Solvent effects on the viscoelastic behavior of porcine submaxillary mucin

Rheological methods have been used to investigate the intermolecular interactions of porcine submaxillary mucins (PSM) in solution. PSM is a high molecular weight glycoprotein consisting of a linear, semi-flexible protein backbone to which a large number of oligosaccharides (1-5 saccharide units) are attached as side chains. Concentrated aqueous solutions of PSM containing different amounts of guanidine hydrochloride (GdnHCl) were subjected to both controlled stress and controlled strain rheological analyses. In the absence of GdnHCl, PSM solutions exhibit viscoelastic properties characteristic of a gel: the storage modulus, G', is much larger than the loss modulus, G", at all deformation frequencies, and the compliance is 100% recoverable at small stresses, indicative of strong intermolecular interactions. In 3.0 M aqueous GdnHCl, PSM forms a viscoelastic solution, with G" > G' at all frequencies and a relatively small recoverable compliance, pointing to disruption of the intermolecular interactions by the chaotropic salt. Intermediate behavior is observed in 1.5 M GdnHCl, characteristic of a marginal gel: G' approximately G" and greater than 50% recoverable compliance. In dilute solution, PSM behaves viscoelastically as a typical polyelectrolyte. However, concentrated solutions are turbid, the turbidity decreasing as GdnHCl is added, indicating that extensive intermolecular association accompanies the gelation process. The results show that although PSM is secreted in nature as a viscous solution, it can form gels that are similar to those of tracheobronchial and gastric mucins, and suggest common features to the gelation mechanism, with the strength of the gel correlated with the length of the oligosaccharide side chains.