Enzyme-Mediated Conjugation of Peptides to Silk Fibroin for Facile Hydrogel Functionalization

Enzymatic crosslinking of tyrosine is a simple and modular method for adding functional peptides to silk fibroin (SF) hydrogels. Silk fibroin is a naturally derived polymer notable for its robust mechanical properties, biological compatibility, and versatility. Hydrogels fabricated from SF are elastic, optically clear, and have tunable moduli, however, they do not contain native biological epitopes for cell interactions. In this work we demonstrate the attachment of peptides to SF hydrogels through crosslinking of tyrosine with horseradish peroxidase (HRP) and hydrogen peroxide (H₂O₂). The goal was to understand the utility of this approach and to study how the addition of peptides affects the SF material properties. SF hydrogels conjugated to model peptides with different molecular weights and hydrophobic properties were studied by liquid chromatography/tandem mass spectroscopy (LC-MS/MS) (bond formation), fluorescent imaging (spatial distribution), Fourier transform infrared spectroscopy (FTIR) (protein secondary structure), and rheology (gelation time, modulus). As a proof of concept using a biologically relevant peptide, a peptide containing the cell binding domain Arg-Gly-Asp (RGD) was conjugated to SF, and the density and morphology of primary human fibroblasts were assessed. This work demonstrates a facile method for adding peptides to silk fibroin that can be adopted for a variety of biomaterials applications.