Effects of Hydration on Structure and Phase Behavior of Pig Gastric Mucin Elucidated by SAXS

In this work small-angle X-ray scattering (SAXS) was used to study hydration and temperature-induced changes of pig gastric mucin (PGM) within the entire concentration range. The scattering is interpreted as originating from PGM fiber-like structures that adopt rod-like bottle-brush conformation in dilute solutions. On the basis of the knowledge about molecular structure of mucins and SAXS data for dilute solutions, we propose a theoretical model for predicting mucin conformation in solution and calculate the corresponding scattering profile. This bottle-brush model comprises a protein backbone with carbohydrate side chains and corresponding structural parameters, such as grafting distance and lengths of the backbone and side chains. It describes the experimental PGM data from dilute solutions in the full q range very well. It furthermore suggests that the carbohydrate side chains are grafted with a regular separation of around 5 nm and a length of 14 nm. The cross-section size with a radius of about 1 nm is also in accordance with the size of the carbohydrate units. Structuring of PGM solutions at higher concentrations was investigated by analyzing semidilute and concentrated PGM samples. Starting at about 20 wt %, Bragg peaks become clearly visible indicating a more ordered mucin system. In very dehydrated and fully dry mucin samples these peaks are not present indicating lack of long-range order. The SAXS data show that the structural change occurring at about 80 wt % mucin and 25 °C corresponds to a glass transition in agreement with our previous calorimetric results. Temperature also has an effect on the phase behavior of mucin. At intermediate levels of hydration, a phase transition is observed at about 60-70 °C. The main Bragg peak appears to split in two, indicating formation of a different structure at elevated temperatures. These findings are used to improve the PGM-water phase diagram.