Heterogeneous structure and distinctive rheological behavior of loach skin mucin aqueous solution

Biological mucus usually has a gel like structure composed of huge amphiphilic mucoproteins, which determines the functional properties of mucus. However, the micro-/nano- structures of mucin, as the gelator of biological mucus, are not yet fully understood. In this work, we isolated mucin from loach epidermal mucus using organic solvents alone, without degrading mucin macromolecules. It reveals that loach skin mucin (LSM) self-assembles into long fibrils (up to 1 μm) in dilute aqueous solution. As the concentration increases, the LSM sequentially exhibits linear fibrils, branched fibrils, and eventually a fibrillar network structure in water due to the lateral hydrophobic association between fibrils. Due to the heterogeneity in lateral packing degree, the fibrillar networks generated comprises two structural elements: tightly packed fibrillar bundles and looser fibrils. The hierarchical structure contributes to the distinctive rheological behavior (e.g. rheopectic and thixotropic response) and potentially influences its mesh size. The mucin-based fibrillar network elucidated here may therefore offer new fundamental insights into the design principles of molecular gels across length scales.