Reversible, Chemically Triggered Water Retention in Disulfide Cross-Linked Synthetic Conserved Anterior Mucus Proteins (SCAMPs)

Abstract

Water-responsive (WR) materials change their morphology and properties in response to variations in relative humidity (RH). Myriad biological processes, including lubrication, adhesion, and ion exchange, rely upon mucus hydrogels, the most prevalent, natural, WR material, and there is substantial interest in mucus hydrogels for biomedical and materials applications. The WR properties of mucus hydrogels arise from the structure of mucins, proteins with O-linked glycans on serine/threonine residues and cysteine (Cys)-mediated cross-linking. However, natural mucuses present significant challenges for materials and biomedical use because of their heterogeneity, instability, and limited scalability. As such, there is a need for synthetic materials that can recreate the structures and WR properties of their natural counterparts. We report a series of polymers, poly(β-Gal-Thr)m-r-poly(Cys-H)n, that are inspired by conserved anterior mucus proteins (CAMPs) found in snail mucus and that possess properties ─including water adsorption, stiffness, and effective work of adhesion─that are similar to natural mucus hydrogels and that are mediated by reversible, biomimetic disulfide cross-linking. The polymerization conditions─involving the ring opening of N-carboxyanhydride monomers to form glycosylated polypeptides─enables independent control over polymer length (m + n) and monomer ratio (m:n). The polymers were characterized by 1H NMR spectroscopy, mass spectrometry, and gel permeation chromatography, and all of the data are consistent with the proposed structures. The tunable mechanical and WR properties were measured using atomic force microscopy (AFM) and dynamic vapor sorption (DVS). Notably, as RH increases, the poly(β-Gal-Thr)m-r-poly(Cys-H)n polymers absorb water, with poly(β-Gal-Thr)78-r-poly(Cys-H)4 doubling in mass with hysteresis between the absorption and desorption profiles, indicative of a porous structure. Cross-linking to form poly(β-Gal-Thr)m-r-poly(Cys-D)n substantially decreases the water adsorption of the polymers, but the water adsorption can be reestablished by reducing the disulfide cross-links. These SCAMPs represent a significant step toward mimicking the complex structures and WR properties of natural mucus using synthetic analogs, and this work has implications for the design and implementation of synthetic mucus in biomaterials, biotechnology, and medicine.