(398bf) Examining Effects on Bending Elasticity and Structure of Phospholipid Bilayer Membrane in Presence of Embedded Surface Functionalized Inorganic Nanoparticles

Nanoparticles (NPs) have unique optical, electronic and thermal properties and have found applications in biomedicine, gene therapy, targeted drug delivery, bioimaging, biosensors, catalysis and a plethora of other important applications. They are also an attractive option due to the straightforward synthesis, stability and precise control of their size and morphology during synthesis. The greatly enhanced use of these engineered nanoparticles has raised a question, whether there are potential health risks and side effects involved due to the unintentional interactions of biological entities with nanoparticles. The interactions between engineered nanoparticles and biological entities like biomembranes can also be intentional, as in the case of designing novel stimuli responsive drug delivery vehicles, imaging and diagnostics agents. Previous researches conducted in this regard have shown the toxic effects of NPs are dependent on the concentration, size, morphology, nature of surface functionalization and charges on the nanoparticles. Among the many toxic effects observed due to nanoparticle membrane interactions, formation of nanopores and unzipping of membrane bilayer are a few. It has also been shown that the membrane disruption due to nanoparticles depends on the phase state of the lipid bilayer. Toxicology studies have mostly been conducted on a symptomatic approach and there has been no clear consensus on the mechanism related to these effects. There is a dire need to study the interaction of these particles with cell membranes systematically and elucidate the interactions under various conditions of bilayer phase states and nanoparticle concentration, shape and surface functionalization.

The cell membrane comprises of a lipid bilayer and is the first biological entity encountered by the nanoparticle. The lipid bilayer comprising of phospholipids are ~5 nm thick. In this treatise, we have studied the interactions of gold nanoparticles (AuNPs) and phospholipid bilayers. We have not changed the surface functionality or morphology of the AuNPs, but only its mean diameter and concentration. We have focused on hydrophobic ligand stabilized gold nanoparticle having diameter less than the bilayer thickness (3 nm) and diameter more than the bilayer thickness (6 nm). A mixture of phospholipids (zwitterionic and anionic) was used to synthesize unilamellar vesicles in the 100 nm diameter size range. These vesicles are convenient models for studying cell membrane properties as most eukaryotic cells comprises of similar lipids. During synthesis of these vesicles in the presence of the hydrophobic nanoparticles, the 3 nm and 6 nm diameter AuNPs preferentially embed into the hydrophobic acyl region of the lipid bilayer. We hypothesize that the 3 nm AuNPs embed easily in the bilayer without compromising its integrity significantly, but the 6 nm AuNPs causes significant disruption of the bilayer due to a mismatch.

To answer our hypothesis, we have examined the bilayer thickness and membrane bending elasticity by utilizing Small Angle Neutron Scattering (SANS) and Neutron Spin Echo (NSE) spectrometry. Bending elasticity is a mechanical property that controls the thermal fluctuations of bilayer. We have attempted to explain the influence of AuNPs of different sizes and concentration on the bending elasticity of the model bilayer at a range of temperatures from the fluid to the gel phase. Most research focuses on measuring structure of bilayers by static methods only like SANS and x-ray scattering. NSE spectroscopy is the most suitable method for our studies because it is a dynamic method and is ideal for measuring thermal fluctuations in lipid bilayers because of its correlation times and length scales overlap with cell membrane fluctuations.

We have supported these findings with various other techniques. We have studied the changes in phase behavior of the bilayer in presence of AuNPs by calorimetric methods like differential scanning calorimetry. We have examined the transport across the bilayer of carboxyfluorescein with temperature to elucidate the effect on diffusion across the bilayer due to the presence of AuNPs in them. We have employed other fluorometric methods to characterize the microenvironment of the bilayer in presence of AuNPs. Diphenylhexatriene is a fluorescent probe which changes the polarization of fluorescence emission with change in lipid ordering in the bilayer. This can be used to measure the transition temperature of the bilayer in presence of AuNPs by correlating membrane fluidity with the phase state. Additionally, Laurdan fluorescent probe has been used to determine the diffusion of solvent inside the bilayer due to increased fluidity beyond its transition temperature. This measurement can also be employed to measure the fluidity induced if any in the presence of the AuNPs.