Fucoidan, a complex polysaccharide found in a variety of brown seaweed species, comprises repeating L-units in the main chain and other monosaccharides and sulfates. Brown algae species are known conventional healthy foods, widely used globally due to their medicinal values. Due to the ubiquitous presence of brown algae, the chemical structures of fucoidan derived from a variety of algae have been extensively researched to expand fucoidan applications. Nevertheless, despite the popularity of brown algae as healthy foods with potential medical value, a thorough understanding of their fundamental physical properties is lacking. This can be attributed to the difficulty in obtaining purified fucoidal samples for effective characterization of the polymer as well as the diversity of the fucoidal chemical structures that present a challenge in determining its common characteristics.
The rheological properties of fucoidan have particularly attracted significant research attention. Such related studies have been mainly performed in aqueous solutions because their functions as medicinal foods mostly emerge in water. Previous results revealed electrostatic force between fucoidan chains dissolved in water and the significant effect of the nearby chain constraints and the entanglement coupling on the polymer chain motion. Moreover, there is seemingly no solvent apart from water for the preparation of fucoidan solutions, and this presents another challenge in the study of their rheological properties. Interestingly, some ionic solutions are speculated to be better solvents than water for the preparation of fucoidans. Ionic liquids exhibit numerous advantages like performing rheological measurements and are a promising solution for studying the rheological properties of fucoidan.
On this account, Professor Jun-ichi Horinaka, Ayuna Kimura (undergraduate student), Professor Toshikazu Takigawa from Kyoto University investigated the rheological behaviors of fucoidan solutions both in an ionic liquid medium and in water. Their main objective was to explore the differences in the flow behaviors between the ionic liquid and aqueous solutions in the concentrated regime. Their research work is published in the research journal, Polymer.
In their approach, fucoidal solutions were first derived from Cladosiphon okamuranus, a type of edible seaweed containing six fucose units and glucuronic acids as monosaccharide units. The concentrated solutions exhibiting entanglement coupling were then prepared using water and ionic liquid 1-butyl-3-methylimidazolium acetate (BmimAc) as solvents. Next, the dynamic viscosity of the fucoidal solutions was studied at different concentrations of the ionic liquid solutions, and the resulting data was used to determine the molecular weight between the entanglements. Additionally, the weight molecular entanglement for the molten state was also estimated based on the concentration dependence of the entanglement.
Results confirmed the existence of electrostatic interactions between the aqueous solutions and the fucoidan chains. However, it disappeared in the ionic liquid BmimAc solution, indicating the differences between the flow behaviors of the solutions. In both the solvents and at relatively higher concentrations, the rubbery plateau demonstrated the entanglement coupling. For fucoidan in the BmimAc solution, the molecular weight was determined at different concentrations using plateau modulus. Based on the concentration dependence of molecular weight between the entanglements, the molecular weight for fucoidan in the molten state was determined to be .
In a nutshell, the study investigated the dynamic viscosity of concentrated Cladosiphon okamuranus fucoidal solutions in ionic liquid and water to provide more insights into the rheological properties of fucoidan. The rheological data obtained was very useful in understanding the electrostatic interactions between the fucoidan chains and determining the molecular weight between the entanglements in different solutions. The value of the molecular weight for fucoidan in the molten state corresponded to the specific chain characteristics of fucoidan. In a statement to Advances in Engineering, Professor Jun-ichi Horinaka noted that the study provided a comprehensive understanding of the rheological properties of solutions and would benefit exploration and applications of its medicinal value.
Horinaka, J., Kimura, A., & Takigawa, T. (2020). Rheological properties of concentrated solutions of fucoidan in water and in an ionic liquid. Polymer, 211, 123090.