An application of small-angle neutron scattering: A study of the composition, structure, and conformation of canine tracheal mucus glycoprotein

Small-angle neutron scattering was used to study mucus glycoproteins, water-soluble biological copolymers whose intra- and intermolecular interactions in solution give rise to the gel structure and rheological properties necessary for the effective functioning of mucus. Canine tracheal mucus glycoprotein was purified from raw mucus secretions by density gradient ultracentrifugation and gel permeation chromatography. Contrast variation neutron scattering experiments were performed on mucus glycoprotein solutions by exchanging D$\sb2$O for H$\sb2$O in the solvent buffer. The intensity of neutrons scattered from the solutions was measured as a function of scattering vector. The chemical composition of the sample was investigated and the presence of a significant lipid component was indicated by matching point analysis. Stuhrmann analysis lead to the derivation of an extended low resolution structure. Scattering functions for simple geometric objects were fit to the data obtained from solvents of different isotopic ratios to develop models for molecular structure. The results were consistent with two structures--concentric oblate ellipsoids, consisting of a glycoprotein core surrounded by a lipid shell, and linear random coil glycoproteins associated with lipid micelles. The effect of the solvent environment upon the conformation of mucus glycoprotein in solution was investigated by comparing models developed for different conditions. Increasing ionic strength resulted in the contraction of the ellipsoid model and the lipid micelle. The dimensions of the glycoprotein chain were relatively unchanged. The effect of a specific ion, calcium, at constant total ionic strength was also studied. The glycoprotein core of the concentric ellipsoid was unaffected by calcium but the lipid shell became thicker. At low calcium levels the glycoprotein chain contracted, however as the concentration was increased further, it expanded. The size of the lipid micelle was independent of calcium. The structures of both purified and delipidated mucus glycoproteins and changes in conformation following the extraction of associated lipids were examined but the results were inconclusive