Elektrotehniˇ ski vestnik 76(3): 92–96, 2009 Electrotechnical Review, Ljubljana, Slovenija Simulations of Inverted Hexagonal Lipid Structures ˇ S´ arka Perutkov´ a, Aleˇ s Igliˇ c Laboratory of Physics, Faculty of Electrical Engineering, Trˇ zaˇ ska 25, SI-1000 Ljubljana, Slovenia E-mail: sarka.perutkova@fe.uni-lj.si, ales.iglic@fe.uni-lj.si Abstract. Phospholipid molecules are composed of multipolar headgroup and two electrically neutral hydrocarbon tails. The inverted hexagonal phase (HII ) belongs to the biologically most significant non-lamellar phospholipid phases in biomembranes. Hence the geometric properties and conditions of transition to the HII phase are nowadays widely studied. In our derivation of the free energy of lipid monolayers we assume that phospholipid molecules are in general anisotropic with respect to the axis perpendicular to the membrane plane. In our model the expression for the phospholipid monolayer free energy consists of two energy contributions: the bending energy which involves also deviatoric term, and the interstitial energy which describes the deformation energy due to stretching of the phospholipid molecule chains. On the basis of the derived expression for the phospholipid monolayer free energy we theoretically predict optimal geometry and physical conditions for the stability of the inverted hexagonal phase. Key words: inverted hexagonal phase, self-assembly of phospholipids, biomembranes, lamellar lipid phase, non-lamellar lipid phases Simulacije invertnih heksagonalnih lipidnih struktur Povzetek. Fosfolipidne molekule sestavljajo multipolna glava in dva nepolarna repa. Invertne heksagonalne faze (H II ) spadajo med najpomembnejˇ se fosfolipidne faze v bioloˇ skih membranah. Zato so geometrijske lastnosti ter pogoji faznega prehoda v H II fazo v zadnjem ˇ casu ohran- jajo ˇ siroko zanimanje. V naˇ sem teoretiˇ cnem modelu H II fosfolipidne faze upoˇ stevamo, da sestavljata prosto en- ergijo fosfolipidne enojne plasti dva prispevka: natezna energija repov fosfolipidnih molekul in upogibna en- ergija fosfolipidne plasti. Pri tem upoˇ stevamo, da so fos- folipidne molekule na sploˇ sno anizotropne glede na nji- hovo vzdolˇ zno glavno os. S pomoˇ cjo minimizacije proste energije fosfolipidne enojne plasti teoretiˇ cno napovemo optimalno geometrijo ter pogoje stabilnosti invertne hek- sagonalne faze (H II ) fosfolipidnih molekul. Kljuˇ cne besede: invertne heksagonalne faze, samoz- druˇ zevanje fosfolipidnih molekul, lamelarna lipidna faza, nelamelarne lipidne faze Received 3 February 2009 Accepted 18 March 2009 1 Introduction One of the main components of biological membranes are phospholipids. They have amphiphatic character, i.e. they comprise a polar head group as well as non-polar hydro- carbon chains in one molecule. Such molecules in aque- ous solution undergo a self-assembling process and form various structures. Biologically important lipid/water sys- tems are known for their rich polymorphism [1]. The driving force of this process is predominantly the hy- drophobic effect where the hydrophilic (polar) surfaces are in contact with water solution while the hydrophobic (non-polar) parts composed of lipid head-groups are hid- den from water [2]. The most common and biologically the most relevant phase is the fluid lamellar lipid bilayer phase (L α ), see Fig. 1. The bilayer of lipid molecules represents the basic building block of the plasma mem- brane, which encloses the cell interior. Nevertheless, non- lamellar model membranes are subject of increasing inter- est [1, 3, 4, 5], due to their importance in living organisms and due to their promising technical applications such as in drug delivery [6, 7], gene transport and nanotechnol- ogy [8]. The bicontinuous cubic phase, inverse hexagonal phase and inverse micellar cubic phase belong to the bi-