Electrical conductivity of bilayer lipid membranes in electrolytic solution

The electrical conductivity of bilayer lipid membranes (BLM) of oxidized cholesterol has been measured separately in bathing solutions of sodium sulphate, sodium chloride, sodium bromide, sodium iodide and also in bathing solutions of iodine and iodine containing these salts. An attempt has been made to explain the conduction of electric current across the membranes.     

The influence of cholesterol on head group mobility in phospholipid membranes

The dielectric dispersion in the MHz range of the zwitterionic dipolar phosphocholine head groups has been measured from 0–70°C for various mixtures of $\text{1,2-}\text{dipalmitoyl}\text{-sn-}\text{glycero-3-phosphocholine}$ (DPPC) and cholesterol. The abrupt change in the derived relaxation frequency $\text{f}{}_2$ observed for pure DPPC at the gel-to-liquid crystalline phase transition at 42°C reduces to a more gradual increase of frequency with temperature as the cholesterol content is increased. In general the presence of cholesterol increases the DPPC head group mobility due to its spacing effect. Below 42°C no sudden changes in $\text{f}{}_2$ are found at 20 or 33 mol% cholesterol, where phase boundaries have been suggested from other methods. Above 42°C, however, a decrease in $\text{f}{}_2$ at cholesterol contents up to 20–30 mol% is found. This is thought to be partly due to an additional restricting effect of the cholesterol on the number of hydrocarbon chain conformations and consequently on the area occupied by the DPPC molecules.     

Radiofrequency interaction with conductive colloids: Permittivity and electrical conductivity of single‐wall carbon nanotubes in saline

Conductive nanoparticles may enhance tissue heating during radiofrequency (RF) irradiation. Specific absorption rate (SAR) is known to rise with the electrical conductivity of tissue. However, no studies to date have measured the relationship between complex permittivity and nanoparticle concentration in tissue‐like samples. The complex permittivities of colloids containing single‐wall carbon nanotubes (SWCNTs) in normal (0.9%) saline were measured from 20 MHz to 1 GHz. Carbon concentrations ranged from 0 to 93 mM (0.06% volume), based on SWCNT weight per volume. Measurements were made with 0.02% Pluronic F108 surfactant added to the colloids to prevent SWCNT flocculation. The data were fit to the Cole–Cole relaxation model with an added constant phase angle element to correct for electrode polarization effects at low RF frequencies. Electrode polarization effects increased with carbon concentration. The real parts of the permittivities of the colloids increased with carbon concentration. The static conductivity rose linearly with carbon concentration, doubling from 0 to 93 mM. The SAR of the colloids is expected to increase with RF frequency, based on the properties of the imaginary part of the permittivity. Bioelectromagnetics 31:582–588, 2010. © 2010 Wiley‐Liss, Inc.     

Control of membrane fusion by phospholipid head groups II. The role of phosphatidylethanolamine in mixtures with phosphatidate and phosphatidylinositol

Membrane fusion induced by Ca²⁺ and Mg²⁺ in large unilamellar vesicles composed of mixtures of phosphatidylethanolamine with phosphatidate and phosphatidylinositol was studied by means of a fluorescence assay for the intermixing of internal aqueous contents of the vesicles. The threshold concentrations of Ca²⁺ or Mg²⁺ required for fusion increased only moderately when up to 80 mol% phosphatidylethanolamine was included with phosphatidate at pH 7.4, but no fusion could be detected in vesicles containing 70 mol% phosphatidylcholine even at high concentrations of Ca²⁺ or Mg²⁺. Phosphatidate-phosphatidylethanolamine (1 : 4) vesicles could be induced to fuse by 0.1 mM Ca²⁺ in the presence of a Mg²⁺ concentration which alone was insufficient for fusion. When equimolar amounts of phosphatidylethanolamine was included with phosphatidylinositol, the vesicles were susceptible to fusion by Ca²⁺, although pure phosphatidylinositol vesicles themselves merely aggregate and do not fuse (Sundler, R. and Papahadjopoulos, D. (1981) Biochim. Biophys. Acta 649, 743–750, accompanying paper). The role of phosphatidylethanolamine acyl chains, and hence the possible involvement of the bilayer-hexagonal (H_II) transition in membrane fusion, was examined by the temperature dependence of Ca²⁺-induced fusion in phosphatidylinositol-dimyristoylphosphatidylethanolamine (1 : 1) vesicles. Fusion was strictly dependent on the gel-liquid crystalline transition of the mixture and not on the phase behavior of the phosphatidylethanolamines. Comparable fusion rates were obtained for both egg yolk phosphatidylethanolamine and dimyristoylphosphatidylethanolamine at 50°C. As the dimyristoylphosphatidylethanolamine does not convert to a non-bilayer phase in this temperature range, we conclude that the bilayer-hexagonal transition is not necessary for membrane fusion. We propose that the dehydration characteristics of the phospholipids and their metal ion complexes are the critical factors determining fusion suceptibility of phospholipid membranes.     

Surface activity and interaction of StarD7 with phospholipid monolayers

StarD7 protein forms stable Gibbs and Langmuir monolayers at the air-buffer interface showing marked surface activity. The latter is enhanced by penetration into phospholipid films at an initial surface pressure above the protein’s own equilibrium adsorption surface pressure to a lipid-free interface. The protein-phospholipid stabilizing interactions at the interface depend on the lipid, with preference for phosphatidylserine, cholesterol, and phosphatidylglycerol, and the increases of lateral surface pressure generated are comparable to those of other membrane-active proteins. The surface activity of StarD7 is strong enough to thermodynamically drive and retain StarD7 at the lipid membrane interface where it may undergo lipid-dependent reorganization as indicated by changes of surface pressure and electrostatics.     

Reliability, repeatability and accuracy of the falling head method for hydraulic conductivity measurements under laboratory conditions

The aim of this study was to verify under lab conditions the reliability, repeatability and accuracy of the falling head method (FHM) for hydraulic conductivity measurements. The FHM is a reliable procedure that has slight variations (less than 10%) in repeated measurements and turns out to be a reliable technique to record the hydraulic conductivities typically described for clogged and unclogged subsurface-flow constructed wetlands (from 4 to ca. 360 m/day). The accuracy of the method is acceptable considering difficulties in the measurement of hydraulic conductivity in highly conductive media. Accordingly, results show measurement deviations of 20% when compared with a laboratory constant head method for highly conductive media (higher than 250 m/day), and 80% for media with low hydraulic conductivity (lower than 50 m/day). The main conclusion of the present paper is that of the FHM is a reliable and repeatable technique for hydraulic conductivity measurements and it is accurate enough for on-site clogging assessment in full-scale constructed wetlands.     

Molecular dynamics simulations of the interaction of phospholipid bilayers with polycaprolactone

The molecular interaction between common polymer chains and the cell membrane is unknown. Molecular dynamics simulations offer an emerging tool to characterise the nature of the interaction between common degradable polymer chains used in biomedical applications, such as polycaprolactone, and model cell membranes. Herein we characterise with all-atomistic and coarse-grained molecular dynamics simulations the interaction between single polycaprolactone chains of varying chain lengths with a phospholipid membrane. We find that the length of the polymer chain greatly affects the nature of interaction with the membrane, as well as the membrane properties. Furthermore, we next utilise advanced sampling techniques in molecular dynamics to characterise the two-dimensional free energy surface for the interaction of varying polymer chain lengths (short, intermediate, and long) with model cell membranes. We find that the free energy minimum shifts from the membrane-water interface to the hydrophobic core of the phospholipid membrane as a function of chain length. Finally, we perform coarse-grained molecular dynamics simulations of slightly larger membranes with polymers of the same length and characterise the results as compared with all-atomistic molecular dynamics simulations. These results can be used to design polymer chain lengths and chemistries to optimise their interaction with cell membranes at the molecular level.     

Relationships between soil bulk electrical conductivity and the principal component analysis of topography and soil fertility values

Measures of soil electrical conductivity (EC) and elevation are relatively inexpensive to collect and result in dense data sets which allow for mapping with limited interpolation. Conversely, soil fertility information is expensive to collect so that relatively few samples are taken and mapping requires extensive interpolation with large estimation errors, resulting in limited usefulness for site-specific applications in precision agriculture. Principal component (PC) analysis and cokriging can be applied to create meaningful field scale summaries of groups of attributes and to decrease the estimation error of maps of the summarized attributes. Deep (0–90 cm) and shallow (0–30 cm) EC, elevation, and soil fertility attributes were measured in fields under corn (Zea mays L.) and soybean (Glycine max L.) rotations, at two sites in Illinois (IL) and two sites in Missouri (MO). Soil fertility and topography attributes were summarized by PC analysis. The first topography PC (TopoPC1) contrasted flow accumulation against elevation and curvature, to describe the main topographic pattern of the fields. The first soil fertility PC (SoilPC1) consistently grouped together cation exchange capacity (CEC), Ca, Mg, and organic matter (OM). SoilPC1 was well correlated to soil EC for all sites and cokriging with EC had higher r ² in the crossvariogram models compared to ordinary kriging. The second and third soil fertility PCs (SoilPC2 and SoilPC3) were concerned with soil pH and P, and reflected historic land use patterns. Maps of SoilPC2 and SoilPC3 had little relationship to soil EC or topography and so could not be improved by cokriging.     

The relatioship between plasma membrane lipid composition and physical-chemical properties II. Effect of phospholipid fatty acid modulation on plasma membrane physical properties and enzymatic activities

The fatty acid composition of plasma membrane phospholipids of the murine T lymphocyte tumor EL4 were systematically modified in an attempt to understand the relationship between lipid bilayer composition and plasma membrane physical and biological properties. Two plasma membrane enzyme activities, adenylate cyclase and ouabain-sensitive ($\text{Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}$)-ATPase, were measured in normal and fatty acid-substituted EL4 plasma membrane fractions. The fatty acid effect on enzyme activities was similar to previously reported effects of fatty acids on cytotoxic T cell function. The activity of both enzymes was inhibited by saturated fatty acids, while unsaturated fatty acids had a moderate enhancing effect on both enzyme activities. Using two different nitroxide derivatives of stearic acid, the order parameter and approximate rotational correlation times were calculated from ESR spectra of normal and fatty acid-modified plasma membranes. No significant difference was found in either parameter in these membranes. These results, in conjunction with earlier data from our laboratory and others, suggest that caution should be exercised in inferring changes in membrane ‘fluidity’ based on lipid modulation of biological membranes.     

The interaction of phospholipid membranes with poly(ethylene glycol). Vesicle aggregation and lipid exchange

(1) The water soluble polymer, poly(ethylene glycol), causes aggregation of sonicated vesicles of dimyristoylphosphatidylcholine in a manner consistent with a steric exclusion mechanism. (2) Poly(ethylene glycol) promotes the exchange of lipids between multilamellar vesicles of dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine when the lipids are in the liquid-crystalline state. (3) ³¹P-NMR studies demonstrate that the bilayer configuration of smectic mesophases of dipalmitoylphosphatidylcholine is substantially maintained in the presence of poly(ethylene glycol).     

Poly-l-lysines and poly-l-arginines induce leakage of negatively charged phospholipid vesicles and translocate through the lipid bilayer upon electrostatic binding to the membrane

Poly-l-lysines (PLL) and poly-l-arginines (PLA) of different polymer chain lengths interact strongly with negatively charged phospholipid vesicles mainly due to their different electrical charges. 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) and their mixtures (1/1 mol/mol) with the respective phosphatidylcholines of equivalent chain length were chosen as model membrane systems that form at room temperature either the fluid L_α or the gel phase L_β lipid bilayer membranes, respectively. Leakage experiments revealed that the fluid POPG membranes are more perturbed compared to the gel phase DPPG membranes upon peptide binding. Furthermore, it was found that pure PG membranes are more prone to release the vesicle contents as a result of pore formation than the lipid mixtures POPG/POPC and DPPG/DPPC. For the longer polymers (≥ 44 amino acids) maximal dye-release was observed when the molar ratio of the concentrations of amino acid residues to charged lipid molecules reached a value of R_P = 0.5, i.e. when the outer membrane layer was theoretically entirely covered by the polymer. At ratios lower or higher than 0.5 leakage dropped significantly. Furthermore, PLL and PLA insertions and/or translocations through lipid membranes were analyzed by using FITC-labeled polymers by monitoring their fluorescence intensity upon membrane binding. Short PLL molecules and PLA molecules of all lengths seemed to translocate through both fluid and gel phase lipid bilayers. Comparison of the PLL and PLA fluorescence assay results showed that PLA interacts stronger with phospholipid membranes compared to PLL. Isothermal titration calorimetry (ITC) measurements were performed to give further insight into these mechanisms and to support the findings obtained by fluorescence assays. Cryo-transmission electron microscopy (cryo-TEM) was used to visualize changes in the vesicles' morphology after addition of the polypeptides.     

The interaction of the brominated flame retardant: Tetrabromobisphenol A with phospholipid membranes

Tetrabromobisphenol A (TBBPA) is one of the most widely used members of the family of brominated flame retardants (BFRs). BFRs, including TBBPA have been shown to be widely distributed within the environment and there is growing evidence of their bio-accumulation within animals and man. Toxicological studies have shown that TBBPA can be harmful to cells by modulating a number of cell signalling processes. In this study, we employed fluorescence spectroscopy and differential scanning calorimetry to investigate the interaction of TBBPA with phospholipid membranes, as this is the most likely route for it to influence membrane-associated cellular processes. TBBPA readily and randomly partitions throughout all regions of the phospholipid bilayer with high efficacy {partition coefficient (Log K_p) = 5.7 ± 0.7}. A decrease in membrane fluidity in both liquid-crystalline and gel-phase membranes was detected at concentrations of TBBPA as low as 2.5 μM. TBBPA also decreases the phase transition temperature of dipalmitoyl phoshatidylcholine (DPPC) membranes and broadened transition peaks, in a fashion similar to that for cholesterol. TBBPA, however, also prefers to partition into membrane regions not too highly enriched with cholesterol. Our findings therefore suggests that, the toxic effects of TBBPA, may at least in part, be due to its lipid membrane binding/perturbing effects, which in turn, could influence biological processes involving cell membranes.     

A method to evaluate the effect of liposome lipid composition on its interaction with the erythrocyte plasma membrane

Lipid aggregates are considered promising carriers for macromolecules and toxic drugs. In order to fulfill this function, aggregates should have properties that ensure the efficient delivery of their cargo to the desired location. One of these properties is their stability in blood when accumulating in the targeted tissue. This stability may be affected by a number of factors, including enzymatic activity, protein adsorption, and non-specific lipid exchange between the aggregate and morphological blood components. Since blood cells in the majority consist of erythrocytes, their interaction with aggregates should be carefully analyzed. In this paper, we present a method that allows the exchange of lipid between liposomes and the erythrocyte plasma membrane to be evaluated. The extent of this exchange was measured in terms of the toxicity of a cationic lipid (DOTAP) incorporated into the liposome lipid bilayer, evaluated by plasma membrane mechanical properties. After liposomes were formed from DOTAP/PC or DOTAP/PE mixtures, erythrocyte plasma membranes were destabilized in a manner dependent on DOTAP concentration. A constant quantity of DOTAP mixed with various proportions of SM caused no such effect, indicating very limited lipid exchange with the cell membrane for such liposome formulations.     

Phase behavior of hydrated lipid bilayer composed of binary mixture of phospholipids with different head groups

Phase behavior of hydrated lipid bilayer was investigated for the mixtures of two phospholipid species chosen from phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG) with the same acyl chains. The pseudo-binary phase diagrams constructed by a differential scanning calorimetry (DSC) were analyzed based on a thermodynamic model applying the Bragg–Williams approximation for non-ideality of mixing. The interchange energy parameters, ρ₀, derived from this approach were positive for all mixture systems in both gel and liquid–crystalline phase bilayers, and increased in the order PG/PE<PC/PA<PC/PE<PG/PA with a few exception. This suggests that the energetical disadvantage for the mixed-pair formation relative to the like-pair formation in the hydrated bilayer increases in this order. In addition, the ρ₀ values increased with the increase in the acyl chain length of the phospholipids. These experimental results were discussed in terms of an intermolecular interaction of the phospholipid species in hydrated bilayer.     

Comparative regional-scale soil salinity assessment with near-ground apparent electrical conductivity and remote sensing canopy reflectance

Soil salinity is recognized worldwide as a major threat to agriculture, particularly in arid and semi-arid regions. Producers and decision makers need updated and accurate maps of salinity in agronomically and environmentally relevant ranges (i.e., <20 dS m⁻¹, when salinity is measured as electrical conductivity of the saturation extract, EC_e). State-of-the-art approaches for creating accurate EC_e maps beyond field scale (i.e., 1 km²) include: (i) Analysis Of Covariance (ANOCOVA) of near-ground measurements of apparent soil electrical conductivity (EC_a) and (ii) regression modeling of multi-year remote sensing canopy reflectance and other co-variates (e.g., crop type, annual rainfall). This study presents a comparison of the two approaches to establish their viability and utility. The approaches were tested using 22 fields (total 542 ha) located in California’s western San Joaquin Valley. In 2013 EC_a-directed soil sampling resulted in the collection of 267 soil samples across the 22 fields, which were analyzed for EC_e, ranging from 0 to 38.6 dS m⁻¹. The ANOCOVA EC_a-EC_e model returned a coefficient of determination (R²) of 0.87 and root mean square prediction error (RMSPE) of 3.05 dS m⁻¹. For the remote sensing approach seven years (2007–2013) of Landsat 7 reflectance were considered. The remote sensing salinity model had R² = 0.73 and RMSPE = 3.63 dS m⁻¹. The robustness of the models was tested with a leave-one-field-out (lofo) cross-validation to assure maximum independence between training and validation datasets. For the ANOCOVA model, lofo cross-validation provided a range of scenarios in terms of RMSPE. The worst, median, and best fit scenarios provided global cross-validation R² of 0.52, 0.80, and 0.81, respectively. The lofo cross-validation for the remote sensing approach returned a R² of 0.65. The ANOCOVA approach performs particularly well at EC_e values <10 dS m⁻¹, but requires extensive field work. Field work is reduced considerably with the remote sensing approach, but due to the larger errors at low EC_e values, the methodology is less suitable for crop selection, and other practices that require accurate knowledge of salinity variation within a field, making it more useful for assessing trends in salinity across a regional scale. The two models proved to be viable solutions at large spatial scales, with the ANOCOVA approach more appropriate for multiple-field to landscape scales (1–10 km²) and the remote sensing approach best for landscape to regional scales (>10 km²).     

Insertion selectivity of antimicrobial peptide protegrin-1 into lipid monolayers: Effect of head group electrostatics and tail group packing

The ability to selectively target the harmful microbial membrane over that of the host cell is one of the most important characteristics of the antimicrobial peptides (AMPs). This selectivity strongly depends on the chemical and structural properties of the lipids that make up the cell membrane. A systematic study of the initial membrane selectivity of protegrin-1 (PG-1), a β-sheet AMP, was performed using Langmuir monolayers. Constant pressure insertion assay was used to quantify the amount of PG-1 insertion and fluorescence microscopy was employed to observe the effect of PG-1 on lipid ordering. Charge and packing properties of the monolayer were altered by using lipids with different head groups, substituting saturated with unsaturated lipid tail group(s) and incorporating spacer molecules. PG-1 inserted most readily into anionic films composed of phosphatidylglycerol (PG) and lipid A, consistent with its high selectivity for microbial membranes. It also discriminated between zwitteranionic phospholipids, inserting more readily into phosphatidylcholine (PC) monolayers than those composed of phosphatidylethanolamine, potentially explaining why PG-1 is hemolytic for PC-rich human erythrocytes and not for the PE-rich erythrocytes of ruminants. Increased packing density of the monolayer by increased surface pressure, increased tail group saturation or incorporation of dihydrocholesterol diminishes the insertion of PG-1. Fluorescence microscopy shows that lipid packing is disordered upon PG-1 insertion. However, the presence of PG-1 can still affect lipid morphology even with no observed PG-1 insertion. These results show the important role that lipid composition of the cell membrane plays in the activity of AMPs.     

The size and phospholipid composition of lipid droplets can influence their proteome

The proteomic makeup of lipid droplets (LDs) is believed to regulate the function of LDs, which are now recognized as important cellular organelles that are associated with many human metabolic disorders. However, factors that help determine LD proteome remain to be identified and characterized. Here we analyzed the phospholipid and protein composition of LDs isolated from wild type (WT) yeast cells, and also from fld1Δ, cds1, and ino2Δ mutant cells which produce ‘supersized’ LDs. LDs of fld1Δ and WT cells exhibited similar phospholipid profiles, whereas LDs of cds1 and ino2Δ strains had a higher (cds1) or lower (ino2Δ) percentage of phosphatidylcholine than those of WT, respectively. Unexpectedly, the presence of most known LD resident proteins was greatly reduced in the LD fraction isolated from cds1 and ino2Δ, including neutral lipid hydrolases. Consistent with this result, mobilization of neutral lipids was seriously impaired in these two strains. Contrary to the reduction of LD resident proteins, the Hsp90 family molecular chaperones, Hsc82 and Hsp82, were greatly increased in the LD fractions of cds1 and ino2Δ strains without changes at the level of expression. These data demonstrate the impact of LD phopholipids and size on the makeup of LD proteome.     

Microwave-induced increase of water and conductivity in submaxillary salivary gland of rats

Hypersalivation is an important mechanism for heat dissipation by animals without sweat glands. The water content and conductivity (at 20 kHz) in sub-maxillary salivary gland (SSG) and in other tissues were investigated in adult male rats exposed to microwaves (2880 MHz, 1.5 μs pulses at 1000 Hz) or to conventional heat at 40 °C. Eighty rats in one series were exposed, one at a time, for 30 min to microwaves producing a specific absorption rate (SAR) of 4.2,6.3,6.8,8.4,10.8 or 12.6 W/kg. Fifty rats were sham-exposed under similar environmental conditions. In the second series, ten rats were sham-exposed, 33 rats were exposed, one at time, for 15, 30 or 60 min to microwaves at a SAR of 9.5 W/kg, and 32 rats were exposed for similar periods to conventional heat at 40 °C. In rats of the first series colonic temperatures were elevated significantly at a SAR of 4.2 W/kg, while SSG water content and conductivity increased significantly at SAR values of 6.3 W/kg and higher. In the second series of experiments increases in colonic temperature and SSG water content were greater after 15 and 30 min of microwave exposure than after exposure to heat. Also, SSG conductivity was significantly depressed by heat and significantly increased by microwaves after exposure for 15 or 30 min. The results support the hypothesis that water content and conductivity of SSG of rats can be used as a sensitive specific test of a microwave induced thermal response.     

A dendrogram of archaea based on lipid component parts composition and its relationship to rRNA phylogeny

The results of two objective and quantitative, computer-assisted analyses of the lipid component parts distribution pattern among various archaeal organisms belonging to Euryarchaeota are reported. One was a cluster analysis and the other a selection of unique combinations of lipid component parts found exclusively in a given taxon. The cluster analysis revealed that the distribution of lipid component parts was correlated with phylogeny based on small subunit rRNA sequences, although there was some discrepancy with rRNA phylogeny. A hypothesis that may explain the reason for the correlation and the discrepancy is proposed. In our scenario, we assumed that random and independent mutations on the rRNA and lipid biosynthesis genes may result largely in coincided evolution. The fact that RNA and lipid are semantide and episemantic molecules, respectively, is the fundamental difference between the phylogeny of RNA and lipid. Moreover, different selective pressures on RNA and lipids exert different effects on their evolution. Unique lipid component parts were detected for eight out of nine orders, 14 families, and 22 genera of the Euryarchaeota analyzed. A unique lipid component parts combination pattern characterized the taxon. The results confirm and extend a previously reported conclusion based on a more statistical basis.