Precision cosmological measurements: Independent evidence for dark energy

Using recent precision measurements of cosmological parameters, we re-examine whether these observations alone, independent of type Ia supernova surveys, are sufficient to imply the existence of dark energy. We find that best measurements of the age of the Universe t₀$t_0$, the Hubble parameter H₀$H_0$ and the matter fraction Ωm${\Omega }_m$ strongly favor an equation of state defined by (w<−1/3$w<-1/3$). This result is consistent with the existence of a repulsive, acceleration-causing component of energy if the Universe is nearly flat.     

Mixing of perfluorooctanesulfonic acid (PFOS) potassium salt with dipalmitoyl phosphatidylcholine (DPPC)

Perfluorooctane-1-sulfonic acid (PFOS) is emerging as an important persistent environmental pollutant. To gain insight into the interaction of PFOS with biological systems, the mixing behavior of dipalmitoylphosphatidylcholine (DPPC) with PFOS was studied using differential scanning calorimetry (DSC) and fluorescence anisotropy measurements. In the DSC experiments the onset temperature of the DPPC pretransition (Tp) decreased with increasing PFOS concentration, disappearing at XDPPC < or = 0.97. The main DPPC phase transition temperature showed a depression and peak broadening with increasing mole fraction of PFOS in both the DSC and the fluorescence anisotropy studies. From the melting point depression in the fluorescence anisotropy studies, which was observed at a concentration as low as 10 mg/L, an apparent partition coefficient of K = 5.7 x 10(4) (mole fraction basis) was calculated. These results suggest that PFOS has a high tendency to partition into lipid bilayers. These direct PFOS-DPPC interactions are one possible mechanism by which PFOS may contribute to adverse effects, for example neonatal mortality, in laboratory studies and possibly in humans.     

Bilayer heating in magnetite nanoparticle-liposome dispersions via fluorescence anisotropy

Temperature measurements have been made within magnetite (Fe(3)O(4)) nanoparticle-liposome dispersions subjected to electromagnetic field at radiofrequency (RF) heating based on the fluorescence anisotropy of diphenylhexatriene (DPH) embedded within the bilayer. Incorporating cholesterol within dipalmitoylphosphatidylcholine (DPPC) bilayers broadened the anisotropy window associated with lipid melting. Cryogenic transmission electron microscopy showed that the dispersions contained magnetoliposomes with nanoparticle aggregates at both low and high encapsulation densities. RF heating results demonstrated the ability to measure the temperature of the ML bilayer with on/off RF cycles using DPH anisotropy. These measurements reflected the temperature of the bulk aqueous phase, which is consistent with previous work showing rapid heat dissipation from a nanoparticle surface during RF heating and a negligible difference between surface and bulk temperature.     

Surface activity of lysozyme and dipalmitoyl phosphatidylcholine vesicles at compressed and supercritical fluid interfaces

The surface activities of lysozyme and dipalmitoyl phosphatidylcholine (DPPC) vesicles at aqueous/compressed fluid interfaces are examined via high-pressure interfacial tension measurements using the pendant drop technique. The density and interfacial tension in compressible fluid systems vary significantly with pressure, providing a versatile medium for elucidating interactions between biomolecules and fluid interfaces and a method to elicit pressure-dependent interfacial morphological responses. The effects of lysozyme concentration (0.0008, 0.01, and 1 mg/mL) and pressure (> or = 7 MPa) on the dynamic surface response in the presence of ethane, propane, N2, and CO2 at 298 K were examined. Interfacial lysozyme adsorption reduced the induction phase and quickly led to interfacial tensions consistent with protein conformational changes and monolayer saturation at the compressed fluid interfaces. Protein adsorption, as indicated by surface pressure, correlated with calculated Hamaker constants for the compressed gases, denoting the importance of dispersion interactions. For DPPC at aqueous/compressed or aqueous/supercritical CO2 interfaces (1.8-20.7 MPa, 308 K), 2-3-fold reductions in interfacial tension were observed relative to the pure binary fluid system. The resulting surface pressures infer pressure-dependent morphological changes within the DPPC monolayer.     

Solvent-dependent permeability in asymmetric ceramic membranes with tortuous or non-tortuous mesopores

Solvent-dependent transport and the role of surface interactions were examined in commercial mesoporous ceramic membranes using permeability and thermoporometry measurements. The membranes chosen were titania (TiO₂) with tortuous interconnected pores (1, 5, and 50 kDa, corresponding to pore diameters of ca. 8.2, 18.3, and 33.2 nm, respectively) and alumina (Al₂O₃) with non-tortuous 20 nm cylindrical pores. A pre-water/solvent/post-water permeability cycle was employed to account for structural differences between membranes and to gauge the effect of residual solvent on water permeability at different temperatures. Our results suggest that in both types of membranes, restricted permeability of 1-butanol and cyclohexane was due to a combination of surface sorption and an increase in disjoining pressure due to solvation forces. Sorption and solvation forces were prevalent as their length scales were on the same order of magnitude as the pore radii. For 1-butanol, chemisorption changed the surfaces from hydrophilic to hydrophobic, and led to a significant decrease in post-water permeability. While Darcy's law could not describe 1-butanol and cyclohexane permeability, it did apply to water and 1,4-dioxane in the 20 nm alumina membranes. Thermoporometry, coupled with permeability, was further used to evaluate surface wetting within the mesopores.     

A parallel multi-configuration self-consistent field algorithm

A scalable multi-configuration self-consistent field (MCSCF) algorithm is described. The method for optimizing the orbital and configurational parameters is based upon the two-step Newton–Raphson approach with an augmented orbital Hessian matrix. A single copy of the two-electron integrals in the molecular orbital basis is distributed over the memory of all processors. Storage of the augmented Hessian is avoided by re-computing its elements as needed. A replicated data approach is used to parallelize the configuration interaction step. Scalability to 1024 processors is demonstrated.     

Liposome fluidization and melting point depression by pressurized CO2 determined by fluorescence anisotropy

The influence of CO2 on the bilayer fluidity of liposomes, which are representative of model cellular membranes, was examined for the first time at the elevated pressures (up to 13.9 MPa) associated with CO2-based processing of liposomes and microbial sterilization. Fluidization and melting point depression of aqueous dipalmitoylphosphatidylcholine (DPPC) liposomes by pressurized CO2 (present as an excess phase) were studied by steady-state fluorescence anisotropy using the membrane probe 1,6-diphenyl-1,3,5-hexatriene (DPH). Isothermal experiments revealed reversible, pressure-dependent fluidization of DPPC bilayers at temperatures corresponding to near-gel (295 K) and fluid (333 K) phases at atmospheric pressure, where the gel-to-fluid phase transition (Tm) occurs at approximately 315 K. Isobaric measurements (PCO2 =1.8, 7.0, and 13.9 MPa) of DPH anisotropy demonstrate substantial melting point depression (DeltaTm = -4.8 to -18.5 K) and a large broadening of the gel-fluid phase transition region, which were interpreted using conventional theories of melting point depression. Liposome fluidity is influenced by CO2 accumulation in the hydrocarbon core and polar headgroup region, as well as the formation of carbonic acid and/or the presence of buffering species under elevated CO2 pressure.     

Cationic gel-phase liposomes with "decorated" anionic SPIO nanoparticles: morphology, colloidal, and bilayer properties

The assembly and complexation of oppositely charged colloids are important phenomena in many natural and synthetic processes. Liposome-nanoparticle assemblies (LNAs) represent an interesting hybrid system that combines "soft" and "hard" colloidal materials. This work describes the formation and characterization of gel-phase LNAs formed by the binding of anionic superparamagnetic iron oxide (SPIO) nanoparticles to cationic dipalmitoylphosphatidylcholine (DPPC)/dipalmitoyltrimethylammonium propane (DPTAP) liposomes. Particles were examined with hydrodynamic diameters below (16 nm) and above (30 nm) the cutoff reported for supported lipid bilayer formation. LNA formation with 16 nm particles was entropically driven and particles bound individually to yield "decorated" structures. In this case, increasing nanoparticle concentration yielded colloidal LNA aggregates and eventual charge inversion. In contrast, LNA formation with 30 nm particles was enthalpically driven, and the nanoparticles aggregated at the bilayer interface. These aggregates led to significant LNA aggregation and large bilayer sheets due to liposome rupture despite minimal charge screening of the liposome surface. In this case SLBs were present, but these structures were not dominant. Differences in LNA structure were also revealed through the lipid phase transition behavior. This work infers size-dependent nanoparticle binding and LNA formation mechanisms that can be used to tailor colloidal and bilayer properties. Analogies are made to polyelectrolyte patch charge heterogeneities and DNA complexation with cationic liposomes.