Proteins are capable of incredibly complex tasks enabled by their structure and exact display of chemical functional groups. Non-native materials applications of proteins are often limited owing to their sensitivity to denaturation upon chemical modification. Unlike most proteins, shorter peptide sequences are able to undergo denaturing and refolding cycles. This work investigated use of a peptide-based material that possesses the exact display of chemistry and self-assembling properties of proteins but is readily modified without negatively impacting the hierarchical structure. In previous work, 29-amino acid sequence peptides were computationally designed to form four-peptide coiled-coil assemblies or bundlemers. The bundlemers were stable over a range of temperatures and pH as an individual unit and were modified with maleimide and thiol functional groups to polymerize through a thiol-Michael reaction to form rigid-rods. Here, we investigate the effect of truncating the 29-amino acid coiled-coil sequence to 15-amino acids. Circular dichroism revealed that the 15-amino acid sequence only forms random-coils; however, subsequent bioconjugation of two 15-amino acid peptides at their N-termini resulted in extraordinarily long rigid-rods (2nm wide by multiple microns long), as observed by cryo-TEM. The observed polymerization is hypothesized to follow a chain-growth assembly mechanism that relies on the transient formation of unstable 15-amino acid coiled-coils that initiate/nucleate stable rigid-rod formation via overhanging 15 amino-acid random-coils. This pathway is unique from the previous 29-amino acid sequences, which form through a step-growth assembly pathway. The newly developed 15-amino acid peptide sequence accesses new assembly pathways for the creation of extraordinarily rigid polymers for broad applications ranging from nanowires to ultra-strong and ultra-light fibers.

Truncated coiled-coil peptides as a building block for hierarchical material synthesis

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