(64d) Complex Formation between Stereoregular Polyelectrolyte and Protein
Investigations of protein-polyelectrolyte complexes are important in the design of many biomedical applications that include enzyme immobilization, drug delivery, biosensor design, and in bioprocessing where complexation with polyelectrolytes is used to prevent protein aggregation and for protein purification. Predicting complexation requires understanding how this process depends on solution parameters such as pH, and the ionic strength of solution, and structural parameters such as polyelectrolyte chain stiffness, charge density, and hydrophobicity, as well as protein charge distribution, size, and possibly shape. However, isolating the effects of each of these parameters is difficult. In this work, we choose to study the effect of polyelectrolyte stereochemistry on the interactions between sulfonated polystyrene (PSS) and a set of different proteins. The advantage of studying the different stereo-forms of the polymers is that each of the polymers has a different persistence length and three-dimensional structure, whereas the hydrophobicity and charge density of the polyions are the same. Consequently, effects due to the former can be isolated from those due to the latter. We have sulfonated polystyrene with a unique stereochemical structure or tacticity, i.e (a) atactic polystyrene (aPS), in which the phenyl groups are located randomly on both sides of the polymer backbone chain, and (b) isotactic polystyrene (iPS), in which the phenyl groups are located on the same side of the polymer backbone chain. aPS is a typical amorphous polymer and the stereochemistry of atactic polystyrene shows some long range syndiotacticity. iPS is a crystalline polymer and in solid state, it has a conformation of 3-1 helix while in gel state it is more stretched and has a helix of 12-1 conformation. The sulfonated states of aPS and iPS, ie, aPSS and iPSS are soluble in water with local different structure. UV-vis, Static light scattering and ITC to monitor binding constants, SAXS to monitor conformations, and also, we use DSC to determine the effect of the complexation on the protein stability. The measurements are made for a range of proteins (such as ovalbumin, myoglobin and Cytochrome c) and for solvent conditions, such as pH, salt concentration, and salt type. Initial results show that the binding affinity to proteins of aPSS is larger than that of iPSS obtained with the overlapping binding model. Theoretical models and simulations are used to complement these studies and further delineate the different contributions to the interaction. Models will be especially useful in probing the structural details of the interaction and in elucidating the effect of polymer structure on the polymer-protein interactions.
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