Analytical Ultracentrifugation Study of Complex Formation between Like-Charged Glycinin and Polyelectrolytes: Impact of Ionic Strength and Chain Length

Abstract

In food systems, complexes formed between soybean protein and polyelectrolytes are extensively utilized. However, the mechanisms governing their interactions remain elusive, especially when they possess similar net charges. This study investigates the complexation between like-charged glycinin (11S) and diblock copolymers of polyethylene glycol-block-poly(sodium styrene sulfonate) (PEG-b-PSS) using analytical ultracentrifugation (AUC), a first-principle technique that examines macromolecular characteristics such as molecular weight, stoichiometry, and binding properties. The results reveal that 11S can interact with polyelectrolytes of the same net charges. At ionic strengths above 0.1 M, under a specific concentration of PEG-b-PSS with a PSS degree of polymerization of 33, reducing ionic strength augments the binding strength between PEG-b-PSS and 11S while enhancing the electrostatic repulsion between complexes. Consequently, association occurs as ionic strength decreases from 0.5 to 0.3 M due to the predominance of binding strength, and the association disappears at 0.1 M because electrostatic repulsion becomes predominant. Moreover, PEG-b-PSS with a higher degree of polymerization in the PSS blocks facilitates the formation of insoluble complexes of 11S via soluble intermediates. At an ionic strength of 0.025 M, PEG-b-PSS induces the dissociation of 11S into subunits and peptides. This study demonstrates that by controlling the ionic strength and the degree of polymerization of the polyelectrolyte, it is possible to regulate the binding strength between 11S and polyelectrolyte, as well as the structural changes during binding. Furthermore, this study contributes to understanding the complex formation mechanism between soybean protein and polyelectrolytes, and further expands the application of AUC in studying their binding.