Characterising the Macromolecular Interactions between Proteins and Polysaccharides

Abstract

The major focus of this thesis is to investigate the macromolecular interactions between model protein βLgA and model polysaccharide pectin when they were oppositely charged and the microstructures of the complexes formed between the two biopolymers. Firstly, interactions between βLgA and pectins were studied in solution. We found that βLgA only interacted with pectins with an adequate amount of charge, and that the complexation between βLgA and pectin was a two-step process that lead to the formation βLgA-pectin complexes with two different structures. It was also revealed that the complex coacervates were made by the compact primary particles comprising overlapped pectin chains and βLgA-rich clusters that brought individual protein-binding pectin chains together. Both the overall the local charge densities of pectin have impacts on the distribution or orientation of βLgA-rich clusters within the primary particles. We then simplified the structural heterogeneity of the protein-polysaccharide system by investigating βLgA binding to the oligo-galacturonic acids (OGA). We found that βLgAOGA interaction was critically dependent on the length of the OGA, a minimum length of 7 or 8 sugar residues was required in order to exhibit appreciable exothermic interactions with βLgA. Two structural regimes were identified during βLgA to OGA titration. The interactions between βLgA and pectins with various charge properties were also investigated at the solid-liquid interface by building layer-by-layered (LbL) structures on hydrophilic silica surface in a ‘protein-polysaccharide-protein’ fashion. We found that the βLgA/pectin LbL assembly was structurally homogeneous with individual βLgA and pectin layers interpenetrated one and another. The compactness or density of the multilayered film were controlled by the charge properties of the pectin. Finally, different from the previous studies on the characterisation of protein and polysaccharide interactions, covalently linked protein-polysaccharide conjugates were synthesized as potential scaffolds for drug delivery systems. In this study, we conjugated hydrophobic protein zein with hydrophilic pectin polysaccharide through a specific covalent linkage between the reducing end of pectin and the amine groups of zein. It was anticipated that the conjugates could be used for developing dual-delivery system for both hydrophobic and hydrophilic bioactives.