The organisation of cholesterol and ergosterol in lipid bilayers based on studies using non-perturbing fluorescent sterol probes.

The fluorescence properties of dehydroergosterol and cholesta-5,7,9-trien-3 beta-ol have been studied in organic solution, in aqueous dispersions and incorporated into aqueous lipid dispersions. The absorption spectra of aqueous dispersions of the probes are very different to those in organic solution, and aqueous dispersions are non-fluorescent. This can be attributed to micelle formation with dimerisation and/or aggregation in the micelles. Concentration quenching also occurs when sterols are incorporated into lipid bilayers, but relatively high fluorescence is observed even at a 1 : 1 steroid:lipid molar ratio. Further, the fluorescence is still polarized at these high molar ratios. We attribute this to the formation of ordered arrays of sterol molecules in the lipid bilayers. In these arrays the sterol molecules are organised in an end-to-end fashion, and face-to-face overlap of the sterols is prevented by the lipid molecules. Possible structures for 1 : 1 mixtures are presented.     

Nonlamellar-Phase-Promoting Colipids Enhance Segregation of Palmitoyl Ceramide in Fluid Bilayers

Ceramide is an important intermediate in sphingolipid homeostasis. We examined how colipids, with negative intrinsic curvature and which may induce curvature stress in the bilayers, affected the segregation of palmitoyl ceramide (PCer). Such colipids include 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and tetra-linoleoyl cardiolipin (CL). In 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers, PCer formed ordered, gel-like domains at concentrations above 10 mol% at 23°C, as evidenced by the change in the average lifetime of the trans-parinaric acid emission. When POPE or DOPE were included in the DOPC bilayer (at 20:80 or 40:60 POPE or DOPE to DOPC, by mol), the lateral segregation of PCer was facilitated in a concentration-dependent manner, and less PCer was required for the formation of the ordered ceramide-rich domains. Inclusion of CL in the DOPE bilayer (at 10:90 or 20:80 CL to PC, by mol) also caused a similar facilitation of the lateral segregation of PCer. The PCer-rich domains formed in the presence of POPE, DOPE, or CL in DOPC bilayers were slightly more thermostable (by 2–10°C) when compared to PCer-rich domains in DOPC-only bilayers. Nonlamellar phases were not present in bilayers in which the effects of POPE or DOPE on PCer segregation were the largest, as verified by ³¹P NMR. When palmitoyl sphingomyelin was added to the different bilayer compositions at 5 mol%, relative to the phospholipids, PCer segregated into gel domains at lower concentrations (2–3 mol% PCer), and the effect of POPE on PCer segregation was eliminated. We suggest that the effects of POPE, DOPE, and CL on PCer segregation was in part influenced by their effects on membrane curvature stress and in part because of unfavorable interactions with PCer due to their unsaturated acyl chains. These lipids are abundant in mitochondrial membranes and are likely to affect functional properties of saturated ceramides in them.     

Abiotic Photophosphorylation Model Based on Abiogenic Flavin and Pteridine Pigments

A model for abiotic photophosphorylation of adenosine diphosphate by orthophosphate with the formation of adenosine triphosphate was studied. The model was based on the photochemical activity of the abiogenic conjugates of pigments with the polymeric material formed after thermolysis of amino acid mixtures. The pigments formed showed different fluorescence parameters depending on the composition of the mixture of amino acid precursors. Thermolysis of the mixture of glutamic acid, glycine, and lysine (8:3:1) resulted in a predominant formation of a pigment fraction which had the fluorescence maximum at 525 nm and the excitation band maxima at 260, 375, and 450 nm and was identified as flavin. When glycine in the initial mixture was replaced with alanine, a product formed whose fluorescence parameters were typical to pteridines (excitation maximum at 350 nm, emission maximum at 440 nm). When irradiated with the quasi-monochromatic light (over the range 325–525 nm), microspheres in which flavin pigments were prevailing showed a maximum photophosphorylating activity at 375 and 450 nm, and pteridine-containing chromoproteinoid microspheres were most active at 350 nm. The positions and the relative height of maxima in the action spectra correlate with those in the excitation spectra of the pigments, which point to the involvement of abiogenic flavins and pteridines in photophosphorylation.     

Influence of Nanoemulsion Addition on the Stability of Conventional Emulsions

Interest in using nanoemulsions as delivery systems for lipophilic food ingredients is growing due to their high optical clarity, good physical stability, and ability to increase bioavailability. Nanoemulsion-based delivery systems may need to be incorporated into food matrices that also contain conventional emulsions. The aim of this work was to evaluate the effect of adding nanoemulsions (d?<?200 nm) to conventional emulsions (d?>?200 nm) on the creaming stability and microstructure of the mixed systems. Droplet flocculation and rapid creaming was observed when the nanoemulsion concentration exceeded a particular level: the critical flocculation concentration (CFC) was 3.75 % and 0.25 % (v/v) for conventional emulsions with average droplet diameters of 350 and 250 nm, respectively. Confocal microscopy indicated that there was appreciable droplet flocculation, and the fraction of individual droplets with diameters?<?100 nm decreased after 14 days storage, which was probably due to Ostwald ripening and/or coalescence. The results of the present study might have important implications for the incorporation of nanoemulsion-based delivery systems into food products containing larger fat droplets, such as dressings, sauces, or beverages.     

Membrane-modifying properties of the pore-forming peptaibols saturnisporin SA IV and harzianin HA V

Abstract Harzianin HA V and saturnisporin SA IV are α-amino isobutyric-containing peptides with 18- and 20-residue chain length, respectively. They were isolated from in vitro cultures of Trichoderma species and their sequences were determined by the combined use of positive ion FAB mass spectrometry and NMR. In organic solvent solution, both peptides exhibited the same predominant α-helical secondary structure including a hinge at the level of the central Pro residue, as deduced from NMR data. Their interaction with neutral phospholipid bilayers was shown to induce leakage of the material entrapped in small unilamellar vesicles composed of egg phosphatidylcholine/cholesterol (7/3). When incorporated into neutral planar lipid bilayers, they promoted voltage-gated channels. The concentration- and voltage-dependences of the ionic conductances induced by these peptides were studied in macroscopic current-voltage experiments. Single-channel measurements showed that whilst SA IV developed non-integral multi-open states similar to those induced by alamethicins, but with faster kinetics, the shorter analogue, HA V promoted much smaller-sized conducting aggregates in agreement with macroscopic conductance data.     

Effects of amantadine on the dynamics of membrane-bound influenza A M2 transmembrane peptide studied by NMR relaxation

The molecular motions of membrane proteins in liquid-crystalline lipid bilayers lie at the interface between motions in isotropic liquids and in solids. Specifically, membrane proteins can undergo whole-body uniaxial diffusion on the microsecond time scale. In this work, we investigate the ¹H rotating-frame spin-lattice relaxation (T _1ρ) caused by the uniaxial diffusion of the influenza A M2 transmembrane peptide (M2TMP), which forms a tetrameric proton channel in lipid bilayers. This uniaxial diffusion was proved before by ²H, ¹⁵N and ¹³C NMR lineshapes of M2TMP in DLPC bilayers. When bound to an inhibitor, amantadine, the protein exhibits significantly narrower linewidths at physiological temperature. We now investigate the origin of this line narrowing through temperature-dependent ¹H T _1ρ relaxation times in the absence and presence of amantadine. Analysis of the temperature dependence indicates that amantadine decreases the correlation time of motion from 2.8 ± 0.9 μs for the apo peptide to 0.89 ± 0.41 μs for the bound peptide at 313 K. Thus the line narrowing of the bound peptide is due to better avoidance of the NMR time scale and suppression of intermediate time scale broadening. The faster diffusion of the bound peptide is due to the higher attempt rate of motion, suggesting that amantadine creates better-packed and more cohesive helical bundles. Analysis of the temperature dependence of $ { \ln }\left( {T_{1\rho }^{ - 1} } \right) $ indicates that the activation energy of motion increased from 14.0 ± 4.0 kJ/mol for the apo peptide to 23.3 ± 6.2 kJ/mol for the bound peptide. This higher activation energy indicates that excess amantadine outside the protein channel in the lipid bilayer increases the membrane viscosity. Thus, the protein-bound amantadine speeds up the diffusion of the helical bundles while the excess amantadine in the bilayer increases the membrane viscosity.     

Single-lipid tracking on nanoscale membrane buds: The effects of curvature on lipid diffusion and sorting

Nanoscale membrane curvature in cells is critical for endocytosis/exocytosis and membrane trafficking. However, the biophysical ramifications of nanoscale membrane curvature on the behavior of lipids remain poorly understood. Here, we created an experimental model system of membrane curvature at a physiologically-relevant scale and obtained nanoscopic information on single-lipid distributions and dynamics. Supported lipid bilayers were created over 50 and 70 nm radius nanoparticles to create membrane buds. Single-molecule localization microscopy was performed with diverse mixtures of fluorescent and non-fluorescent lipids. Variations in lipid acyl tales length, saturation, head-group, and fluorescent labeling strategy were tested while maintaining a single fluid lipid phase throughout the membrane. Monte Carlo simulations were used to fit our experimental results and quantify the effects of curvature on the lipid diffusion and sorting. Whereas varying the composition of the non-fluorescent lipids yielded minimal changes to the curvature effects, the labeling strategy of the fluorescent lipids yielded highly varying effects of curvature. Most conditions yield single-population Brownian diffusion throughout the membrane; however, curvature-induced lipid sorting, slowing, and aggregation were observed in some conditions. Head-group labeled lipids such as DPPE-Texas Red and POPE-Rhodamine diffused >2.4× slower on the curved vs. the planar membranes; tail-labeled lipids such as NBD-PPC, TopFluor-PPC, and TopFluor-PIP2, as well as DiIC₁₂ and DiIC₁₈ displayed no significant changes in diffusion due to the membrane curvature. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.     

Uptake of Fluorescent Iron Oxide Nanoparticles by Oligodendroglial OLN-93 Cells

To investigate the cellular accumulation and intracellular localization of dimercaptosuccinate-coated iron oxide nanoparticles (D-IONPs) in oligodendroglial cells, we have synthesized IONPs that contain the fluorescent dye BODIPY (BP) in their coat (BP-D-IONPs) and have investigated the potential effects of the absence or presence of this dye on the particle uptake by oligodendroglial OLN-93 cells. Fluorescent BP-D-IONPs and non-fluorescent D-IONPs had similar hydrodynamic diameters and ζ-potentials of around 60 nm and ?58 mV, respectively, and showed identical colloidal stability in physiological media with increasing particle size and positivation of the ζ-potential in presence of serum. After exposure of oligodendroglial OLN-93 cells to BP-D-IONPs or D-IONPs in the absence of serum, the specific cellular iron content increased strongly to around 1,800 nmol/mg. This strong iron accumulation was lowered for both types of IONPs by around 50 % on exposure of the cells at 4 °C and by around 90 % on incubation in presence of 10 % serum. The accumulation of both D-IONPs and BP-D-IONPs in the absence of serum was not affected by endocytosis inhibitors, whereas in the presence of serum inhibitors of clathrin-dependent endocytosis lowered the particle accumulation by around 50 %. These data demonstrate that oligodendroglial cells efficiently accumulate IONPs by an endocytotic process which is strongly affected by the temperature and the presence of serum and that BP-D-IONPs are a reliable tool to monitor by fluorescence microscopy the uptake and cellular fate of D-IONPs.     

Mode of binding of cytochrome b5 to phospholipid bilayers in lamellar structure

X-ray diffraction studies were made on the multilamellar systems produced by incubation of phospholipid bilayers and the membrane protein, cytochrome b₅, or non-membrane proteins (albumin, ovalbumin and β-lactoglobulin A) at pH 8.1 in aqueous 5 mM CaCl₂ solutions.Detergent-extracted cytochrome b₅ (soluble aggregate) forms two types of lamellar phase with dipalmitoyl phosphatidylcholine bilayers, depending upon the incubation temperature. One type, which has a repeat distance of 114Å, is formed above 34°C, where the binding of cytochrome b₅ to the bilayers is hydrophobic. The other type, with a repeat distance of 153 Å, is formed below 34°C, where the binding is electrostatic. It is also suggested that cytochrome b₅ is monomeric in the former phase but remains aggregated in the latter phase.When dimyristoyl phosphatidylcholine is used, the boundary temperature for the two types shifts to 12°C. These boundary temperatures coincide with the thermal pretransition points of hydrated dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine, respectively.Trypsin-treated cytochrome b₅ (monomeric) and the three non-membrane proteins exhibit only binding of the electrostatic type to the bilayers, independently of the incubation temperature. The observed repeat distances suggest that in these cases two layers of protein molecules are incorporated between the bilayers.     

Constant surface tension molecular dynamics simulations of lipid bilayers with trehalose

Surface areas and fluctuations evaluated from 50 ns molecular dynamics simulations of fully hydrated dipalmitoylphosphatidylcholine (DPPC) bilayers in a 1:2 trehalose:lipid ratio carried out at surface tensions 10, 17 and 25 dyn/cm/leaflet are compared with those of pure bilayers under the same conditions. Trehalose increases the surface area, as consistent with the surface tension lowering observed in simulations at constant area. The system bulk elastic modulus K _b  = 1.5 ± 0.3 × 10¹⁰ dyn/cm². It is independent of bilayer surface area and trehalose content within statistical error. In contrast, the area elastic modulus K _a shows a strong area dependence. At 64 Å²/lipid (the experimental surface area), K _a  = 138 ± 26 dyn/cm for a pure DPPC bilayer and 82 ± 10 dyn/cm for one with trehalose; i.e. trehalose increases fluidity of the bilayer surface at this area per lipid.     

Photocatalytic antibacterial effects on TiO2-anatase upon UV-A and UV-A/VIS threshold irradiation

Photocatalysis mediated by the anatase modification of titanium dioxide (TiO₂) has shown antibacterial effects in medical applications. The aim of this study was to investigate the possibility of expanding the excitation wavelengths for photocatalytic antibacterial effects from ultraviolet (UV) into the visible light range. After deposition of salivary pellicle and adhesion of Streptococcus gordonii on anatase, different irradiation protocols were applied to induce photocatalysis: ultraviolet A (UV-A) > 320 nm; ultraviolet/visible (UV-A/VIS) light > 380 nm and > 390 nm; and VIS light 400–410 nm. A quartz crystal microbalance with dissipation (QCM-D) tests and microscopic examination were used to observe the photoinduced antibacterial effects. Salivary pellicle could be photocatalytically decomposed under all irradiation protocols. In contrast, effective photocatalytic attack of bacteria could be observed by UV-A as well as by UV-A/VIS at 380 nm < λ < 390 nm only. Wavelengths above 380 nm show promise for in situ therapeutic antifouling applications.     

Minimum Membrane Bending Energies of Fusion Pores

Membranes fuse by forming highly curved intermediates, culminating in structures described as fusion pores. These hourglass-like figures that join two fusing membranes have high bending energies, which can be estimated using continuum elasticity models. Fusion pore bending energies depend strongly on shape, and the present study developed a method for determining the shape that minimizes bending energy. This was first applied to a fusion pore modeled as a single surface and then extended to a more realistic model treating a bilayer as two monolayers. For the two-monolayer model, fusion pores were found to have metastable states with energy minima at particular values of the pore diameter and bilayer separation. Fusion pore energies were relatively insensitive to membrane thickness but highly sensitive to spontaneous curvature and membrane asymmetry. With symmetrical bilayers and monolayer spontaneous curvatures of ?0.1 nm^?1 (a typical value) separated by 6 nm (closest distance determined by repulsive hydration forces), fusion pore formation required 43–65 kT. The pore radius of ~2.25 nm fell within the range estimated from conductance measurements. With bilayer separation >6 nm, fusion pore formation required less energy, suggesting that protein scaffolds can promote fusion by bending membranes toward one another. With nonzero spontaneous monolayer curvature, the shape that minimized the energy change during fusion pore formation differed from the shape that minimized its energy after it formed. Thus, a nascent fusion pore will relax spontaneously to a new shape, consistent with the experimentally observed expansion of nascent fusion pores during viral fusion.     

Calorimetric detection of curvature strain in phospholipid bilayers.

Phospholipids in biological membranes are arranged as bilayers. When constrained to pack into planar bilayers, certain phospholipids will form unstable structures as a consequence of their molecular shape and noncovalent bonding. This produces curvature strain which may provide energy for certain membrane processes. We demonstrate that an exothermic process associated with the relief of curvature strain can be detected calorimetrically. The enthalpy for the incorporation of a few percent lysophosphatidylcholine into large unilamellar vesicles of monomethyldioleoylphosphatidylethanolamine at pH 7.4 is exothermic but it is endothermic for stable bilayers such as this same lipid at pH 9 or dioleoylphosphatidylcholine at pH 7.4 or 9. The addition of lysophosphatidylcholine to monomethyldioleoylphosphatidylethanolamine at pH 7.4 is exothermic only for the addition of the first few percent of lysophosphatidylcholine and then it becomes endothermic. The size of the exothermic heat change is sensitive to changes in temperature, while the endothermic processes are relatively temperature-insensitive. The exothermic heat is also larger when 1 or 2 mol % of diolein is incorporated into vesicles of monomethyldioleoylphosphatidylethanolamine. These results are all consistent with the exothermic process corresponding to the relief of curvature strain in bilayers having a tendency to convert to the hexagonal phase. It provides a demonstration that considerable energy may be released upon the incorporation of certain molecules into membranes which have a low radius of spontaneous curvature.     

Folding and stability of outer membrane protein A (OmpA) from Escherichia coli in an amphipathic polymer, amphipol A8-35

Amphipols are a class of amphipathic polymers designed to maintain membrane proteins in aqueous solutions in the absence of detergents. Denatured β-barrel membrane proteins, like outer membrane proteins OmpA from Escherichia coli and FomA from Fusobacterium nucleatum, can be folded by dilution of the denaturant urea in the presence of amphipol A8-35. Here, the folding kinetics and stability of OmpA in A8-35 have been investigated. Folding is well described by two parallel first-order processes, whose half-times, ~5 and ~70 min, respectively, are independent of A8-35 concentration. The faster process contributed ~55–64 % to OmpA folding. Folding into A8-35 was faster than into dioleoylphosphatidylcholine bilayers and complete at ratios as low as ~0.17 g/g A8-35/OmpA, corresponding to ~1–2 A8-35 molecules per OmpA. Activation energies were determined from the temperature dependence of folding kinetics, monitored both by electrophoresis, which reports on the formation of stable OmpA tertiary structure, and by fluorescence spectroscopy, which reflects changes in the environment of tryptophan side chains. The two methods yielded consistent estimates, namely ~5–9 kJ/mol for the fast process and ~29–37 kJ/mol for the slow one, which is lower than is observed for OmpA folding into dioleoylphosphatidylcholine bilayers. Folding and unfolding titrations with urea demonstrated that OmpA folding into A8-35 is reversible and that amphipol-refolded OmpA is thermodynamically stable at room temperature. Comparison of activation energies for folding and unfolding in A8-35 versus detergent indicates that stabilization of A8-35-trapped OmpA against denaturation by urea is a kinetic, not a thermodynamic phenomenon.     

The thickness of monoolein lipid bilayers as determined from reflectance measurements

Measurements of the reflectance of monoolein $\text{n-}\text{alkane}$ and monoolein/squalene lipid bilayers have been made. The total thickness of the bilayer was calculated from the dependence of reflectance on the refractive index of the aqueous salt or sucrose solution surrounding the bilayer. The total thickness was then compared to the thickness of the hydrocarbon chain region as determined from capacitance measurements. From this comparison, we found that the thickness of each polar region of the bilayers in salt solutions was $\text{0.5 ± 0.1}\text{nm}$, independent of the hydrocarbon solvent used. When the aqueous solutions contained sucrose, each polar region was approx. 0.9 nm thick. When $\text{n-}\text{tetradecane}$ and $\text{n-}\text{hexadecane}$ were used as solvents, microlenses of solvent trapped in the monoolein bilayer increased the reflectance. After about one hour, the coalescence of microlenses into larger lenses allowed the reflectance of the bilayer alone to be measured. The use of reflectance to measure the thickness of monoolein bilayers appears to be consistent with other methods and to give useful information about the structure of lipid bilayers.     

Non-equilibrium molecular dynamics study of nanoscale thermal contact resistance

Interfaces play an important role in microscale and nanoscale heat transfer processes with molecular dynamics (MD) simulations often used to study these interfacial phenomena. In this study, two models were used to simulate thermal conduction across micro contact points and the thermal contact resistance using non-equilibrium molecular dynamics simulations with consideration of the near field radiation. When the ratio of the length of the micro contact to the length of the conduction region is less than 0.125, the influence of the near field radiation should be considered; but when the ratio is larger than 0.2, it can be neglected. When the computational domain sizes are 8.50 × 10.62 × 8.50 nm and 10.62 × 10.62 × 10.62 nm, the MD results show that the thermal contact resistance exponentially increases with decreasing area of the micro contact point and increases with increasing micro contact layer thickness. The MD thermal contact resistances in nanoscale are much larger than that of the classical thermal analysis since the material thermal conductivity reduction is ignored in the classical model. The results also show that material defects increase the thermal resistance.     

Localized Surface Plasmon Resonance (LSPR) Biosensor for the Protein Detection

In this paper, we investigate the ability of the gold nanorods (GNRs) to detect some proteins and demonstrate their potential to be used as plasmonic nanobiosensors. The GNRs were synthesized by a two-step seed-mediated growth procedure at room temperature. Firstly, a seed solution of gold nanoparticles was synthesized in the presence of cetyltrimethylammonium bromide surfactant and, subsequently, incorporated with appropriate amount of silver nitrate and tetrachloroauric acid solutions to grow GNRs with average length of 50 nm and diameter of 14 nm. We study the interaction of GNRs with proteins whose molecular weight varies from 6.5 up to 75 kDa. We investigate the resulting solutions by means of UV–vis absorption spectroscopy to determine the effect of the proteins characteristics on the shift of the localized surface plasmon resonance (LSPR). We show that for the case when proteins are in large excess compared to the GNRs concentration, whatever the protein is, the LSPR shift is constant and does not depend on the protein molecular weight. Moreover, we have been able to demonstrate that the sensitivity of such LSPR sensor is around 10^–9 M/nm on a concentration range from 10^–10 to 10^–8 M. Some comparison with finite-difference time-domain simulations have also shown that the number of proteins adsorbed at the GNRs surface is around 40.     

Molecular dynamics simulations of palmitoyloleoylphosphatidylglycerol bilayers

Phosphatidylglycerol (PG) is one of the important components of biological membranes, but there is a paucity of experimental data to test the accuracy of molecular dynamics (MD) simulations. This work consists of testing the accuracy of the CHARMM36 (C36) lipid force field on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) lipid bilayers. MD simulations of POPG lipid bilayers are compared to recently available X-ray and neutron scattering and deuterium NMR measurements. Overall, the C36 lipid force field accurately represents the X-ray and neutron form factors, bilayer and hydrocarbon thicknesses and chain deuterium order parameters. The surface area per lipid from MD simulations with C36 (67.7 ± 0.2 Å²) is in excellent agreement with the experimentally determined value of 66.0 ± 1.3 Å². C36 outperforms the lipid force field developed by Berger et al. [15] and suggests that past studies with this force field may result in lateral areas that are too small. Moreover, our studies give some insight into the structural model used in experiments and suggest that the functional form for the head group may not be Gaussian-like. Based on our simulations, the POPG lipid in the C36 lipid force field is well parameterised and can be used for other PG lipids and membrane models with mixed lipids.     

Stabilization of liposome bilayers to freezing and thawing: Effects of cryoprotective agents and membrane proteins

Lipid bilayer vesicles (liposomes) with and without glycoprotein incorporated into the membranes were tested for stability during freezing and thawing, in presence and absence of the cryoprotective agents (CPA) glycerol and dimethyl sulfoxide. Changes in turbidity and loss of energy transfer between fluorescent probes present in the bilayers were used to estimate membrane integrity.Freezing caused a 30 to 40% destruction of protein-free liposomes, in absence of CPA. CPA at 10 to 20% concentration prevented such losses, but at higher concentrations destabilized liposomes even without freezing. Protein-containing liposomes suffered no loss on freezing in absence or presence of CPA at moderate concentrations.Lowering of the storage temperature of frozen samples within the range of ?5 to ?27 °C increased the freeze damage. Slower rates of cooling and warming caused a slightly greater loss.The results are interpreted in terms of the liquid mosaic model for lipid bilayers. CPA at higher concentrations destabilize bilayers by dissolving phospholipids. At moderate concentrations, however, they prevent the damaging effect of dehydration of the lipid on freezing. Proteins appear to stabilize bilayers by providing increased hydration at the membrane surface, and by additional hydrophobic binding in the membrane interior.     

Dispersion and bilayer interaction of single-walled carbon nanotubes modulated by covalent and noncovalent PEGylation

Single-walled carbon nanotubes (SWNTs) covalently functionalised with polyethylene glycol (PEG) or noncovalently coated with PEGylated lipids were simulated in water and in lipid bilayers at different PEG sizes and grafting densities using coarse-grained force fields. Starting with the random position of three SWNT–PEG complexes in water, larger PEGs at higher grafting densities more significantly inhibit the aggregation of SWNTs because of larger radii of gyration and hydrodynamic radii of the SWNT–PEG complex, which influence the thickness and the wrapping extent of PEG layer. In particular, PEG-functionalised SWNTs, where PEGs are evenly grafted along the SWNT, disperse, while PEG-coated SWNTs aggregate because SWNTs are less covered by randomly adsorbed PEGylated lipids. Simulations of SWNT–PEGs in lipid bilayers show that PEG (M_w = 550 and 2000)-functionalised SWNTs bind to the bilayer surface but do not insert into the bilayer, while PEG-coated SWNTs insert into the bilayer because PEGylated lipids detach from SWNTs and mix with bilayer lipids. These findings support recent experiments at the same PEG size and density, which suggested that PEG-coated SWNTs may form bundles and thus cannot be easily excreted through the renal route, while PEG-functionalised SWNTs may remain individual and thus show more renal excretion.