Mixtures of supported and hybrid lipid membranes on heterogeneously modified silica nanoparticles

Lipid Packing in Lipid-Wrapped Nanoparticles

Equilibrium simulations of lipid-wrapped nanoparticles (LNP) were performed using a hybrid molecular dynamics/Monte Carlo (MD/MC) approach. The radius, R, of a spherical nanoparticle (NP) core was adjusted with MC moves while a surrounding lipid bilayer was treated with MD. A wide range of LNP sizes, with the largest R ∼ 40 nm, were studied to determine the average NP radius for a given total number of lipids, N, the number of lipids in each layer, and configurational information. A three-bead lipid model was used to allow large N. A nonequilibrium Jarzynski free energy calculation of the optimal R for a given N, was also demonstrated validating the MD/MC method. An order/disorder transition was described, unique to LNP and distinct from lamellar bilayers, that is weak and continuous with small N, but sharpens to a first order transition with N > 10000 at T ≈ 1.1, shifting to higher T with increasing N. The radius and the overlap of the inner and outer layers were used as order parameters charactering the whole system, and the density vs distance from the origin served to describe the transition in individual layers. The ordering effect of the core on the inner layer, and the disordering effect of curvature, are evident. Excellent fits for the number of lipids in the inner and outer layers vs R are presented, based on the idea that the inner layer is described as usual by an area and area/lipid but that the outer layer is slaved to the inner. The most ordered states exhibit interdigitation of the inner head groups with themselves.

Single Lipid Molecule Dynamics on Supported Lipid Bilayers with Membrane Curvature

The plasma membrane is a highly compartmentalized, dynamic material and this organization is essential for a wide variety of cellular processes. Nanoscale domains allow proteins to organize for cell signaling, endo- and exocytosis, and other essential processes. Even in the absence of proteins, lipids have the ability to organize into domains as a result of a variety of chemical and physical interactions. One feature of membranes that affects lipid domain formation is membrane curvature. To directly test the role of curvature in lipid sorting, we measured the accumulation of two similar lipids, 1,2-Dihexadecanoyl-sn-glycero-3-phosphoethanolamine (DHPE) and hexadecanoic acid (HDA), using a supported lipid bilayer that was assembled over a nanopatterned surface to obtain regions of membrane curvature. Both lipids studied contain 16 carbon, saturated tails and a head group tag for fluorescence microscopy measurements. The accumulation of lipids at curvatures ranging from 28 nm to 55 nm radii was measured and fluorescein labeled DHPE accumulated more than fluorescein labeled HDA at regions of membrane curvature. We then tested whether single biotinylated DHPE molecules sense curvature using single particle tracking methods. Similar to groups of fluorescein labeled DHPE accumulating at curvature, the dynamics of single molecules of biotinylated DHPE was also affected by membrane curvature and highly confined motion was observed.

Silver nanoparticles functionalized with a fluorescent cyclic RGD peptide: a versatile integrin targeting platform for cells and bacteria

A green fluorescent RGD peptide–silver nanoparticle platform to target integrin receptors in cells and bacterial studies.

Using the Localized Surface Plasmon Resonance of Gold Nanoparticles to Monitor Lipid Membrane Assembly and Protein Binding

Gold nanoparticles provide a template for preparing supported lipid layers with well-defined curvature. Here, we utilize the localized surface plasmon resonance (LSPR) of gold nanoparticles as a sensor for monitoring the preparation of lipid layers on nanoparticles. The LSPR is very sensitive to the immediate surroundings of the nanoparticle surface and it is used to monitor the coating of lipids and subsequent conversion of a supported bilayer to a hybrid membrane with an outer lipid leaflet and an inner leaflet containing hydrophobic alkanethiol. We demonstrate that both decanethiol and propanethiol are able to form hybrid membranes and that the membrane created over the shorter thiol can be stripped from the gold along with the lipid leaflet using β-mercaptoethanol. The sensitivity of the nanoparticle LSPR to the refractive index (RI) of its surroundings is greater when the shorter thiol is used (37.8 ± 1.5 nm per RI unit) than when the longer thiol is used (27.5 ± 0.5 nm per RI unit). Finally, C-reactive protein binding to the membrane is measured using this sensor allowing observation of both protein-membrane and nanoparticle-nanoparticle interactions without chemical labeling of protein or lipids.