Julia Genova

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.

Influence of nanoparticle-membrane electrostatic interactions on membrane fluidity and bending elasticity

The aim of this work is to investigate the effect of electrostatic interactions between the nanoparticles and the membrane lipids on altering the physical properties of the liposomal membrane such as fluidity and bending elasticity. For this purpose, we have used nanoparticles and lipids with different surface charges. Positively charged iron oxide (γ-Fe2O3) nanoparticles, neutral and negatively charged cobalt ferrite (CoFe2O4) nanoparticles were encapsulated in neutral lipid 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine and negatively charged 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine lipid mixture. Membrane fluidity was assessed through the anisotropy measurements using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Though the interaction of both the types of nanoparticles reduced the membrane fluidity, the results were more pronounced in the negatively charged liposomes encapsulated with positively charged iron oxide nanoparticles due to strong electrostatic attractions. X-ray photoelectron spectroscopy results also confirmed the presence of significant quantity of positively charged iron oxide nanoparticles in negatively charged liposomes. Through thermally induced shape fluctuation measurements of the giant liposomes, a considerable reduction in the bending elasticity modulus was observed for cobalt ferrite nanoparticles. The experimental results were supported by the simulation studies using modified Langevin-Poisson-Boltzmann model.

Mechanoformation of neutral giant phospholipid vesicles in high ionic strength solution

We present new and simple method for formation of giant unilamellar vesicles (GUVs) in high ionic strength solutions, such as phosphate buffered saline (PBS). Mechanoformation method is an alternative method to electroformation method. The advantage of the mechanoformation procedure is that there are no limitations with respect to the ionic strength of the aqueous solutions, because there is no applied electric potential thus no current flow through the formation cell and no electrolysis is induced.

A study on the interaction of nanoparticles with lipid membranes and their influence on membrane fluidity

In recent years, liposomes encapsulated with nanoparticles have found enormous scopes in various biomedical fields such as drug design, transport, imaging, targeted delivery and therapy. These applications require a clear understanding about the interaction of nanoparticles with cell membranes. The present work aims to investigate the effect of encapsulation of uncharged and positively charged nanoparticles in three different types of lipids such as1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC),1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine and1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine(SOPC-POPS) mixture and archaeal lipids. Through the temperature dependent fluorescence anisotropy measurements, we have found that the entrapment of nanoparticles in the bilayer has decreased the lipid transition temperature and increased the membrane fluidity of all three types of lipid vesicles. The results were more predominant in SOPC-POPS mixture because of high density encapsulation of nanoparticles in the vesicles due to electrostatic interaction between negatively charged membrane and positively charged iron oxide nanoparticles.

Does maltose influence on the elasticity of SOPC membrane

Thermally induced shape fluctuations of giant quasi-spherical lipid vesicles are used to study the influence of the disaccharide maltose, dissolved in the aqueous solution, on the curvature elasticity kc of a lipid membrane. The influence of the carbohydrate solute is investigated throughout a considerably wide interval of concentrations. The values of the bending elastic modulus for 200 mM and 400 mM of maltose in the water solution are obtained. The data for kc in presence of maltose is compared with previously obtained results for this constant for the most popular hydrocarbons: monosaccharides glucose and fructose and disaccharides sucrose and trehalose. It is shown that the presence of maltose, dissolved in the aqueous phase surrounding the membrane does not influence on the bending elasticity with the increase of its concentration in the aqueous solution. Up to our knowledge this is the first sugar that does not show decrease of the bending elastic modulus of the lipid membrane, when present in the water surrounding it in concentration up to 400mM.

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.

Bending elasticity of lipid membranes in presence of beta 2 glycoprotein I in the surrounding solution

Thermally induced shape fluctuations of giant quasi/spherical lipid vesicles are used to study the bending elasticity modulus kc of a phospholipid (PHLP) membranes in presence of beta 2 glycoprotein I (β2/GPI) in the aqueous solution which surrounds the vesicles membrane. The bending elastic modulus kc of PHLP - protein membrane was obtained for different mass concentrations of β2/GPI for pure neutral SOPC membranes and for mixed SOPC: Cardiolipin negatively charged membranes. The experimental results for the bending elastic modulus kc of the PHLP membranes does not show dependence on the concentration of β2/GPI in the range from 5.5 to 55 >g/ml, when β2/GPI is present in the aqueous solution surrounding the vesicles membrane. Obtained results are in good agreement with predictions, based on different experiments, explaining the mechanism of binding of β2/GPI to neutral membranes.

Influence of Cholesterol on the Elastic Properties of Lipid Membranes

Thermally induced shape fluctuations of giant quasi-spherical lipid vesicles are used to study the influence of the cholesterol, incorporated in the lipid membranes, on the bending elasticity modulus kc of the lipid membrane. The influence of cholesterol is investigated throughout a considerably wide interval of concentrations. The values of the bending elastic modulus for 10, 20, 30 and 50 mol% of cholesterol in the SOPC membrane are obtained as a mean weighted value of 6–11 vesicles for each system. The dependence of the bending elasticity modulus on the concentration of cholesterol in the lipid membrane is obtained. At low concentration of cholesterol in the SOPC membrane (10 mol %) a decrease of the bending elasticity modulus is observed, compared to pure SOPC membrane. At high cholesterol content (50 mol% and above) a twofold increase of the bending modulus is obtained. The data for kc for mixed SOPC – cholesterol membrane is compared to the results obtained by different methods on different lipid matrices.

Chapter One – Marin Mitov Lectures: Measuring the Bending Elasticity of Lipid Bilayers

Abstract The thermally induced shape fluctuation method was developed both theoretically and experimentally in our laboratory under the supervision of Marin Mitov. He devoted his entire scientific career to the refinement of the theoretical derivation of the elastic deformation measurement, construction and realization of the experimental setup and developing a whole set of software programs for image processing, and experimental determination of elastic properties of lipid membranes by means of the thermally induced shape fluctuation method. The aim of this chapter in the Marin Mitov memorial edition was to put together his Lectures, containing the basic parts of the theoretical and experimental investigations of the developed method, together with the detailed description of the entire experimental setup with the realized stroboscopic illumination, and the data processing procedure for extracting the bending elasticity modulus of the lipid membrane developed by him.The thermally induced shape fluctuation method was developed both theoretically and experimentally in our laboratory under the supervision of Marin Mitov. He devoted his entire scientific career to the refinement of the theoretical derivation of the elastic deformation measurement, construction and realization of the experimental setup and developing a whole set of software programs for image processing, and experimental determination of elastic properties of lipid membranes by means of the thermally induced shape fluctuation method. The aim of this chapter in the Marin Mitov memorial edition was to put together his Lectures, containing the basic parts of the theoretical and experimental investigations of the developed method, together with the detailed description of the entire experimental setup with the realized stroboscopic illumination, and the data processing procedure for extracting the bending elasticity modulus of the lipid membrane developed by him.