Understanding Charged Polymer-Lipid Interactions in Model Membranes Revealed by EPR and Solid-State NMR: Implications...

Polymers such as Styrene Maleic Acid (SMA) have become valuable tools for studying membrane proteins within lipid bilayers due to their ability to solubilize membrane proteins in their native environments. However, limitations in the use of SMA copolymers have led to the development of SMA derivatives. In this study, we investigate the impact of the charge properties of several SMA derivatives on various lipid systems, which may also carry charges. We employed neutral, positively charged, and negatively charged SMA-derivative copolymers (SMA-Neut, SMA-Pos, SMA-AE, SMA Glu, and SMA-BZ30) to investigate their impact on bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), and a 9:1 molar ratio mixture of POPC and POPG. No single technique, however, is effective across the wide array of different MPs, and there are often significant discrepancies between structures determined by different methods if the protein's environment is far from a native lipid bilayer. In this study, we utilized biophysical techniques, including dynamic light scattering (DLS), electron paramagnetic resonance (EPR) spectroscopy, and solid-state nuclear magnetic resonance (ssNMR) spectroscopy, to analyze the interactions between polymers and lipid bilayers. Our EPR findings indicate that all the polymers preserved the POPC/POPG lipid bilayer. Meanwhile, our ssNMR result analysis suggests that neutral polymers may disrupt the bilayer less than charged polymers do. This implies that the electrostatic interactions between charged polymers and lipids can significantly alter the native environment of the bilayer, potentially impacting the stability and dynamics of encapsulated membrane proteins. Although this work primarily focuses on lipid systems, our results underscore the importance of considering polymer-lipid electrostatic interactions when selecting SMA derivatives and polymers for biophysical studies of membrane proteins, as the choice of polymer can profoundly affect the integrity and behavior of the lipid bilayer system.