Correlation of chemical composition and optical rotation of water-soluble galactomannans

A quantitative relationship can be obtained between the rotatory power and the molar ratio mannose/galactose for galactomannans having a structure consisting of a backbone of β-(1 → 4)-linked d -mannopyranose units with side chains of α-(1 → 6)-linked d -galactopyranose residues, by using an equation based on the principle of optical superposition and derived originally by Timell for native xylans. By adopting the usually accepted errors for the determination of optical rotations (±5° for polysaccharides) and the percentages of the sugar units (±5%), the natural, water-soluble galactomannans can be grouped into two classes, namely, those for which the differences between the experimental and calculated rotations lie in the range of the experimental error, and those for which the above differences are higher than the experimental error. For the last class, the existence of structural details not considered in the basic pattern used for the correlation is suggested.