Characterization, Exchange Dynamics, and Supported Bilayer Formation of Phosphatidylcholine Vesicles

Abstract

Liposomes and supported lipid bilayers (SLBs) are used as surrogates for cell membranes in a number of applications, including basic studies, in biotechnology applications, as nanoreactors, as sensors, and in environmental applications. It is crucial that the behavior of liposomes and supported lipid bilayers be understood in order that they may be better utilized in future applications. The effect of lamellarity and size on calorimetric phase transition in single component phosphatidylcholine vesicles was studied using nano-differential scanning calorimetry, dynamic light scattering and cryo-transmission electron microscopy. It was determined that the two phase transition peaks, appearing as a doublet, are attributed to mixtures of oligolamellar and unilamellar vesicles. The mechanism of supported bilayer formation of lipids on silica nanoparticles was investigated for a system of ~100 nm silica nanoparticles and zwitterionic lipids in the form of ~100 nm small unilamellar vesicles (SUVs) at high and low ionic strength, as a function of the surface area ratios of the SUVs and silica. The effect of defects, ionic strength, and size on lipid exchange and transfer on nanoparticle supported bilayers was also studied. Exchange kinetics for SUVs decreased with increasing ionic strength, a trend that was even more pronounced for SLBs. Defects on SLBs were found to occur at phase boundaries between gel and fluid phases, in bare regions of exposed silica, and during cycling due to changes in lipid area at Tm. Lastly, the behavior of styryl dyes in lipid bilayers was explored. Styrl dyes are a useful tool in a variety of biophysical application due to their properties depending on their external environment, such as a dramatic increase in their fluorescence in a hydrophobic environment as opposed to their fluorescence in the aqueous phase. Despite many applications of styrl dyes, the method of incorporation of the dyes into cell membranes, or vesicle model systems, is not resolved. Nano-differential scanning calorimetry and dynamic light scattering were used to investigate the incorporation of the styrl dyes into multilamellar (MLVs) and small unilamellar (SUVs) vesicles composed of DMPC.