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The effects of adding 1 mol % of gramicidin A to the well-known DOPC/DSPC/cholesterol lipid mixtures were investigated. 4-component giant unilamellar vesicles (GUV) were prepared using our recently developed Wet-Film method. The phase boundary of liquid-ordered and liquid-disordered (Lo-Ld) coexisting region was determined using video fluorescence microscopy. We found that if cares were not taken, light-induced domain artifacts could significantly distort the measured phase boundary. After testing several fluorescence dyes, we found that the emission spectrum of Nile Red is quite sensitive to membrane composition. By fitting the Nile Red emission spectra at the phase boundary to the spectra in the Lo-Ld coexisting region, the thermodynamic tie-lines were determined. As an active component of lipid membranes, gramicidin not only partitions favorably into the liquid-disordered (Ld) phase, it also alters the phase boundary and thermodynamic tie-lines. Even at as low as 1 mol %, gramicidin decreases the cholesterol mole fraction of Ld phase and increases the area of Lo phase.

Structural and optical studies are reported of yttrium orthoborate YBO3 when tri-doped with Eu3+, Ce3+, and Tb3+, focusing on the role of terbium concentration. Incorporation of Tb3+ affects emission properties for photoluminescence (PL) excited by near ultraviolet light. For constant cerium and europium concentrations, increasing the Tb3+ results in diminished PL from the Ce3+ and Tb3+ color centers. Simultaneously, the PL excitation bands related to both Ce3+ and Tb3+ increase in intensity for red emission from the Eu3+. Results are consistent with a Ce3+ → (Tb3+)n → Eu3+ energy transfer scheme, where (Tb3+)n denotes a chain incorporating n terbium ions. A high red to orange PL intensity ratio is obtained, ranging from 1.34 to 2.09. Raman vibrational bands show a systematic change, with Tb3+ concentration, in the B3O9 ring terminal oxygen bending mode coordinated with the yttrium site where dopant ions substitute. The structural changes are interpreted as variations in the local neighborhood of these si...

Hybrid lipids (HL) are phospholipids with one saturated and one unsaturated chains. Although it is believed that hybrid lipids can act as linactants (i.e., 2D surfactants) in cell membranes to reduce line tension of lipid domains, as a group, their fundamental roles in membrane heterogeneity have not yet been systematically investigated. In this study, three hybrid lipids of different degrees of chain unsaturation (i.e., 16:0-18:1PC (POPC), 16:0-18:2PC (PLPC), and 16:0-20:4PC (PAPC)) are compared in their abilities to alter the composition, order and compactness of lipid domains. The liquid-ordered and liquid-disordered (Lo+Ld) phase boundaries of HL/di18:0PC(DSPC)/cholesterol systems were determined from giant unilamellar vesicles (GUV). We found that the Lo and Ld lipid domains in PAPC/DSPC/CHOL and PLPC/DSPC/CHOL mixtures are micron-sized, and only POPC/DSPC/CHOL system has nanoscopic domains. The results show that some poly-unsaturated HLs essentially behave like the double-unsaturated di18:1PC (DOPC), and the mono-unsaturated POPC clearly displays both properties of unsaturated lipid and linactant. The obtained phase boundaries indicate that both POPC and PLPC have significant partition in the Lo phases. Our MD simulations reveal that these hybrid lipids decrease the order and compactness of Lo domains. Thus, hybrid lipids distinguish themselves from other lipids in this combined “partitioning and loosening” ability in Lo domains, and this ability could facilitate membrane proteins to partition into lipid-raft-like domains in biomembranes. Our line tension measurement and Monte Carlo simulation both show that even in the best cases, HLs are weak linactants with only modest ability in reducing line tension. Furthermore, the reduction is through two separated mechanisms: one through reducing the compositional differences of Lo and Ld domains, and the other through weak linactant action.

Hybrid lipids are thought to be able to perform the function of linactants at membrane domain interfaces: They can reduce line tension and stabilize nanoscopic lipid raft domains in biomembranes. Hybrid lipids are lipids with one saturated chain and one unsaturated chain. Here we provide evidences that only certain hybrid lipids behave like linactants. In this study, we compared three hybrid lipids (i.e., 16:0-181PC (POPC), 16:0-18:2PC, and 16:0-22:4PC) in their abilities to reduce lipid domain size and shift phase boundary. The Lo-Ld phase boundaries of hybrid-lipid/di18:0PC(DSPC)/cholesterol systems were determined from giant unilamellar vesicles (GUV) using fluorescence microscopy. We found that 16:0-22:4PC behaves similarly to a fluid-phase lipid: The Lo and Ld lipid domains in 16:0-22:4PC/DSPC/CHOL mixtures are macroscopic and the phase coexisting region is very wide. On the other hand, 16:0-18:2PC/DSPC/CHOL system has a much narrower Lo-Ld phase coexisting region; however, the lipid domains are still macroscopic. Only POPC/DSPC/CHOL system contains nanoscopic lipid domains. These results were compared with Monte Carlo simulations. Based on the magnitudes of interaction energies, it appears that only mono-unsaturated hybrid lipids behave like linactants, while poly-unsaturated hybrid lipids behave more or less like fluid-phase lipids.

Although shales constitute about 75% of most sedimentary basins, the studies dealing with their seismic response are relatively few, particularly for the organic rich shale gas. Mapping distribution of shale gas and identifying their maturation level and organic carbon richness is critically important for unconventional gas field exploration and development. This study analyses the sensitivity of acoustic and elastic parameters of shales to variations in pore fluid content. Based on the effective medium theory a rock physics model has been made by inversion of the shale stiffness tensor from sonic, density, porosity and clay content logs. Due to the lack of a generally agreed upon fluid substitution model for shale, a statistical approach to Gassmann’s Model using effective porosity in the near boundary conditions, has been developed to account for shale. Fluid substituted logs—for a variety of maturation levels—and gas saturations were generated and used to make the layered earth models. AVO and seismic forward modelling were performed using the rock physics modelled and the fluid substituted logs on layered models. As part of seismic forward modelling, simultaneous inversion is performed for each model to generate P-impedance, S-impedance and density volumes. The sensitivity of the models were analysed by histogram, cross plotting, cross section highlighting, and body checking techniques. This study showed a dramatic hydrocarbon content effect—specifically gas—in the seismic response of shales.

Giant unilamellar vesicles (GUVs) containing cholesterol often have a wide distribution in lipid composition. In this study, GUVs of 1,2-dioleoyl-sn-glycero-3-phosphocholine(DOPC)/1,2-distearoyl-sn-glycero-3-phosphocholine(DSPC)/cholesterol and 1,2-diphytanoyl-sn-glycero-3-phosphocholine(diPhyPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine(DPPC)/cholesterol were prepared from dry lipid films using the standard electroformation method as well as a modified method from damp lipid films, which are made from compositional uniform liposomes prepared using the Rapid Solvent Exchange (RSE) method. We quantified the lipid compositional distributions of GUV by measuring the miscibility transition temperature of GUVs using fluorescence microscopy, since a narrower distribution in the transition temperature should correspond to a more uniform distribution in GUV lipid composition. Cholesterol molecules can demix from other lipids in dry state and form cholesterol crystals. Using optical microscopy, micron-sized crystals were observed in some dry lipid films. Thus, a major cause of GUV lipid compositional heterogeneity is the demixing of lipids in the dry film state. By avoiding the dry film state, GUVs prepared from damp lipid films have a better uniformity in lipid composition, and the standard deviations of miscibility transition temperature are about 2.5 times smaller than that of GUVs prepared from dry lipid films. Comparing the two ternary systems, diPhyPC/DPPC/cholesterol GUVs has a larger cholesterol compositional heterogeneity, which directly correlates with the low maximum solubility of cholesterol in diPhyPC lipid bilayers (40.2 ± 0.5 mol%) measured by light scattering. Our data indicate that cholesterol interacts far less favorably with diPhyPC than it does with other PCs. The damp lipid film method also has a potential of preparing GUVs from cell membranes containing native proteins without going through a dry state.