Influence of lipid saturation on the structural properties of styrene maleic acid lipid nanoparticles (SMALPs)

Membrane bilayers are complex three-dimensional structures whose molecular events in the deep dimensions of membrane lipids are crucial for understanding membrane function. This study investigates the interaction of coexisting membrane domains in terms of hydrophobicity, alkyl chain order, and fluidity using Styrene Maleic Acid (SMA) copolymers as membrane mimics. We employed continuous wave electron paramagnetic resonance spectroscopy (CW-EPR) to characterize the structural dynamic properties of membrane domains without separation. Lipid-spin probe vesicles were prepared using phospholipids with varying degrees of saturation (DOPC, POPC, DMPC, and DSPC) and doxyl spin-labeled phospholipids at different depths (5, 12, and 16-doxyl PC) as membrane probes. These vesicles were titrated with two SMA polymers of different hydrophobic tail lengths. Dynamic light scattering (DLS) confirmed the formation of Styrene Maleic Acid lipid nanoparticles (SMALPs). CW-EPR spectroscopy was used to characterize the dynamic properties of vesicles incorporated into the SMALP systems. Analysis of the CW-EPR spectral line shape data revealed that the hydrophobic tail of SMA, the degree of lipid saturation, and the length of phospholipids significantly affect membrane fluidity and alkyl chain ordering, as well as lipid interactions. Notably, samples containing DSPC, a fully saturated longer-chain phospholipid, and those containing SMA exhibited increased rigidity of motion, reduced fluidity, and improved ordering of the alkyl chain in the membrane. This study provides crucial insights into the molecular dynamics of membrane bilayers and the impact of SMA copolymers on membrane properties, contributing to our understanding of fundamental membrane functions such as lateral movement of proteins and lipids.