Isak Bivas

Temperature and Chain Length Effects on Bending Elasticity of Phosphatidylcholine Bilayers

A strong decrease of the bilayer bending elastic modulus, kc, has been experimentally established for dimyristoyl and dipalmitoyl phosphatidylcholine, when the liquid-crystal-gel phase transition temperature is approached from above. This phenomenon may be due to local-curvature-induced changes in the transition temperature. In the liquid-crystal phase, the bending modulus, kc, significantly increases as a function of the lipid chain length for dilauryl, dimyristoyl and dipalmitoyl phosphatidylcholine.

Sugars in the Aqueous Phase Change the Mechanical Properties of Lipid Mono- and Bilayers

ABSTRACT We applied two independent methods to measure the bending elasticity of SOPC lipid membranes in the presence of different sucrose concentrations in the aqueous phase. The micropipette technique was used to study the membrane bending rigidity in the concentration range of (0.11–0.30) mol/l of sucrose, while for sucrose concentrations 0 mol/l and 0.05 mol/l the thermal fluctuation analysis of quasi-spherical vesicles was applied. Both methods revealed a strong reduction of the bending elastic modulus, when sucrose is present in the water. Using micromanipulation of emulsion droplets, we investigated the sucrose influence on the stretching elasticity of SOPC lipid monolayers at the oil-water interface. Our results showed an almost two-fold reduction of the stretching elasticity modulus of the lipid monolayer in the presence of 0.18 mol/l of sucrose in the aqueous phase. The experimental results, reported here, reveal a strong influence of sugar molecules on the elasticity of lipid mono- and bilayers.

Permeability and the hidden area of lipid bilayers

The passive water permeability of a lipid vesicle membrane was studied, related to the hydrostatic (not osmotic) pressure difference between the inner and the outer side of the vesicle in a water environment without additives. Each pressure difference was created by sucking a vesicle into a micropipette at a given sucking pressure. The part of the membrane sucked into the micropipette (the projection length) was measured as a function of time. The time dependence can be divided into two intervals. We put forward the idea that smoothing of membrane defects, accompanied by an increase of the membrane area, takes place during the initial time interval, which results in a faster increase of the projection length. In the second time interval the volume of the vesicle decreases due to the permeability of its membrane and the increase of the projection length is slower. The hidden area and the water permeability of a typical lipid bilayer were estimated. The measured permeability, conjugated to the hydrostatic pressure difference, is an order of magnitude higher than the known value of the permeability, conjugated to the osmotic pressure difference. A hypothesis, based on pore formation, is proposed as an explanation of this experimental result.

Flexoelectric and steric interactions between two bilayer lipid membranes resulting from their curvature fluctuations

Abstract Steric repulsion of lipid membranes due to their thermally stimulated out-of-plane fluctuations has been calculated by Helfrich. In the present paper another method of calculation has been used and the fluctuating curvature-induced (flexoelectric) polarization of the membranes has been taken into account as well. This polarization leads to an attractive contribution to the interaction force. At a great distance the flexoelectric effect has modified the Helfrichs law of repulsion while at a short distance and big enough value of the flexoelectric coefficient a first order phase transition, similar to the onset of condensation in a non-ideal gas under higher pressure, has been predicted.

Electrical resistivity of the liquid phase of vesicular suspensions prepared by different methods

Giant lipid vesicles are obtained mainly by two methods of formation: (i) electroformation and (ii) gentle hydration (spontaneous swelling). Very often the electoformation is carried out in experimental cells consisting of indium-tin oxide (ITO) coated plates as electrodes and various polymer spacers. In the present work, the influence of the ITO coatings and the polymer spacers on the electrical resistivity of the liquid medium of electroformed vesicle suspensions is examined by means of electrochemical impedance spectroscopy (EIS). Our study is intended to point out possible implications of the electroformation method, especially in cases when phenomena, related to electric properties of the vesicle membranes, are investigated.

THERMAL SHAPE FLUCTUATIONS OF A QUASI SPHERICAL LIPID VESICLE WHEN THE MUTUAL DISPLACEMENTS OF ITS MONOLAYERS ARE TAKEN INTO ACCOUNT

Abstract In most of the previous theories dealing with shape thermal fluctuations of quasi spherical vesicles, the membrane was considered as a homogeneous shell characterized by its bending moduli. Our description considers the fluctuations to be followed by a lateral redistribution of the molecules within the bilayer and by an intermonolayer friction. These phenomena change the expressions for both the mean square values and the dynamic behavior of the fluctuation mode amplitudes. The corrections are expressed through the bending moduli of blocked and free exchange of the molecules within the monolayers. Numerical estimations show that such effects can be observed when shape fluctuations of giant quasi spherical vesicles are analyzed.

Influence of the Electric Double Layers of the Membrane on the Value of its Flexoelectric Coefficient

Abstract The potential jump due to the electric double layers across a spherically deformed charged membrane is calculated. The equations of Langmuir adsorption are used to estimate the change in the surface density of the membrane. The flexoelectric coefficient due to the electric double layers of the membrane is obtained for the cases of blocked and free flip-flop.

Chapter 11 Elasticity and Electrostatics of Amphiphilic Layers: Current State of the Theory and the Experiment

Abstract A critical review is made of the existing theoretical approaches for determination of the relationship between the electrostatic interactions and the mechanical properties of charged membranes composed of amphiphilic molecules (lipids, detergents, etc). Special attention is paid to the Gouy–Chapman theory of the electric double layer and its application for the calculation of the electrostatic corrections to the moduli of membrane elasticity. Some experimental data on the elasticity, tension, surface pressure, and lateral condensation of charged lipid bilayers and monolayers are compared with the theoretical predictions.

Digital holographic microscopy as a tool to study the thermal shape fluctuations of lipid vesicles

The bending elasticity modulus of lipid membranes is obtained by applying for the first time, to the best of our knowledge, a novel experimental technique based on digital holographic microscopy. The fluctuations of the radius with time were extracted by tracking and measuring the optical thickness at the vesicle poles. The temporal autocorrelation function of the vesicle diameter computed for each of the studied vesicles was then fitted with the theoretical expression to deduce the membranes tension and bending constant. For the bending elasticity modulus of SOPC bilayers, the value of (0.93 ± 0.03) × 10(-12) erg was obtained. This result is in accordance with values previously obtained by means of other conventional methods for the same type of lipid membrane in the presence of sugar molecules in aqueous medium. The obtained results encourage the future development of the digital holographic microscopy as a technique suitable for the measurement of the bending elasticity of lipid membranes.

Bending Elasticity Modulus of Giant Vesicles Composed of Aeropyrum Pernix K1 Archaeal Lipid

Thermally induced shape fluctuations were used to study elastic properties of giant vesicles composed of archaeal lipids C25,25-archetidyl (glucosyl) inositol and C25,25-archetidylinositol isolated from lyophilised Aeropyrum pernix K1 cells. Giant vesicles were created by electroformation in pure water environment. Stroboscopic illumination using a xenon flash lamp was implemented to remove the blur effect due to the finite integration time of the camera and to obtain an instant picture of the fluctuating vesicle shape. The mean weighted value of the bending elasticity modulus kc of the archaeal membrane determined from the measurements meeting the entire set of qualification criteria was (1.89 ± 0.18) × 10−19 J, which is similar to the values obtained for a membrane composed of the eukaryotic phospholipids SOPC (1.88 ± 0.17) × 10−19 J and POPC (2.00 ± 0.21) × 10−19 J. We conclude that membranes composed of archaeal lipids isolated from Aeropyrum pernix K1 cells have similar elastic properties as membranes composed of eukaryotic lipids. This fact, together with the importance of the elastic properties for the normal circulation through blood system, provides further evidence in favor of expectations that archaeal lipids could be appropriate for the design of drug delivery systems.