Spontaneous nickel-catalyzed dityrosine crosslinking to form highly tunable elastin-like polypeptide hydrogels

Various crosslinking methods have been used to form elastin-like polypeptide (ELP) hydrogels. Tyrosine-based crosslinking creates ELP scaffolds that are mechanically strong and tunable. While nickel-catalyzed dityrosine crosslinking has been used for small-scale peptide oligomerization, its potential for crosslinking large polymeric networks remains unexplored. Here, we adapted this chemistry to synthesize macroscopic ELP hydrogels. Specifically, the recombinant ELP monomer was designed to contain numerous tyrosine residues. Upon the addition of an oxidizer, Ni(II) ions, and the tripeptide GGH to ELPs, spontaneous crosslinking occurs. The histidine in the GGH tripeptide forms a coordination complex with Ni(II) to act as a catalyst. We optimized reactant concentrations using a one-factor-at-a-time (OFAT) approach. Because the GGH-Ni(II) complex remains in the liquid phase and is not covalently integrated into the solid hydrogel, it can be extracted post-gelation. To address the inherent toxicity of the residual metal, we engineered a continuous washing process that drastically improved biocompatibility by removing up to 99% of the Ni(II) ions. Subsequent evaluation of the physical, biological, and mechanical properties demonstrated highly tunable matrix mechanics. Overall, this work presents a novel Ni(II)-based ELP hydrogel synthesis method that can be tailored for a wide variety of biomedical applications, adding a versatile new reaction to the dityrosine crosslinking toolkit that can be applied to other phenol-containing polymers.