Adsorption behaviors of DPPC/MO aggregates on SiO2 surfaces

Nanometer-scale molecular organization in lipid membranes studied by time-of-flight secondary ion mass spectrometry

The organization of lipid membranes plays an important role in a wide range of biological processes at different length scales. Herein, the authors present a procedure based on time-of-flight secondary ion mass spectrometry (ToF-SIMS) to characterize the nanometer-scale ordering of lipids in lipid membrane structures on surfaces. While ToF-SIMS is a powerful tool for label-free analysis of lipid-containing samples, its limited spatial resolution prevents in-depth knowledge of how lipid properties affect the molecular assembly of the membrane. The authors overcome this limitation by measuring the formation of lipid dimers, originating in the same nanometer-sized primary ion impact areas. The lipid dimers reflect the local lipid environment and thus allow us to characterize the membrane miscibility on the nanometer level. Using this technique, the authors show that the chemical properties of the constituting lipids are critical for the structure and organization of the membrane on both the nanometer and micrometer length scales. Our results show that even at lipid surface compositions favoring two-phase systems, lipids are still extracted from solid, gel phase, domains into the surrounding fluid supported lipid bilayer surrounding the gel phase domains. The technique offers a means to obtain detailed knowledge of the chemical composition and organization of lipid membranes with potential application in systems where labeling is not possible, such as cell-derived supported lipid bilayers.

Phase Behavior of Mixed Lipid Monolayers on Perfluorocarbon Nanoemulsions and its Effect on Acoustic Contrast

Lipid-stabilized nanoemulsions containing a volatile liquid perfluorocarbon core have been studied as ultrasound contrast agents owing to their ability to transform into high-contrast microbubbles when subjected to high intensity focused ultrasound (HIFU). However, while there have been several studies on the effect of acoustic parameters on contrast, the effect of the droplet's stabilizing shell has not been studied as extensively. Inspired by previous studies showing lateral phase separation in microbubbles and vesicles, nanodroplets were formulated with a perfluorohexane core and a shell composed of varying amounts of saturated (DPPC) phospholipids, unsaturated (DOPC) phospholipids, and cholesterol, which were fractionated to obtain nanodroplets of mean diameter 300-400 nm and were stable over one week. When the DOPC content was increased to 40 mol%, ultrasound contrast increased by about one order of magnitude over DPPC-only droplets. Based on fluorescence microscopy results of lateral lipid phase separation on the droplet surface, the various combinations of DPPC, DOPC, and cholesterol were assigned to three regimes on the ternary phase diagram: solid-liquid ordered (low contrast), liquid ordered-liquid disordered (medium contrast), and solid-liquid disordered (high contrast). These regimes were confirmed by TEM analysis of nanoscale droplets. Droplets containing mixed lipid monolayers were also found to produce a significantly greater yield than single-component droplets. The discovery of the dependence of acoustic response on lipid phase separation will help to understand the formulation, behavior, and vaporization mechanism of acoustically-responsive nanoemulsions.

Impact of monoolein on aquaporin1-based supported lipid bilayer membranes

Aquaporin (AQP) based biomimetic membranes have attracted considerable attention for their potential water purification applications. In this paper, AQP1 incorporated biomimetic membranes were prepared and characterized. The morphology and structure of the biomimetic membranes were characterized by in situ atomic force microscopy (AFM), infrared absorption spectroscopy, fluorescence microscopy, and contact angle measurements. The nanofiltration performance of the AQP1 incorporated membranes was investigated at 4 bar by using 2 g l^-1 NaCl as feed solution. Lipid mobility plays an important role in the performance of the AQP1 incorporated supported lipid bilayer (SLB) membranes. We demonstrated that the lipid mobility is successfully tuned by the addition of monoolein (MO). Through in situ AFM and fluorescence recovery after photo-bleaching (FRAP) measurements, the membrane morphology and the molecular mobility were studied. The lipid mobility increased in the sequence DPPC < DPPC/MO (R_MO = 5/5) < DOPC/MO (R_MO = 5/5) < DOPC, which is consistent with the flux increment and salt rejection. This study may provide some useful insights for improving the water purification performance of biomimetic membranes.

Hydroxytyrosol and tyrosol esters partitioning into, location within, and effect on DOPC liposome bilayer behavior

The phenols hydroxytyrosol and tyrosol made abundantly available through olive oil processing were enzymatically transesterified into effective lipophilic antioxidants with cuphea oil. The hydroxytyrosyl and tyrosyl esters made from cuphea oil were assessed for their ability to partition into, locate within and effect the bilayer behavior of 1,2-dioloeoylphosphatidylcholine liposomes and compared to their counterparts made from decanoic acid. Partitioning into liposomes was on the same scale for both hydroxytyrosyl derivatives and both tyrosyl derivatives. All were found to locate nearly at the same depth within the bilayer. Each was found to affect bilayer behavior in a distinct manner.

Formation of supported lipid bilayers on silica: relation to lipid phase transition temperature and liposome size

DPPC liposomes ranging from 90 nm to 160 nm in diameter were prepared and used for studies of the formation of supported lipid membranes on silica (SiO2) at temperatures below and above the gel to liquid-crystalline phase transition temperature (Tm = 41 °C), and by applying temperature gradients through Tm. The main method was the quartz crystal microbalance with dissipation (QCM-D) technique. It was found that liposomes smaller than 100 nm spontaneously rupture on the silica surface when deposited at a temperature above Tm and at a critical surface coverage, following a well-established pathway. In contrast, DPPC liposomes larger than 160 nm do not rupture on the surface when adsorbed at 22 °C or at 50 °C. However, when liposomes of this size are first adsorbed at 22 °C and at a high enough surface coverage, after which they are subject to a constant temperature gradient up to 50 °C, they rupture and fuse to a bilayer, a process that is initiated around Tm. The results are discussed and interpreted considering a combination of effects derived from liposome-surface and liposome-liposome interactions, different softness/stiffness and shape of liposomes below and above Tm, the dynamics and thermal activation of the bilayers occurring around Tm and (for liposomes containing 33% of NaCl) osmotic pressure. These findings are valuable both for preparation of supported lipid bilayer cell membrane mimics and for designing temperature-responsive material coatings.

Fluorescence correlation spectroscopy study of protein transport and dynamic interactions with clustered-charge peptide adsorbents

Ion-exchange chromatography relies on electrostatic interactions between the adsorbent and the adsorbate and is used extensively in protein purification. Conventional ion-exchange chromatography uses ligands that are singly charged and randomly dispersed over the adsorbent, creating a heterogeneous distribution of potential adsorption sites. Clustered-charge ion exchangers exhibit higher affinity, capacity, and selectivity than their dispersed-charge counterparts of the same total charge density. In the present work, we monitored the transport behavior of an anionic protein near clustered-charge adsorbent surfaces using fluorescence correlation spectroscopy. We can resolve protein-free diffusion, hindered diffusion, and association with bare glass, agarose-coated, and agarose-clustered peptide surfaces, demonstrating that this method can be used to understand and ultimately optimize clustered-charge adsorbent and other surface interactions at the molecular scale.

Investigation of the interface in silica-encapsulated liposomes by combining solid state NMR and first principles calculations

In the context of nanomedicine, liposils (liposomes and silica) have a strong potential for drug storage and release schemes: such materials combine the intrinsic properties of liposome (encapsulation) and silica (increased rigidity, protective coating, pH degradability). In this work, an original approach combining solid state NMR, molecular dynamics, first principles geometry optimization, and NMR parameters calculation allows the building of a precise representation of the organic/inorganic interface in liposils. {(1)H-(29)Si}(1)H and {(1)H-(31)P}(1)H Double Cross-Polarization (CP) MAS NMR experiments were implemented in order to explore the proton chemical environments around the silica and the phospholipids, respectively. Using VASP (Vienna Ab Initio Simulation Package), DFT calculations including molecular dynamics, and geometry optimization lead to the determination of energetically favorable configurations of a DPPC (dipalmitoylphosphatidylcholine) headgroup adsorbed onto a hydroxylated silica surface that corresponds to a realistic model of an amorphous silica slab. These data combined with first principles NMR parameters calculations by GIPAW (Gauge Included Projected Augmented Wave) show that the phosphate moieties are not directly interacting with silanols. The stabilization of the interface is achieved through the presence of water molecules located in-between the head groups of the phospholipids and the silica surface forming an interfacial H-bonded water layer. A detailed study of the (31)P chemical shift anisotropy (CSA) parameters allows us to interpret the local dynamics of DPPC in liposils. Finally, the VASP/solid state NMR/GIPAW combined approach can be extended to a large variety of organic-inorganic hybrid interfaces.