoribbons

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ACS Fall 2022

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