Chemical and Microstructural Characterization of pH and [Ca2+] Dependent Sol-Gel Transitions in Mucin Biopolymer

Abstract

Mucus is responsible for controlling transport and barrier function in biological systems, and its properties can be significantly affected by compositional and environmental changes. In this study, the impacts of pH and cacl 2 were examined on the solution-to-gel transition of mucin, the primary structural component of mucus. Microscale structural changes were correlated with macroscale viscoelastic behavior as a function of pH and calcium addition using rheology, dynamic light scattering, zeta potential, surface tension, and ftiR spectroscopic characterization. Mucin solutions transitioned from solution to gel behavior between pH 4-5 and correspondingly displayed a more than tenfold increase in viscoelastic moduli. Addition of CaCl 2 increased the sol-gel transition pH value to ca. 6, with a twofold increase in loss moduli at low frequencies and tenfold increase in storage modulus. changing the ionic conditions-specifically [H + ] and [ca 2+ ]-modulated the sol-gel transition pH, isoelectric point, and viscoelastic properties due to reversible conformational changes with mucin forming a network structure via non-covalent cross-links between mucin chains. Mucin is a polyelectrolyte glycoprotein found in mucus, the structured complex fluid found in all types of organisms from bacteria to humans. In vertebrates, mucus is produced by mucus membranes and can be found lining the eyelids, mouth, and nose, as well as gastrointestinal, respiratory, and genital tracts. Since the 1970s 1 , animal mucus and mucin solutions have been well studied, especially the roles mucus plays in drug delivery 2-6 , disorders like cystic fibrosis 7,8 , and its protective biological functions 9-12. Mucin glycoproteins are present in mucus at concentrations of 1-5%, along with electrolytes (ca. 1%), lipids (1-2%), other proteins (1-2%), and water (90-95%) 13. Despite being present in low concentrations, mucin glycoproteins are primarily responsible for the protective and lubricative functions of mucus within the body. Solubilized mucin behaves as a complex fluid with changing viscoelastic and structural properties in response to its environmental conditions. Mucin glycoproteins are responsible for the majority of the physical properties of mucus, as mucin conformation, intra-and inter-strand bonding, and microstructure changes in response to environmental factors such as pH 6,7,12,14 , temperature 2,15 , and ion content 1,14,16. All these factors affect the reversible supramolecular bonding between functional groups within the protein strands, which modifies the microstructure as well as the mechanical and transport properties of the mucin network. In aqueous solutions, biopolymers like mucin form networks that can change dynamically due to non-covalent crosslinks, including physical entanglements and supramolecular bonding. Mucins resemble a bottle brush polymer with a protein backbone and oligosaccharide (carbohydrate) side chains arranged radially from the backbone. The protein backbone of the mucin biopolymer has areas that are dense w...

A Bio‐synthetic Hybrid Hydrogel Formed under Physiological Conditions Consisting of Mucin and a Synthetic Polymer Carrying Boronic Acid

“…1-100 Pa, which is close to that of animalmucus. [6,7,16] To elucidate the mechanism of hydrogelation, mucin and pDMA-2APBA polymer solutions were mixed in the presence of competitive sugars. Mucin (ca.…”

“…Mucins are categorized into various types including membrane-bound mucin, secreted mucin, and others. Mucin and other proteins are cross-linked through noncovalent bonds, e.g., disulfide bonds, ionic bonds, and hydrogen bonds [6,7] to form mucus. The main roles of animal mucins are: (1) physical barrier preventing tissues and organs from drying, harmful substances, and viruses, [8,9] (2) intercellular communication through molecular recognition, [10] and (3) living environment for symbiotic bacteria.…”

“…[12] Recently, mucin as a naturally derived macromolecule having such a unique structure and properties has been regarded as a building block of biomaterials, [10] as represented by coating for cell-repellent [13] or cell-selective [14] surfaces, and a component of therapeutic agent for wound healing. [15] Animal mucus forms through the interaction of mucin and nonmucin proteins via dynamic bonds, forming hydrogel having shear moduli in the order of 1-100 Pa. [6,7,16] Similarly, interactions between mucin and various macromolecules including cationic or mucoadhesive proteins, [17,18] polysaccharides, [19][20][21][22] and lectins [23,24] have been extensively studied. Generally, chemical modification is needed for mucins to be used as building blocks of bio-synthetic hybrid materials.…”

Modulation of Gastrointestinal Mucus Properties with Ionic Liquids for Drug Delivery

“…[41,42] pH changes induced by the addition of IL are important because of the reported effects of pH on mucus rheology, wherein acidic pH generally reduces the repulsions between mucins and increases mucus viscosity, while basic pH results in decreased viscosity. [43,44] This phenomenon is explained by the polyelectrolyte effect in mucus, wherein raising pH above the pK a of acidic amino acids on mucin results in deprotonation, and a rush of counterions to the exposed charges. The resultant solute concentration gradient leads to an influx of water, causing swelling of the mucus structure and a reduction of viscosity.…”

Double-Cross-Linked Networks Based on Methacryloyl Mucin

“…Considering the well-researched influence of a medium’s pH on mucin behavior [ 31 , 32 , 33 ], in this work, hydrogels are prepared using media with different pH values. The synthesized SN systems are further incubated in a TA solution, and the properties of the obtained DCN hydrogels are compared with those of the SN counterparts.…”