Molecular weight dependent structure of the exopolysaccharide levan.

Levan is a bacterial homopolysaccharide, which consists of β-2➔6 linked β-(D)-fructose monomers. Because of its structural properties and its health promoting effects, levan is a promising functional ingredient for food, cosmetic and pharmaceutical products. The properties of levan have been reported to be linked to its molecular weight. For a better understanding of how the molecular weight determines its polymer conformation in aqueous solution, levan produced by the food grade acetic acid bacterium Gluconobacter albidus TMW 2.1191 was analysed over a broad molecular weight range using dynamic and static light scattering and viscometry. Low molecular weight levan exhibit a compact random coil structure. As the molecular weight increases, the structure transforms into a compact non-drained sphere. The density of the sphere continues to increase with increasing molecular weight. This resulted in a negative exponent in the Mark-Houwink-Sakurada Plot. For the first time, an increase in molecular density with increasing molecular weight, as determined by a negative Mark-Houwink-Sakurada exponent, can be shown for biopolymers. Our results reveal the unique properties of high-molecular weight levan and confirm the need of further systematic studies on the structure-function relationship of levan for its targeted use in food, cosmetic and pharmaceutical applications.