A synthetic lecithin containing branched-chain fatty acids: physical properties and membrane studies

Effect of Cobalt Ferrite Nanoparticles in a Hydrophilic Shell on the Conductance of Bilayer Lipid Membrane

We measured the conductance of bilayer lipid membranes of diphytanoylphosphatidylcholine induced by interaction with cubic magnetic nanoparticles (MNPs) of cobalt ferrite 12 and 27 nm in size and coated with a hydrophilic shell. The MNP coating is human serum albumin (HSA) or polyethylene glycol (PEG). The interaction of nanoparticles added to the bulk solution with the lipid bilayer causes the formation of metastable conductive pores, which, in turn, increases the integral conductance of the membranes. The increase in conductance with increasing MNP concentration was practically independent of the particle size. The dependence of the bilayer conductance on the concentration of PEG-coated MNPs was much weaker than that on the concentration with a shell of HSA. Analyzing the current traces, we believe that the conductive pores formed as a result of the interaction of nanoparticles with the membrane can change their size, remaining metastable. The form of multilevel current traces allows us to assume that there are several metastable pore states close in energy. The average radius of the putative cylindrical pores is in the range of 0.4-1.3 nm.

Calmodulin Modulates the Gating Properties of Voltage-Dependent Anion Channel from Rat Brain Mitochondria

Calmodulin (CaM) is a key signaling protein that plays a decisive role in mitochondrial Ca2+ homeostasis and signaling and modulates the mitochondrial membrane properties. We propose that voltage-dependent anion channel 1 (VDAC1), one of the most abundant outer mitochondrial membrane (OMM) proteins, could be its possible target or site of action. VDAC1 is known to play a crucial role in the mitochondrial Ca2+ signaling mechanism. Bilayer electrophysiology experiments show that CaM significantly reduces VDAC1's conductivity and modulates its gating as well as permeability properties. Also, spectrofluorimetric analysis indicates the possibility of binding CaM with VDAC1. Theoretical analysis of fluorescence data shows that the aforementioned protein-protein interaction is not linear, but rather it is a complex nonlinear process. In VDAC1, CaM binding site has been predicted using various bioinformatics tools. It is proposed that CaM could interact with VDAC1's outer-loop region and regulate its gating properties. Our findings suggest that VDAC1-CaM interaction could play a crucial role in the transport of ions and metabolites through the OMM and the regulation of the mitochondrial Ca2+ signaling mechanism through alteration of VDAC1's gating and conductive properties.

Memristive plasticity in artificial electrical synapses via geometrically reconfigurable, gramicidin-doped biomembranes

It is now known that mammalian brains leverage plasticity of both chemical and electrical synapses (ES) for collocating memory and processing. Unlike chemical synapses, ES join neurons via gap junction ion channels that permit fast, threshold-independent, and bidirectional ion transport. Like chemical synapses, ES exhibit activity-dependent plasticity, which modulates the ionic conductance between neurons and, thereby, enables adaptive synchronization of action potentials. Many types of adaptive computing devices that display discrete, threshold-dependent changes in conductance have been developed, yet far less effort has been devoted to emulating the continuously variable conductance and activity-dependent plasticity of ES. Here, we describe an artificial electrical synapse (AES) that exhibits voltage-dependent, analog changes in ionic conductance at biologically relevant voltages. AES plasticity is achieved at the nanoscale by linking dynamical geometrical changes of a host lipid bilayer to ion transport via gramicidin transmembrane ion channels. As a result, the AES uniquely mimics the composition, biophysical properties, bidirectional and threshold-independent ion transport, and plasticity of ES. Through experiments and modeling, we classify our AES as a volatile memristor, where the voltage-controlled conductance is governed by reversible changes in membrane geometry and gramicidin channel density. Simulations show that AES plasticity can adaptively synchronize Hodgkin-Huxley neurons. Finally, by modulating the molecular constituents of the AES, we show that the amplitude, direction, and speed of conductance changes can be tuned. This work motivates the development and integration of ES-inspired computing devices for achieving more capable neuromorphic hardware.

Diphytanoyl lipids as model systems for studying membrane-active peptides

The branched chains in diphytanoyl lipids provide membranes with unique properties, such as high chemical/physical stability, low water permeability, and no gel-to-fluid phase transition at ambient temperature. Synthetic diphytanoyl phospholipids are often used as model membranes for electrophysiological experiments. To evaluate whether these sturdy lipids are also suitable for solid-state NMR, we have examined their interactions with a typical amphiphilic peptide in comparison with straight-chain lipids. First, their phase properties were monitored using ³¹P NMR, and the structural behaviour of the antimicrobial peptide PGLa was studied by ¹⁹F NMR and circular dichroism in oriented membrane samples. Only lipids with choline headgroups (DPhPC) were found to form stable lipid bilayers in oriented samples, while DPhPG, DPhPE and DPhPS display non-lamellar structures. Hence, the experimental temperature and hydration are crucial factors when using supported diphytanoyl lipids, as both parameters must be maintained in an appropriate range to avoid the formation of non-bilayer structures. For the same reason, a high content of other diphytanoyl lipids besides DPhPC in mixed lipid systems is not favourable. Unlike the situation in straight-chain membranes, we found that the α-helical PGLa was not able to insert into the tightly packed fluid bilayer of DPhPC but remained in a surface-bound state even at very high peptide concentration. This behaviour can be explained by the high cohesivity and the negative spontaneous curvature of the diphytanoyl lipids. These characteristic features must therefore be taken into consideration, both, in electrophysiological studies, and when interpreting the structural behaviour of membrane-active peptides in such lipid environment.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Lipid mixing during freeze-thawing of liposomal membranes as monitored by fluorescence energy transfer

A new pair of fluorescence-energy-transferring probes, dansylphosphatidylethanolamine and dioctadecylindocarbocyanine, were incorporated separately into phospholipid vesicles to monitor intervesicle lipid mixing under various conditions. The transfer efficiencies of mixtures of sonicated vesicles labeled with 2 wt% donor dansylphosphatidylethanolamine (DnsPE) or with 1 wt% acceptor dioctadecylindocarbocyanine (DiI-C18) were negligible, but increased to about 25% after the vesicles had been frozen in a solid CO2/ethanol bath, thawed and diluted. The freeze-thaw-induced mixing of lipids between vesicles, signified by energy transfer, was dependent on lipid concentration and was promoted by 0.5-1.5 M KCl, 0.5 M potassium trichloroacetate and 5 mM sodium acetate (pH 4) and inhibited by 0.5 M LiCl, 0.5 M glycerol, 0.5 M sucrose, 0.15 M KCl and 0.15-1.5 M NaCl. These results support and complement previously reported measurements of the trapped volumes, turbidities and population size distributions of similarly treated liposomes. Comparison of the responses of paucilamellar vesicles with those of multilamellar vesicles suggests that lipid mixing during freeze-thawing can occur either during interaction of the outermost bilayers of vesicles or during interaction of all bilayers, possibly as a result of breakdown and reformation of bilayer structure.

Models for slow anisotropic rotational diffusion in saturation transfer electron paramagnetic resonance at 9 and 35 GHz

Model systems of cholestane and 5-doxylstearic acid analogue spin probes in lipid bilayer dispersions of dipalmitoylphosphatidylcholine and cholesterol (9:1 w/w) are used to analyze saturation transfer electron paramagnetic resonance spectral behavior for slow rotational diffusion in an anisotropic medium. Measurements are made at both 9 and 35 GHz to provide enhanced spectral resolution for different types of motion. Parameter correlation plots of spectral parameters from different regions of the saturation transfer spectra appear to be potentially useful in characterizing different types of motion. Anisotropic rotational diffusion about a symmetry axis coincident with the nitroxide y principal axis is clearly distinguishable from isotropic rotational diffusion and may be distinguishable from rotational diffusion about the nitroxide z principal axis. Approximate anisotropic rotational diffusion about a symmetry axis coincident with the nitroxide z principal axis is distinguishable from isotropic rotational diffusion under some, but not all, conditions.

Lipid phase states influence glycophorin reconstitution

Reconstitution of glycophorin into dimyristoyl phosphatidylcholine and sphingomyelin vesicles was sub-maximal below the phase transition temperatures of these lipids. Reconstitution of glycophorin into diisostearoyl phosphatidylcholine and dioleoyl phosphatidylcholine liposomes was maximal within a range of temperatures below the phase transition temperatures of dimyristoyl phosphatidylcholine and sphingomyelin but above the phase transition temperatures of diisostearoyl phosphatidylcholine and dioleoyl phosphatidylcholine. These findings indicate a greater tendency for reconstitution of glycophorin into fluid as opposed to solid lipid phases.

Lipid vesicle-cell interactions. I. Hemagglutination and hemolysis

The interaction of lipid vesicles (liposomes) of several different compositions with erythrocytes has been investigated. Lecithin liposomes, rendered positively charged with stearylamine, exhibit potent hemagglutination activity in media containing low concentrations of electrolytes. The hemagglutination titer is found to be a linear function of the zeta potential of the lipid vesicles. Hemagglutination is reduced when the surface potential of the cells is made more positive by pH adjustment or enzyme treatment. Similarly, hemagglutination is reduced by increasing concentrations of electrolytes. Hemagglutination is examined theoretically and is shown to be consistent with vesicle-cell interactions that are due to only electrostatic forces. Vesicles containing lysolecithin in addition to lecithin and stearylamine cause lysis of erythrocytes, provided the lipids of the vesicles are above the crystal-liquid crystal phase transition temperature. In addition, hemolysis requires close juxtaposition of the vesicle to the cell membrane; vesicles precoated with antibodies exhibit severely diminished hemolytic activities, only a small fraction of which can be attributed to a reduction in hemagglutination titer. Evidence is presented indicating that a single vesicle is sufficient to lyse one cell. With regard to hemagglutination and hemolysis, lipid vesicles of simple composition mimic paramyxoviruses such as Sendai virus.