Influence of different polymer belts on lipid properties in nanodiscs characterized by CW EPR spectroscopy

With this study we aim at comparing the well-known lipid membrane model system of liposomes and polymer-encapsulated nanodiscs regarding their lipid properties. Using differential scanning calorimetry (DSC) and continuous-wave electron paramagnetic resonance (CW EPR) spectroscopy, we characterize the temperature-dependent lipid behavior within 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes and nanodiscs made from such liposomes by application of various polymers based on styrene-co-maleic acid (SMA), diisobutylene-alt-maleic acid (DIBMA), and styrene-co-maleic amide sulfobetaine (SMA-SB), a new SMA-derived copolymer containing sulfobetaine side chains. By incorporating a spin label doxyl moiety into the lipid bilayer in position 16 or 5 we were able to study the micropolarity as well as rotational restrictions onto the lipids in the apolar bilayer center and the chain region adjacent to the carbonyl groups, respectively. Our results suggest that all polymers broaden the main melting transition of DMPC, change the water accessibility within the lipid bilayer, and exhibit additional constraints onto the lipids. Independent of the used polymer, the rotational mobility of both spin-labeled lipids decreased with DIBMA exerting less restraints probably due to its aliphatic side chains. Our findings imply that the choice of the solubilizing polymer has to be considered an important step to form lipid nanodiscs which should be included into research of lipid membranes and membrane proteins in the future.