Complex formation by alpha-lactalbumin and polysaccharide copolymers
Colloidal delivery systems, such as surfactant micelles, emulsions, liposomes, and nanoparticles, are widely used in food and pharmaceuticals to improve the dispersion of poorly-soluble ingredients or modify their bioaccessibility within the body. As a delivery system for medical- and health-relevant bioactive components, surfactant micelles have potential toxicity due to the disruption of cell membranes and interference with cell membrane transporters. Complexes formed among proteins and polysaccharides, as renewable biopolymers, are drawing attention for their potential application in the food, pharmaceutical, and cosmetic industries due to their non-toxicity and higher potential for rapid biodegradation. In our studies, carboxymethyl-dextran-block-poly(ethylene glycol) (CMD-b-PEG) and chitosan-graft-poly(ethylene glycol) (CH-PEG) were successfully prepared from dextran or chitosan (CH) by covalently attaching a poly(ethylene glycol) chain, which was confirmed by nuclear magnetic resonance spectroscopy and chromatography. Complex formation between polysaccharides and the globular dairy protein alpha-lactalbumin was investigated following acid or base titration, where complexes with unmodified chitosan (CH) or carboxymethyl-dextran (CMD) were used as controls. The successful assembling of complexes between alpha-lactalbumin and polysaccharides at different mixing ratios was confirmed through turbidity, light scattering, colloidal charge, and electron microscopy. Complex formation also occurred at a higher pH value between α-lac/CMD-b-PEG when compared to α-lac/CMD. This insensitivity of phase separation to r and the increased pH of complex formation implied that addition of the PEG chain to CMD increased the favorability of forming complexes with α-lac in a manner that was insensitive to the quantity of added CMD-b-PEG. It was proposed that α-lac/CMD-b-PEG formed a C3M structure that was surrounded by the non-ionic PEG chains, flocculated and separated from the continuous phase. Covalent attachment of the PEG chains to CH was found to decrease the turbidity of complexes in mid and lower molecular weights with all protein-polysaccharide ratios, while size of the formed complexes was reduced in specific protein-polysaccharide ratios or polysaccharides of certain molecular weights. Diameter of spherical complexes formed with CMD-b-PEG and CH-PEG was found to be about 10–40 nm, and assemblies of 200–500 nm were formed from apparent bridging interactions between spherical complexes. Heat treatment of the complexes, shown in prior studies with unmodified polysaccharides to lead to the formation of protein-rich spherical aggregates, was also studied to determine the influence of the attached PEG chain on the structure of heat-induced aggregates. Similar to the unheated complexes, the covalently-attached PEG chain had minimal influence on the size of formed spherical aggregates, yet the turbidity of the suspensions was reduced. These results demonstrate that non-interactive components of ionic polysaccharides have a significant impact on the formation of complexes with alpha-lactalbumin, which could translate to novel designs of vehicles for the controlled-delivery of active molecules.
Jones, Purdue University.
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