Lipid membrane characterization with second harmonic scattering: surface potentials, ionization, membrane asymmetry and hydration

Membranes, composed of a variety of lipids and other biomolecules, mediate signaling processes between cells and their aqueous environment. To fulfill this function, membranes can vary their composition leaflet-specific and thus alter their surface properties. To fully understand the impact of these processes on the molecular level, it is necessary to develop tools that can access the molecular properties of free-floating model membranes label-free. These tools are ideally surface-specific. In this thesis, we apply the nonlinear optical techniques second harmonic scattering (SHS) and vibrational sum-frequency scattering (SFS) together with electrokinetic measurements to label-free characterize the interfacial properties, hydration structure and surface potentials of liposomes in aqueous solutions. We generalize the nonlinear optical theory to describe the second-order surface response from interfaces with aqueous solutions independent of the ionic strength for reflection, transmission and scattering geometries. Then, we apply this theory to demonstrate that SHS patterns of liposomes and oil droplets contain all necessary information to extract the absolute surface potential of the respective particles without assuming a model for the interfacial structure but analyzing the hydration structure of the interface. Subsequently we quantify hydration and lipid asymmetries in binary mixed lipid membranes using SHS and SFS. Finally, we quantify the surface properties of phosphocholine and phosphoserine containing membranes and analyze the counterion condensation. The work of this thesis demonstrates that biotechnological studies but also very fundamental questions can be equally well adressed with these nonlinear optical techniques.