Victoria Vitkova

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.

Chapter Five - Lipid Bilayers and Membranes: Material Properties☆

Abstract In this chapter, the experimental data about the mechanical and the electromechanical properties of lipid bilayers and native membranes are collected and reviewed together with the existing methodology for their measurements. The emphasis is on the important mechanical property, membrane bending rigidity. The discussion is accompanied by illustrative experimental examples for the curvature elasticity of lipid bilayers with different compositions and in various environments. Particular attention is given to the alteration of the bending stiffness of lipid membranes as a consequence of the presence of biologically significant molecules such as cholesterol, carbohydrates, or amphiphilic peptides.In this chapter, the experimental data about the mechanical and the electromechanical properties of lipid bilayers and native membranes are collected and reviewed together with the existing methodology for their measurements. The emphasis is on the important mechanical property, membrane bending rigidity. The discussion is accompanied by illustrative experimental examples for the curvature elasticity of lipid bilayers with different compositions and in various environments. Particular attention is given to the alteration of the bending stiffness of lipid membranes as a consequence of the presence of biologically significant molecules such as cholesterol, carbohydrates, or amphiphilic peptides.

Dynamics of Lipid Vesicles: From Thermal Fluctuations to Rheology

Deformability is a key feature of the lipid membrane, being of importance for numerous processes taking place in biological cells, as well as for the flow behavior of cells in blood circulation. In the first part of the chapter, the potentials of investigating the dynamics of membrane fluctuations as an experimental tool for probing the membrane material properties are presented and discussed. By analysing the dynamics of thermally induced shape fluctuations of nearly spherical lipid vesicles, important mechanical constants of the bilayer are possible to be extracted, namely bending elasticity modules at free and blocked exchange of molecules between the two monolayers, comprising the lipid membrane, and the intermonolayer friction coefficient of the bilayer. The second part of this contribution is dedicated to the dynamics of unconfined lipid vesicles in linear hydrodynamic fields. The current state of theory and experiment of single vesicle dynamics in simple shear flows is reviewed. Special attention is given to the relation between the overall rheological properties of vesicle suspensions and the individual vesicle dynamics in the flow.Abstract Deformability is a key feature of the lipid membrane, being of importance for numerous processes taking place in biological cells, as well as for the flow behavior of cells in blood circulation. In the first part of the chapter, the potentials of investigating the dynamics of membrane fluctuations as an experimental tool for probing the membrane material properties are presented and discussed. By analysing the dynamics of thermally induced shape fluctuations of nearly spherical lipid vesicles, important mechanical constants of the bilayer are possible to be extracted, namely bending elasticity modules at free and blocked exchange of molecules between the two monolayers, comprising the lipid membrane, and the intermonolayer friction coefficient of the bilayer. The second part of this contribution is dedicated to the dynamics of unconfined lipid vesicles in linear hydrodynamic fields. The current state of theory and experiment of single vesicle dynamics in simple shear flows is reviewed. Special attention is given to the relation between the overall rheological properties of vesicle suspensions and the individual vesicle dynamics in the flow.

Deformation of giant vesicles in AC electric fields —Dependence of the prolate-to-oblate transition frequency on vesicle radius

The electrodeformation of giant vesicles is studied as a function of their radii and the frequency of the applied AC field. At low frequency the shape is prolate, at sufficiently high frequency it is oblate and at some frequency, fc, the shape changes from prolate to oblate. A linear dependence of the prolate-to-oblate transition inverse frequency, 1/fc, on the vesicle radius is found. The nature of this phenomenon does not change with the variation of both the solution conductivity, ?, and the type of the fluid enclosed by the lipid membrane (water, sucrose or glucose aqueous solution). When ? increases, the value of fc increases while the slope of the line 1/fc(r) decreases. For vesicles in symmetrical conditions (the same conductivity of the inner and the outer solution) a linear dependence between ? and the critical frequency, fc, is obtained for conductivities up to ?=114??S/cm. For vesicles with sizes below a certain minimum radius, depending on the solution conductivity, no shape transition could be observed.

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.

Influence of alamethicin on the passive water permeability of model lipid membranes and on the morphology of giant lipid vesicles

The water permeability of the membrane of a lipid vesicle in the presence of the natural peptide alamethicin was studied at various constant hydrostatic pressure differences between the inside and outside of the vesicle. Each pressure difference was created by sucking a vesicle into a micropipette at a given sucking pressure. It was determined that the presence of a 7 μM total alamethicin concentration in the experimental cell increased the membrane permeability towards water by about 15 times. A new experimental technique for the investigation of the morphological behavior of giant vesicles was developed. It was used for the study of the influence of alamethicin on the morphology of giant SOPC vesicles. Strong and reversible changes from a quasi-sphere to a discocyte were observed at 7 μM alamethicin concentration in the aqueous phase.

The aqueous surroundings alter the bending rigidity of lipid membranes

The bending elasticity is the mechanical property that characterizes the deformability of lipid bilayers. In the present study the bending elasticity of phosphatidylcholine lipid membranes is reported in aqueous media with various chemical composition and pH. The bending modulus is obtained from analysis of the thermal shape fluctuations, performed on nearly spherical giant lipid vesicles. Lower bending rigidity of phosphatidylcholine bilayers is measured in aqueous media, containing potassium or sodium chlorides, compared to its value in water without salts. The results reported here for the membrane bending elasticity at three acidic values of pH are compared with the literature data from micromanipulation measurements of giant unilamellar vesicles from the same lipid. In accordance with previous results, further evidences are provided for the softening of lipid bilayers in the presence of sucrose in the aqueous surroundings.

Phospholipase A2-Induced Remodeling Processes on Liquid-Ordered/Liquid-Disordered Membranes Containing Docosahexaenoic or Oleic Acid: A Comparison Study.

Vesicle cycling, which is an important biological event, involves the interplay between membrane lipids and proteins, among which the enzyme phospholipase A2 (PLA2) plays a critical role. The capacity of PLA2 to trigger the budding and fission of liquid-ordered (L(o)) domains has been examined in palmitoyl-docosahexaenoylphosphatidylcholine (PDPC) and palmitoyl-oleoylphosphatidylcholine (POPC)/sphingomyelin/cholesterol membranes. They both exhibited a L(o)/liquid-disordered (L(d)) phase separation. We demonstrated that PLA2 was able to trigger budding in PDPC-containing vesicles but not POPC ones. The enzymatic activity, line tension, and elasticity of the membrane surrounding the L(o) domains are critical for budding. The higher line tension of Lo domains in PDPC mixtures was assigned to the greater difference in order parameters of the coexisting phases. The higher amount of lysophosphatidylcholine generated by PLA2 in the PDPC-containing mixtures led to a less-rigid membrane, compared to POPC. The more elastic L(d) membranes in PDPC mixtures exert a lower counteracting force against the L(o) domain bending.

New optical method for measuring the bending elasticity of lipid bilayers

The knowledge of the elasticity of lipid bilayer structures is fundamental for new developments in biophysics, pharmacology and biomedical research. Lipid vesicles are readily prepared in laboratory conditions and employed for studying the physical properties of lipid membranes. The thermal fluctuation analysis of the shape of lipid vesicles (or flicker spectroscopy) is one of the experimental methods widely used for the measurement of the bending modulus of lipid bilayers. We present direct phase measurements performed on dilute vesicular suspensions by means of a new optical method exploiting holographic microscopy. For the bending constant of phosphatidylcholine bilayers we report the value of 23kBT in agreement with values previously measured by micropipette aspiration, electrodeformation and flicker spectroscopy of giant lipid vesicles. The application of this novel approach for the evaluation of the bending elasticity of lipid membranes opens the way to future developments in the phase measurements on lipid vesicles for the evaluation of their mechanical constants.