Hierarchical Assembly of Conductive Fibers from Coiled-Coil Peptide Building Blocks

Biology provides many sources of inspiration for synthetic and multifunctional nanomaterials. Naturally evolved proteins exhibit specialized, sequence-defined functions and self-assembly behavior. Recapitulating their molecularly defined self-assembly behavior, however, is challenging in de novo proteins. Peptides, on the other hand, represent a more well-defined and rationally designable space with the potential for sequence-programmable, stimuli-responsive design for structure and function, making them ideal building blocks of bioelectronic interfaces. In this work, we design peptides that exhibit stimuli-responsive self-assembly and the capacity to transport electrical current over micrometer-long distances. A lysine-lysine (KK) motif inserted at solvent-exposed positions of a coiled-coil-forming peptide sequence introduces pH-dependent control over a transition from unordered to α-helical peptide structure. The ordered state of the peptide serves as a building block for the assembly of coiled coils and higher-order assemblies. Cryo-EM structures of these structures reveal a hierarchical organization of α-helical peptides in a cross coiled coil (CCC) arrangement. Structural analysis also reveals a β-sheet fiber phase under certain conditions and placements of the KK motif, revealing a complex and sensitive self-assembly pathway. Both solid-state and solution-based electrochemical characterizations show that CCC fibers are electronically conductive. Single-fiber conductive AFM measurement indicates that the solid-state electrical conductivity is comparable with bacterial cytochrome filaments. Solution-deposited fiber films approximately doubled the electroactive surface area of the electrode, confirming their conductivity in aqueous environments. This work establishes a stimuli-responsive peptide sequence element for balancing the order-disorder transitions in peptides to control their self-assembly into highly organized electronically conductive nanofibers.