Interaction and complexation of fucoidan and (3-Lactoglobulin monitored by turbidity and time-resolved small angle X-ray scattering

Abstract

A novel methodology of using turbidity and time-resolved small angle X-ray scattering in combination with D-glucono-δ-lactone (GDL)-induced gradual pH drop was applied for the first time to continuously monitor the dynamic association process of polyelectrolyte and protein. Two fucoidans from brown seaweed species Sargassum mcclurei (Fsm) and Fucus serratus (Ffs) with a backbone of alternating (1 → 3)- and (1 → 4)-linked residues and (1 → 3)-linked α-L-fucopyranose residues, respectively, were used to prepare complexes with β-Lactoglobulin (β-Lg). FT-IR indicated that the complex formation between fucoidan and β-Lg is mainly driven by electrostatic interactions and supported by hydrogen bonding. The successful monitoring by both techniques suggested that the complexation process moved from the molecular/nanoscale association (seen by small angle X-ray scattering) to macroscopic aggregation (seen by turbidity) as the pH drops. Further microstructural characterization of the fucoidan-protein complexes was confirmed via DLS, SEM and TEM. The results indicated that the formation of β-Lactoglobulin-fucoidan complexes is strongly dependent on the fucoidan's structure. The higher Mw, higher sulfate content and simpler structure of Ffs resulted in the early and gradual formation of a large, inhomogeneous complex network, while the lower Mw, lower sulfate content and alternating structure of Fsm exhibited a sharp transition from a small complex state to the rapid formation of large aggregates later in the pH drop. Quantitative analysis using the Debye-Bueche function revealed that the inhomogeneity length, a for Ffs grows from the initial binding stage, whereas Fsm shows no initial inhomogeneity until a critical pH is reached. These results confirm that the structure and molecular weight of the polysaccharide strongly decide the pH-dependent aggregation.