The Structure of Pig Gastric Mucus Conformational Transitions Induced by Salt

The two major mucoprotein components ( s 0 25,w = 18.7 S and 4.9 S, respectively) of a watersoluble extract of hog‐gastric mucus have been investigated as to their behaviour in aqueous KCl solutions of differing concentrations. The values of viscosity for the unfractionated, watersoluble mucus extract undergo two sharp changes, one between 0 and 0.05 M KCl which is attributed to a polyelectrolyte charge‐shielding phenomenon, and one between 0.5 and 1.5 M KCl. The isolated 18.7 S and 4.9 S mucoprotein components of the mucus show no change in molecular weight when measured in 0.2 M KCl and 2.5 M KCl, but both undergo a marked change in hydrodynamic properties in this range of salt concentration. A large decrease in the ratio K s /η, (where K s = (1/ s 0 ) (d s /d c ) and η is the intrinsic viscosity at zero shear stress) is interpreted in terms of a marked increase in asymmetry accompanied by reduced solvation of the mucoproteins. This transition is only partially reversible, indicating the occurrence of definite structural changes within the molecule. The viscosity at zero shear of the 18.7 S mucoprotein becomes anomalously high at concentrations above 3 mg ml −1 , reflecting the ability of mucus to form a gel structure under physiological conditions.