A comparative characterisation of commercially available lipid-polymer nanoparticles formed from model membranes

From the discovery of the first membrane-interacting polymer, styrene maleic-acid (SMA), there has been a rapid development of membrane solubilising polymers. These new polymers can solubilise membranes under a wide range of conditions and produce varied sizes of nanoparticles, yet there has been a lack of broad comparison between the common polymer types and solubilising conditions. Here, we present a comparative study on the three most common commercial polymers: SMA 3:1, SMA 2:1, and DIBMA. Additionally, this work presents, for the first time, a comparative characterisation of polymethacrylate copolymer (PMA). Absorbance and dynamic light scattering measurements were used to evaluate solubilisation across key buffer conditions in a simple, adaptable assay format that looked at pH, salinity, and divalent cation concentration. Lipid-polymer nanoparticles formed from SMA variants were found to be the most susceptible to buffer effects, with nanoparticles from either zwitterionic DMPC or POPC:POPG (3:1) bilayers only forming in low to moderate salinity (< 600 mM NaCl) and above pH 6. DIBMA-lipid nanoparticles could be formed above a pH of 5 and were stable in up to 4 M NaCl. Similarly, PMA-lipid nanoparticles were stable in all NaCl concentrations tested (up to 4 M) and a broad pH range (3-10). However, for both DIBMA and PMA nanoparticles there is a severe penalty observed for bilayer solubilisation in non-optimal conditions or when using a charged membrane. Additionally, lipid fluidity of the DMPC-polymer nanoparticles was analysed through cw-EPR, showing no cooperative gel-fluid transition as would be expected for native-like lipid membranes.

Automated summary

This study compared the solubilisation properties of three common commercial polymers (SMA 3:1, SMA 2:1, and DIBMA) and also characterised a novel polymethacrylate copolymer (PMA). The solubilisation was evaluated under different buffer conditions using absorbance and dynamic light scattering measurements. The results showed that SMA variants formed lipid-polymer nanoparticles that were most affected by the buffer conditions, with nanoparticles from charged membranes only forming under specific salinity and pH conditions. Conversely, DIBMA-lipid nanoparticles were stable under a broader range of conditions, while PMA-lipid nanoparticles exhibited stability across all tested conditions. However, both DIBMA and PMA nanoparticles showed poor solubilisation performance in non-optimal conditions or when using charged membranes. Additionally, the fluidity analysis of DMPC-polymer nanoparticles revealed a lack of cooperative gel-fluid transition observed in native lipid membranes.