Mechanism of Side Chain-Controlled Proton Conductivity in Bioinspired Peptidic Nanostructures.

Bioinspired peptide assemblies are promising candidates for use as proton-conducting materials in electrochemical devices and other advanced technologies. Progress toward applications requires establishing foundational structure-function relationships for transport in these materials. This experimental-theoretical study sheds light on how the molecular structure and proton conduction are linked in three synthetic cyclic peptide nanotube assemblies that comprise the three canonical basic amino acids (lysine, arginine, and histidine). Experiments find an order of magnitude higher proton conductivity for lysine-containing peptide assemblies compared to histidine and arginine containing assemblies. The simulations indicate that, upon peptide assembly, the basic amino acid side chains are close enough to enable direct proton transfer. The proton transfer kinetics is determined in the simulations to be governed by the structure and flexibility of the side chains. Together, experiments and theory indicate that the proton mobility is the main determinant of proton conductivity, critical for the performance of peptide-based devices.

Self-Assembled Peptide Nanotube Films with High Proton Conductivity.

Design flexibility and modularity have emerged as powerful tools in the development of functional self-assembled peptide nanostructures. In particular, the tendency of peptides to form fibrils and nanotubes has motivated the investigation of electron and, more recently, proton transport in their fibrous films. In this study, we present a detailed characterization by impedance spectroscopy of films of self-assembled cyclic octa-d,l-α-peptide self-assembled nanotubes with amine side chains that promote proton transport. We show that the conductivity of the peptide nanotube film, which is in the range of 0.3 mS cm-1, is within the same order of magnitude as that of ultrathin films of Nafion, a benchmark proton conducting polymer. In addition, we show that while slow diffusion processes at the interface are present for both films, additional interface effects occur in the peptide nanotube films at the same rate as their bulk proton transport effects, further limiting charge transport at the interface. Overall, our studies demonstrate the great potential of using peptides as building blocks for the preparation of bioinspired supramolecular proton conducting polymers with improved conductivity with respect to that of natural systems.