Microstructural study of mucin: influence of pH, temperature, concentration and nanoparticles

Abstract

The aim of this research is to examine the nano- and micro-structure of mucin in response to chemicophysical changes within the aqueous solution. By characterizing the mucin structure across a range of length scales relationships can be developed and related to the macroscopic properties. Mucin is a glycoprotein found in mucus at very low concentrations but yet is responsible for the majority of the physical properties of mucus. Therefore, the biological behavior (e.g., barrier properties) of mucus can be related to the reversible structural changes—viable dynamic bonding—that occurs within and between mucin strands. In the present work, the impact of pH, temperature and nanoparticle addition were examined on the structure of porcine gastric mucin (PGM) in aqueous solution. Findings are presented regarding the influence of (i) temperature and concentration on mucin aggregation and gelling properties, (ii) pH on surface tension, aggregation, and network structure, and (iii) addition of silica nanoparticles (SiNPs) and magnetic nanoparticles (MNPs) on mucin aggregation and rheology. The samples were characterized using rheology, dynamic light scattering (DLS) particle measurements, zeta potential (ZP), Fourier-transform infrared spectroscopy (FTIR), and pendant drop surface tension measurements. Rehydrated purified mucin is used in this study as it is structurally comparable to native mucus and readily available. Mucin concentrations were examined at 1, 2, and 5 wt% for the pH and temperature study. For the nanoparticle experiments, the mucin concentration was held constant at 1 wt% and the nanoparticle concentration varied. It is shown that changes in the microstructure of PGM due to concentration, temperature, and pH can be observed through surface tension, rheological, DLS, and ZP measurements. By varying the pH, the PGM solution could be induced to undergo a sol-gel transition, and PGM strands were found to aggregate and form a network structure under acidic conditions. DLS and ZP data showed that at the isoelectric point, PGM displayed the largest mean diameter by number as is expected when there is no longer charge repulsion and aggregation is induced. Increased temperature and concentration also influenced the viscoelastic properties of the mucin solutions. The sol-gel transition was observed to occur at higher frequencies for samples at higher PGM concentrations and temperatures. Finally, nanoparticle interaction with PGM showed an increase in gelling behavior and swelling of mucin in solution.