Ion transport through thin lipid films

Currents through thin lipid films were measured versus temperature, applied voltage, pH value, ionic strength and ionic species. All experimental data tend to fit a two-component current theory, that is, the film current composed of a surface barrier jumping current and a surface recombination current. A good agreement was obtained in surface barrier height between theory and experiment. Breakdown phenomenon was observed and is interpreted as possibly arising from avalanche multiplication of ions, most probably from surface traps.     

Electrical capacity of black lipid films and of lipid bilayers made from monolayers

Planar bilayer membranes were formed from monolayers of a series of monounsaturated monoglycerides and lecithins. The hydrocarbon thickness of these membranes, as calculated from the electrical capacity, increases with the length of the fatty acid chain. The specific capacity of monoolein bilayers was found to be 0.745 μF/cm² which is nearly twice that of a monoolein black film made in the presence of decane, but is close to that obtained after freezing out the solvent from the black film. The hydrocarbon thickness of the bilayer, as calculated with a dielectric constant of 2.1, is considerably less than twice the length of the extended hydrocarbon chain of the monoglyceride.The specific capacity ($\text{C}{}_{\mathrm{<mtext>m</mtext>}}$) of bilayers made from monoolein monolayers showed a negligible voltage dependence, whereas the $\text{C}{}_{\mathrm{<mtext>m</mtext>}}$ increased significantly at a voltage of 150 mV in the case of Mueller-Rudin-type monoolein films with $\text{n-}\text{decane}$ as a solvent.     

Electrochemical investigations of cholesterol enriched glassy carbon supported thin lipid films

The formation and study of stable cholesterol enriched thin lipid layers onto the surface of glassy carbon electrode is reported in this work. The method of formation relies on additional thinning of wetting films by electrostriction. Electrochemical techniques based on the concepts of impedance and voltammetry are used to explore the films' features. The impedance data reveal a substantial change of relaxation characteristics of the modified films. In this respect, opportunities for the evaluation of the films' stage based on the approximation with 'constant phase angle element' are discussed. The possible final structure of the films, as well as, their relevance for development of sensor elements are briefly viewed.     

Nonlinearity in the I-V characteristics in thin lipid films: Effect of AC and DC fields

The current-voltage characteristics of bilayer lipid membranes of oxidized cholesterol separating two bathing solutions have already been extensively studied under a DC electric field. The observed deviation from linearity at high field has been explained by field-induced pore formation, which then act as ion channels in the membrane. Using thin films of oxidized cholesterol and of dipalmitoyl phosphatidylcholine, we have reported for the first time similar deviation from nonlinearity in the DC I-V characteristics when the applied field is above 40 V/cm. Upon application of an AC field, the conductivity increases as square of frequency, while the nonlinear nature of the I-V characteristic curve is still retained at all frequencies up to 5,000 Hz. Our results indicate that besides pore formation, the intrinsic electrical properties of the constituent lipid molecules are also responsible for the observed nonlinearity.     

Water permeability of thin lipid membranes

The osmotic permeability coefficient, P_f, and the tagged water permeability coefficient, P_d, were determined for thin (<100 A) lipid membranes formed from ox brain lipids plus DL-α-tocopherol; their value of approximately 1 x 10^-3 cm/sec is within the range reported for plasma membranes. It was established that P_f = P_d. Other reports that P_f > P_d can be attributed to the presence of unstirred layers in the experimental determination of P_d. Thus, there is no evidence for the existence of aqueous pores in these thin phospholipid membranes. The adsorption onto the membrane of a protein that lowers its electrical resistance by a factor of 10³ was found not to affect its water permeability; however, glucose and sucrose were found to interact with the membrane to modify P_f. Possible mechanisms of water transport across these films are discussed, together with the implications of data obtained on these structures for plasma membranes.     

Second-order susceptibilities of anisotropic liquid-crystal thin films

Using optical second-harmonic generation, we have measured the anisotropic orientation behavior of liquid crystal (LC) molecules in thin films. A high degree of in-plane anisotropy was obtained by depositing the LC films onto rubbed polyimide-covered substrates. The analysis with respect to the relevant second-order susceptibility tensor components is based upon the experimental results obtained in transmission geometry using different angles of incidence for the separation of isotropic and anisotropic contributions.     

The distribution of polyphosphoinositides in lipid films

Fluorescence microscopy of Langmuir films is used to determine the effect of polyphosphoinositides (PPIs) on the structure of phosphatidylcholine-containing monolayers. Dramatic alterations in the texture of these films occur as the fraction of PPI in the film is altered. These changes depend on the ionic strength of the underlying subphase and can be accounted for by considering the electrostatic interactions among PPIs. Surface adsorption of a fluorescent peptide derivative based on the PPI binding site of the protein gelsolin co-localizes with PPI-rich domains. Localization of polyphosphoinositides in domains within the inner leaflet of the plasma membrane is proposed to be a key element in some aspects of intracellular signaling, and these data have implications for explaining the cause of restructuring of such domains.     

Some studies on lipid peroxidation in monomolecular and bimolecular lipid films

Summary Hydrogen peroxide generated from dissolved oxygen through the alloxandialuric acid cycle affected both the permeability and the stability of lipid bilayer membranes. The permeability of the artificial membranes varied directly with the hydrogen peroxide concentration. Membrane stability varied inversely with the hydrogen peroxide concentration. Bilayers formed from solutions containing both phospholipid and the antioxidant vitamin E were less permeable and more stable in the presence of hydrogen peroxide than bilayers generated from solutions containing phospholipid alone. Peroxidation of phospholipid monolayers caused first an expansion of the films presumably through the introduction of peroxide groups. Further oxidation of phospholipid monolayers led to contraction of the films presumably through the formation of water-soluble products. The results of the monolayer studies and a consideration of the possible kinetics for the peroxidation reaction sequence have been used to explain the changes in the permeability and the stability of lipid bilayer membranes. Our data suggest that oxidation of lipid in biological membranes may first increase membrane permeability and then decrease membrane stability.     

Levan nanostructured thin films by MAPLE assembling

Synthesis of nanostructured thin films of pure and oxidized levan exopolysaccharide by matrix-assisted pulsed laser evaporation is reported. Solutions of pure exopolysaccharides in dimethyl sulfoxide were frozen in liquid nitrogen to obtain solid cryogenic pellets that have been used as targets in pulsed laser evaporation experiments with a KrF* excimer source. The expulsed material was collected and assembled onto glass slides and Si wafers. The contact angle studies evidenced a higher hydrophilic behavior in the case of oxidized levan structures because of the presence of acidic aldehyde-hydrogen bonds of the coating formed after oxidation. The obtained films preserved the base material composition as confirmed by Fourier transform infrared spectroscopy. They were compact with high specific surface areas, as demonstrated by scanning electron and atomic force microscopy investigations. In vitro colorimetric assays revealed a high potential for cell proliferation for all coatings with certain predominance for oxidized levan.     

Fracture formation in vitrified thin films of cryoprotectants

As a part of an ongoing effort to study the continuum mechanics effects associated with cryopreservation, the current report focuses on fracture formation in vitrified thin films of cryoprotective agents. The current study combines experimental observations with continuum mechanics analysis. Experimental results have been developed using a new imaging device, termed a "cryomacroscope", which has been recently presented by the current research team. A newly developed liquid nitrogen-based cooling stage is presented in this paper. The samples under investigation are 0.5 ml droplets of cryoprotective agents, having a characteristic diameter of 20 mm and a characteristic thickness of 1.5 mm. Tested samples included dimethyl sulfoxide (DMSO) in a concentration range from 6 to 8.4M, and the cryoprotectant cocktails VS55 and DP6. Some samples contained small bovine muscle segments, having a characteristic dimension of 1mm, in order to study stress concentration effects. Experimental results show that the onset of fracturing in vitrified films of cryoprotectants is very consistent, occurring over a small temperature range. Fracture pattern, however, was affected by the cooling rate. The presence of tissue segments did not affect the onset temperature of fracture, but affected the fracture pattern. The continuum mechanics analysis solidified the hypothesis that fracture is driven by thermal stress, not by temperature per se, and allowed fracture strain to be inferred from observed fracture temperature. In conjunction with the current report, additional photos of fracture formation in thin films are available at .     

Secondary structure of proteins associated in thin films

The solid state secondary structure of myoglobin, RNase A, concanavalin A (Con A), poly(L -lysine), and two linear heterooligomeric peptides were examined by both far-uv CD spectroscopy¹ and by ir spectroscopy. The proteins associated from water solution on glass and mica surfaces into noncrystalline, amorphous films, as judged by transmission electron microscopy of carbon-platinum replicas of surface and cross-fractured layer. The association into the solid state induced insignificant changes in the amide CD spectra of all α-helical myoglobin, decreased the molar ellipticity of the α/β RNase A, and increased the molar ellipticity of all-β Con A with no change in the positions of the bands' maxima. High-temperature exposure of the films induced permanent changes in the conformation of all proteins, resulting in less α-helix and more β-sheet structure. The results suggest that the protein α-helices are less stable in films and that the secondary structure may rearrange into β-sheets at high temperature. Two heterooligomeric peptides and poly (L -lysine), all in solution at neutral pH with “random coil” conformation, formed films with variable degrees of their secondary structure in β-sheets or β-turns. The result corresponded to the protein-derived Chou-Fasman amino acid propensities, and depended on both temperature and solvent used. The ir and CD spectra correlations of the peptides in the solid state indicate that the CD spectrum of a “random” structure in films differs from random coil in solution. Formic acid treatment transformed the secondary structure of the protein and peptide films into a stable α-helix or β-sheet conformations. The results indicate that the proteins aggregate into a noncrystalline, glass-like state with preserved secondary structure. The solid state secondary structure may undergo further irreversible transformations induced by heat or solvent. © 1993 John Wiley & Sons, Inc.     

Free-standing lipid films stabilized by Annexin-A5

Free-standing lipid bilayers in nano- and micro-pores are interesting membrane models and attractive for biotechnological applications. We describe here the controlled preparation of proteo-lipid mono- and bilayers using the Langmuir–Schaefer transfer or Langmuir–Blodgett technique, respectively on hydrophobic and hydrophilic surfaces. We demonstrate the formation of suspended proteo-lipid layers by Transmission Electron Microscopy (TEM) and in situ Atomic Force Microscopy (AFM) imaging. Using Annexin-A5 as a membrane-associated protein, continuous proteo-lipid mono- and bilayers were formed, which span pore arrays over areas of several square-micrometers. The 2D organization of proteins associated to lipid monolayer is well preserved during the transfer process and the protein association is Ca²⁺-dependent and therefore reversible. The simple formation and reliable transfer of stabilized free-standing lipid films is a first crucial step to create biomimetic membranes for biotechnological applications and membrane protein research.     

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.