Historical notes:
The research on lyotropic liquid crystals in Laboratory of Liquid Crystals started in 1976 with the development of the phenomenological model of Petrov and Derzhanski1 describing the bending and stretching elasticity of a lipid monolayer as a constituent of the lipid bilayer. This very intuitive model links the geometrical shape of a lipid molecule, the strength of interactions between hydrophilic heads and between hydrophobic chains of similar molecules with the elastic moduli of a monolayer (a bilayer respectively) constituted of such molecules. The studies on the elasticity of lipid bilayers was continued during the PhD thesis of Bivas2 and Mitov3 under the leadership of professor Derzhanski. In 1984 a scientific collaboration was established with “Centre de Recherche Paul Pascal”, Bordeaux, France. During long term visits in Bordeaux of Bivas and Mitov, in tight collaboration with French colleagues Bothorel, Faucon, Meleard a method for measuring of the bending elastic modulus of lipid bilayers based on thermal shape fluctuations analysis was elaborated4-9. Later on the method of Evans10,11 for measuring the stretching elasticity modulus of lipid membranes by sucking a giant vesicle into a micropipette was adopted and successfully applied to measure not only the stretching elasticity of membranes but the bending elasticity as well11. The fruitful scientific collaboration with the Bordeaux team grew into “Programme International de Cooperation Scientifique” PICS in 1991 for a period of 4 years and then, in 1995, into creation of a French-Bulgarian Laboratory “Vesicles and Membranes” under a contract between CNRS, France, and the Bulgarian Academy of Sciences and Sofia University, Bulgaria. The aim of the creation of this French-Bulgarian Laboratory “without walls” was to help Bulgarian and French scientists to perform concurrent investigations on the leading edge of this scientific domain. The quality of the common work attracted new partners, French, Bulgarian or others. The onset of an international research network can be established.
Methods and equipment:
The purchase of the equipment in use is funded by CNRS, France, Bulgarian Academy of Sciences, Bulgaria, Swiss National Fund, Swiss, Bulgarian National Fund.
Two principal methods and corresponding equipment are used in the Laboratory of Liquid Crystals for measuring the elastic properties of bilayer lipid membranes:
The method of analysis of thermally induced shape fluctuations is jointly developed by members of French-Bulgarian laboratory4,6-9. It consists in observation of thermal shape fluctuations of giant vesicles under phase contrast microscope. The images are visualized by a CCD videocamera and recorded on a S-VHS videorecorder for further analysis. The signal from the videorecorder is fed to a frame grabber for digitalization and storage on the hard disk of a PC. The stored digital images are further analyzed by home made software. Due to the intrinsic limitations of the imaging equipment (the CCD camera) leading to deformation of the extracted shape information a stroboscopic illumination9 is used that permits to obtain almost instant images of the moving (deforming) object. The stroboscopic illumination cancels the effect of image smearing and permits the extraction of precise data. The home made software measures the vesicle radius, calculates geometrical parameters and by a fitting procedure finds the bending elastic modulus kc and the apparent membrane stress s.
The micropipette method of vesicle aspiration10,11 is also jointly developed by members of French-Bulgarian laboratory. It consists in sucking a giant vesicle into a micropipette with a radius smaller then the vesicle radius. The length of that part of the lipid membrane sucked into the pipette is measured as a function of the sucking pressure. At low sucking pressure one can measure the bending elastic constant kc, and at high enough sucking pressure the stretching elastic modulus ks, is measured11. The same equipment can be used to measure the membrane permeability under different transmembrane pressure difference. The visualization of fluorescent vesicles is realized by an image intensifier coupled to the CCD videocamera and recorded on a S-VHS videorecorder for further analysis. The signal from the videorecorder is fed to a frame grabber for digitalization and storage on the hard disk in a PC. The stored digital images are further analyzed by home-made software.
Recent research subjects:
 | Development of the method of analysis of thermally induced shape fluctuations for the measurement of the membrane bending rigidity4,6-9 |
| Temperature and cholesterol influence on the membrane bending rigidity12 |
| Elastic properties of lipid bilayers containing modified lipids13,14,15 |
| Thermal shape fluctuations of a quasi spherical lipid vesicle when mutual displacements of its monolayers are taken into account16 |
| Bending elasticity and bending fluctuations of lipid bilayer containing an additive17 |
| Free energy of a fluctuating vesicle. Influence of the fluctuations on the Laplace law18 |
| Elasticity and shape equation of lipid membrane19 |
| Mechanical properties of a lipid monolayer on the oil-water interface20,21 |
| Electromechanical properties of model membranes22 |
| Bending rigidity of charged membranes studied via micromanipulation of giant lipid vesicles23,24 |
| The permeability of lipid bilayer towards water at different hydrostatic pressure differences25,26,27 |
| The influence of the natural amphiphilic peptide alamethicin on the mechanical properties, permeability and morphology of giant lipid vesicles28 |
| Analysis of the shape fluctuations of quasi-spherical giant vesicles under a stroboscopic illumination29,30 |
| Rheology of vesicular suspensions in a hydrodynamic field31,32 |
References:
1. A. G. Petrov, A. Derzhanski, J.
Physique, C3 (1976) 155.
2. I. Bivas, Intermolecular and Interaggregate Interactions in Lyotropic Systems,
PhD thesis, 1981, ISSP, Sofia.
3. M. D. Mitov, Elasticity ans Stability of Bilayer Lipid Membranes, PhD thesis,
1981, ISSP, Sofia.
4. I. Bivas, P. Meleard, P. Bothorel, P. Lalanne, O. Aguerre-Chariol, J.
Physique, 48 (1987) 855.
5. S. Milner, S. Safran, Phys. Rev. A 36 (1987) 4371.
6. J.-F. Faucon, M. D. Mitov, M. Meleard, I. Bivas, P. Bothorel, J. Physique, 50
(1989) 2389.
7. M. Mitov, J.-F. Faucon, M. Meleard, P. Bothorel, in: Advances in
Supramolecular Chemistry, G. W. Gokel Ed., JAI Press, 1992, 93.
8. P. Meleard, M. D. Mitov, J.-F. Faucon, P. Bothorel, Europhys. Lett. 11 (1990)
355.
9. P. Meleard, J.-F. Faucon, M. D. Mitov, P. Bothorel, Europhys. Lett. 19 (1992)
267.
10. E. Evans, Biophys. J. 13 (1973) 941.
11. E. Evans, W. Rawicz, Phys. Rev. Lett. 64 (1990) 2094.
12. P. Meleard, C. Gerbeaud, T. Pott, L. Fernandez-Puente, I. Bivas, M. D. Mitov,
J. Dufourcq, P. Bothorel, Biophys. J., 72 (1997) 2616.
13. I. Bivas, D. Georgescauld, N. Jeandaine, M. Winterhalter, M. Meleard, G.
Marinov, P. Bothorel, Progr. Colloid Polym. Sci. 104 (1997) 197
14. I. Bivas, M. Winterhalter, P. Meleard, P. Bothorel, Europhys. Lett. 41
(1998) 261.
15. I. Bivas, V. Vitkova, M. D. Mitov, M. Winterhalter, R. Alargova, P. Méléard, P. Bothorel, "Mechanical Properties of Lipid Bilayers, Containing Grafted Lipids", Perspectives in Supramolecular Chemistry, vol. 6, Giant Vesicles, Ed. P. L. Luisi and P. Walde, (John Willey & Sons Ltd., 2000) Chapter 14, pp 207 - 219
16. I. Bivas, P. Meleard, I. Mirtcheva, P. Bothorel, Coll. Surfaces A, 157 (1999)
17. I. Bivas and P. Meleard, Phys. Rev. E, 67, 012901 (2003)
18. I. Bivas, in "Giant Vesicles", P.
Walde and P. L. Luisi eds., John Willey & Sons, (2000) 95.
19. I. Bivas, Elasticity and shape equation of a liquid membrane, Euro. Phys. J.
B 29 pp. 317-322 (2002).
20. V. Vitkova, Elasticite, permeabilite et morphologie des bicouches lipidiques
en presence d’additifs hydrophiles et amphiphiles, PhD thesis, 2002, ENSCR,
Rennes.
21. V. Vitkova, J. Genova, M.D. Mitov, and I. Bivas, “Mechanical Properties of
Lipid Mono- and Bilayers in the Presence of Small Carbohydrates in the Aqueous
Phase”, C. R. Acad. Bulg. Sci. 57 (6) (2004)
22. M. D. Mitov, M. Meleard, M. Winterhalter, M. Angelova, P. Bothorel, Phys.
Rev. E 48 (1993) 628.
23. V. Vitkova, J. Genova, K. Hristova, Y. Ermakov, I. Bivas, M. D. Mitov, 7th
International Frumkin Symposium “Basic Electrochemistry for Science and
Technology”, 23-28 October 2000, Moscow, Russia.
24. V. Vitkova, J. Genova, O. Finogenova, Y. Ermakov, M.D. Mitov, and I. Bivas,
“Surface Charge Effect on the Lipid Bilayer Elasticity”, C. R. Acad. Bulg. Sci.
57 (11) (2004)
25. V. Vitkova, J. Genova and I. Bivas, “Pores – Possible Mechanism of
Communication Between the Two Sides of a Bilayer Under Tension”, Materials for
Information Technology in the New Millennium, edited by J. M. Marshall, A. G.
Petrov, A. Vavrek, D. Nesheva, D. Dimova-Malinovska, J. M. Maud (Bookcraft:
Bath, 2001) pp. 448 – 451
26. V. Vitkova, J. Genova and I. Bivas, “Experimental and Theoretical Study of
Lipid Bilayers Permeability and Hidden Area”, C. R. Acad. Bulg. Sci. 55 (10) pp
15-20 (2002)
27. V. Vitkova, J. Genova and I. Bivas, “Permeability and Hidden Area of Lipid
Bilayers”, Eur. Biophys. J. 33 (8) pp 706-714 (2004)
28. V. Vitkova, J. Genova and P. Méléard, “Influence of alamethicin on the
passive water permeability of model lipid membranes and on the morphology of
giant lipid vesicles”, J. Mater. Sci.: Mater. El. 14 (10-12) pp 819-820 (2003)
29. J. Genova, V. Vitkova, L. Aladgem, P. Meleard, M. D. Mitov, “Using
Stroboscopic Illumination to Improve the Precision of the Bending Modulus
Measured by the Analysis of Thermally Induced Shape Fluctuations of Giant
Vesicles”, Bulg J Phys, 30 (2003)
30. J. Genova, V. Vitkova, L. Aladgem, M. D. Mitov, “The stroboscopic
illumination gives new opportunities and improves the precision of the bending
elastic modulus measurement”, J. Optoel. Adv. Mater. 7 (1), pp. 257-260 (2005)
31. V. Vitkova, M. Mader, and T. Podgorski, “Deformation of vesicles flowing
through a capillary”, Europhys. Lett. 68 (3), pp. 398-404 (2004)
32. V. Vitkova, M. Mader, T. Biben, and T. Podgorski, “Tumbling of Deformable
Lipid Vesicles, Enclosing a Viscous Fluid under a Shear Flow”, J. Optoel. Adv.
Mater. 7 (1), pp. 261-264 (2005)