Identification of subunits

The study of polysaccharide structure usually focuses on the chemical composition, the linkage between the monosaccharide units, and the size and shape of the molecule. The size and shape of a polysaccharide can be ascertained by techniques that are usually applied to large molecules; e.g., the most accurate molecular weight determination measures the sedimentation properties of the molecule in an applied gravitational field (e.g., the rate at which a solid material is deposited from a state of suspension or solution in a liquid). Indications of the shape of polysaccharide molecules in solution are obtained from viscosity measurements, in which the resistance of the molecules to flow (viscosity) is equated with the end-to-end length of the molecule; the viscosity of hyaluronic acid, for example, shows a marked dependence on both concentration of the acid and the salt content of the solution, and, under conditions approximating those found in biological systems, a hyaluronic acid molecule may be thought of as occupying a great deal of space. Alternatively, the compact nature of a glycogen molecule of molecular weight equal to that of a molecule of hyaluronic acid results in its accommodation to a much smaller space than the latter molecule.

The identification of sugars in a mixture resulting from the hydrolytic breakdown of a heteropolysaccharide is most often carried out by chromatography of the mixture on paper, silica gel, or cellulose. Ready separations can be achieved between pentoses, hexoses and, for example, deoxy sugars; closely related compounds such as D-glucose and D-galactose also can be separated using chromatographic techniques. The linkage positions in polysaccharides are usually determined using the methylation procedure described previously. The various monosaccharide methyl ethers produced by the methylation are separated by gas–liquid chromatography.

Detailed statements about polysaccharide structure and function are limited by the statistical nature of some measurements (e.g., branching frequency), the biological variability of parameters such as size and molecular weight, and incomplete information about associative interactions in living things.