Influence of charge density on bilayer bending rigidity in lipid vesicles A combined dynamic light scattering and neutron spin echo study

We report a combined dynamic light scattering and neutron spin echo study on vesicles composed of the uncharged stabilizing lipid 1,2 dimyristoyl sn glycero 3 phosphatidylcholine DMPC and the cationic lipid 1,2 dioleoyl 3 trimethylammonium propane DOTAP . Mechanical properties of a model membrane and thus the corresponding bilayer undulation dynamics can be specifically tuned by changing its composition through lipid headgroup or acyl chain properties. We compare the undulation dynamics in lipid vesicles composed of DMPC DOTAP to vesicles composed of a mixture of the uncharged helper lipid DMPC with the also uncharged reference lipid 1,2 dioleoyl sn glycero 3 phosphocholine DOPC . We have performed dynamic light scattering on the lipid mixtures to investigate changes in lipid vesicle size and the corresponding center of mass diffusion. We study lipid translational diffusion in the membrane plane and local bilayer undulations using neutron spin echo spectroscopy, on two distinct time scales, namely around 25 ns and around 150 ns. Finally, we calculate the respective bilayer bending rigidities kappa for both types of lipid vesicles. We find, that on the local length scale inserting lipid headgroup charge into the membrane influences the bilayer undulation dynamics and bilayer bending rigidity kappa less than inserting lipid acyl chain unsaturation We observe a bilayer softening with increasing inhomogenity of the lipid mixture, which could be caused by a hydrophobic mismatch between the acyl chains of the respective lipid components, causing a lateral phase segregation domain formation in the membrane plan