Investigating Membrane Fluidity in Two-dimensions

Biological membranes allow for compartmentalization within cells, membranes possess unique, complex chemical and morphological features, thus proving it difficult to investigate the processes that occur at the membrane interface. The study of cellular membranes requires suitable models to mimic these unique and complex features and research has developed solid-supported lipid bilayers (SLBs). SLBs are simplified models of biological membranes that allow investigations into processes occurring at the cellular interface The aim of this study is to investigate membrane fluidity in two-dimensions, the outcome of these investigations was measured by FRAP and LSPR. Firstly, the effect of substrate material and treatment upon POPC membrane fluidity was investigated. The data showed that the fluidity of a POPC lipid bilayer can effectively be tuned. On detergent-treated glass, POPC bilayers exhibit a fluidity of 4.73 ± 0.12 µm2 s-1 but drastically decreases to 1.72 ± 0.43 µm2 s-1 on oxidised PDMS, mimicking in vivo membrane fluidity. This produced a trapping mechanism for POPC lipids on PDMS supports and adds a novel key finding to the hydrophobic recovery of oxidised PDMS, that has not been visually reported before. The results inferred the presence of lipid-substrate interactions and was further supported with LSPR data. The rate of POPC bilayer formation was measured for 10 and 20 mol % POPS concentrations with LSPR on plasma-treated sensors. The data showed faster vesicle adsorption/bilayer formation with low POPS concentration. The membrane fluidity of POPC bilayers doped with POPS lipids was also studied and the results show that POPC membrane fluidity is linearly dependent upon POPS concentration. LSPR data inferred the presence of lipid-lipid and lipid-substrate interactions and the possible interactions were explored. Scenarios were supported with existing experimental and simulation data from the literature. Analysis showed the effects of the POPS bound counterion (Na+) on membrane fluidity have not been thoroughly investigated This thesis has demonstrated that 2D membrane fluidity is significantly affected by substrate material, treatment and phospholipid composition. This study adds knowledge to the thermodynamics of binary zwitterionic and negatively charged lipids and novel experimental data which supports existing molecular dynamic simulations.