High Molecular Weight Protein-Like Semiflexible Chains via Bioorthogonal Polymerization of Coiled-Coil Peptides

Peptides capable of forming homotetrameric coiled-coil bundles are utilized as the monomeric building blocks ("bundlemers") to synthesize protein-like hybrid polymers consisting of covalently linked coiled-coil microdomains with regularly spaced ethylene glycol repeats via step-growth polymerization employing the highly efficient, bioorthogonal tetrazine (Tz) ligation with <i>trans</i>-cyclooctene (TCO). Polymerization of Tz and TCO-functionalized peptides in aqueous media under strict stoichiometry at Tz or TCO concentrations of 0.1 to 4.5 mM leads to the establishment of exceptionally long, semiflexible polymer chains with a Kuhn length of 6-7 nm and an apparent molecular weight up to 3 MDa. Bioorthogonal polymerization at bundlemer concentrations above 5 mM gives rise to physical gels through interchain entanglements. Hydrogels prepared at 10 mM exhibit an average elastic modulus of 400 Pa and a strain to failure of 300%. Copolymerization of coiled-coil peptides with distinct composition and thermal stability results in hydrogels that are thermally tunable. Solid-to-fluid transition occurs when one of the coiled-coil repeats melts. Upon cooling, solid-like properties are partially recovered through intermolecular association of the helical peptides. Overall, tetrazine ligation has enabled the covalent polymerization of self-assembled coiled-coil motifs for the establishment of protein-like linear polymers with unprecedented molecular weight.