How Gold Nanoparticles affect the Lipid Packing in Model Membranes

The past decade has witnessed the increasing integration of gold nanoparticles (AuNPs) in the biological systems for biomedical and pharmaceutical applications. This makes it imperative to understand how AuNPs interact with cell membranes, the first barriers to be crossed in order to enter the cellular systems. Here we show, on the simplest possible systems, how unmodified AuNPs of various size affect the lipid packing in model membrane systems constituted of dimyristoyl phosphatidylcholine (DMPC). The goal is to gain insights from the experimental evidences on the fundamental factors that determine the biophysical interactions at bio-nano interface. We will report results from the spectrofluormetry, fluorescence microscopy, and AFM studies. The generalized polarization (GP) functions of the emission spectra of Laurdan, a fluorescent molecule which lodges itself in the acyl chains of phospholipid molecules, will provide evidences on the fluidity changes in lipid membranes induced by AuNPs. The circularity analysis of the fluorescence microscopy images will provide evidences on the shape changes, or the structural integrities, of the lipid vesicles affected by AuNPs. The nanomachanical analysis of AFM studies will show elastic and adhesive property changes in lipid bilayer upon AuNP interrogation. The size of AuNPs and their ratios to phospholipid molecules are two variables in our study. From the spectrofluorometry study, it was found in general that AuNPs tend to increase the fluidity of non-extruded liposomes whereas decrease the fluidity of extruded liposomes, which was confirmed by the fluorescence microcopy analysis. Key factors that determine the interactions between AuNPs and lipid bilayers will be identified from the complementary studies.