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

Biology provides many sources of inspiration for synthetic and multi-functional nanomaterials. Naturally evolved proteins exhibit unique, sequence-defined functions and self-assembly behavior. Some microbial protein filaments are electronically conductive, evolved to support access to remote electron acceptors outside the cell, which serve as a design platform for bioelectronic materials with applications in biosensors and enzymatic electrocatalysis. Recapitulating their self-assembly and conductive 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 random coil to α-helical peptide structure. The ordered state of the peptide serves as a building block for assembly of coiled coils and higher order assemblies. Cryo-EM structures of these structures reveal a novel organization of α-helical peptides in a cross coiled coil (CCC) arrangement that is unprecedented in de novo and natural protein designs. Structural analysis also reveals a -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 characterization show that CCC fibers are electronically conductive. Single-fiber conductive AFM measurement indicate 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.