Synthesis of biomimetic co-polypeptides with tunable degrees of phosphorylation

Phosphorylated polypeptides represent promising biomimetic macromolecules for various regenerative applications. However to date, large-scale synthesis of phosphorylated polypeptides with controlled degrees of phosphorylation has not been achieved, restricting research in phosphorylated proteins to their central roles in biomineralization pathways. Here, we present a co-polypeptide synthesis strategy based on the Ring-Opening Polymerization (ROP) of N-carboxyanhydrides (NCAs), followed by controlled phosphorylation of serine (Ser) residues. The molecular design, including amino acid composition and molecular weight of polypeptides, mimicked the intriguing phosphorylated protein Pc-3 secreted by the sandcastle tube worm Phragmatopoma californica, which is a major constituent of the glue produced by the animal to bind hard particles together for their protective tubes. Pc-3 is comprised of mostly Ser and tyrosine (Tyr), with up to 70% of Ser residues phosphorylated into phospho-serine (pSer) giving rise to the high net negative charge of Pc-3. Three NCA monomers were synthesized, namely Ser with free -OH groups, and Ser and Tyr with protected -OH groups, and subsequently polymerized with various feeding ratios in order to obtain a broad range of final amino acid compositions. In the final step, phosphorylation targeting free -OH groups of Ser was conducted. With this strategy, the degree of phosphorylation is governed by the initial amount of unprotected -OH groups of the precursor Ser-NCA, and the final co-polypeptides contain relative amounts of Tyr and pSer that can be tailored, yielding a composition and molecular weight (MW) that closely match those of Pc-3. This control of phosphorylation leads to polypeptides exhibiting a wide range of zeta potential values between -20 and -50 mV. Using analytical assays, including Dynamic Light Scattering (DLS), Surface Plasmon Resonance (SPR), and Quartz Crystal Microbalance with Dissipation (QCM-D), we demonstrate that these phosphorylated polypeptides exhibit affinity towards divalent ions such as Ca²⁺, thus opening the door for their usage as scaffolds for mineralized tissue repair or as a major component of biocompatible adhesives.