1063-7745/05/5002- $26.00 © 2005 Pleiades Publishing, Inc. 0308 Crystallography Reports, Vol. 50, No. 2, 2005, pp. 308–315. Translated from Kristallografiya, Vol. 50, No. 2, 2005, pp. 344–351. Original Russian Text Copyright © 2005 by Pikin, Kharakoz, Tiktopulo, Pikina. INTRODUCTION Recently, the physical nature of the phase transition “melting–chain ordering” and its relation to interlayer interactions in lipid systems have been intensively stud- ied [1–4]. These problems are of the fundamental importance both from physical and biological stand- points. Each bilayer in a membrane is a quasi-two- dimensional system, which undergoes the so-called main phase transition (chain ordering), and is similar to a first-order liquid–solid phase transition characterized by the jumps of the first derivatives of free energy with respect to temperature [1, 5]. At the same time, in the vicinity of the transition temperature T M , the lipid sys- tems demonstrate the behavior characteristic of second- order phase transitions, i.e., an increase in the density fluctuations in the bilayer plane, a jump in the flexural rigidity, and an anomalous increase in a distance between bilayers in the film (swelling) [6–8]. The cause of anomalous swelling and its relation to the nature of the main transition are the subjects of permanent dis- cussions. There are two alternative approaches to the explanation of this phenomenon. The first is based on the assumption that lipid bilayers are in the state close to critical [7, 8]. The second is based on the concept that the main phase transition is a weak first-order transition and has nothing in common with any critical state [6]. This approach yields the consistent quantitative expla- nation of such properties of a system of packed lipid molecules as density fluctuations, compressibility, and flexural rigidity. The concept of a weak first-order tran- sition turned out to be rather fruitful in description of phase transformations in many solids [9, 10]. There also exist some approaches to the description of phase transitions in smectic liquid crystals with two- and three-dimensional structures which take into account far- and short-range orientational and positional order- ing [11, 12]. Below, we consider the possibility of describing the above main phase transition in lipid systems as a weak orientational first-order phase transition whose charac- teristics are close to those of a second-order phase tran- sition characterized by pretransition phenomena and pronounced fluctuations. A first order transition in this case is explained by interactions between the molecule and density fluctuations at a low shear modulus. It was assumed [9, 10] that a shear modulus μ has the same finite value both in the low- and high-temperature phases. Only the case of a logarithmic temperature behavior of heat capacity was considered. In the present study, when studying dipalmitoyl-phosphatidylcholine (DPPC) and dimyristoyl-phospatidylcholine (DMPC), we considered the case of the zero μ value in the high- temperature phase (of the type of smectic A with the point symmetry group D ∞h ) and the case of a finite μ value in the low-temperature phase with the correlation of tilted molecules in the smectic plane (either of a smectic C or a smectic G with the point group C 2h or C i ). Moreover, the μ value may be a function of temper- LIQUID CRYSTALS Molecular Aspects of the Main Phase Transition in Lipid Systems as a Weak First-Order Phase Transition: 1. Model of Thermodynamic Behavior of Lipid Membranes S. A. Pikin*, D. P. Kharakoz**, L. I. Tiktopulo***, and E. S. Pikina**** * Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiœ pr. 59, Moscow, 119333 Russia e-mail: pikin@ns.crys.ras.ru ** Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences, Pushchino, Moscow oblast, 142292 Russia *** Institute of Proteins, Russian Academy of Sciences, Moscow oblast, Russia **** Institute for Problems of Oil and Gas, Russian Academy of Sciences, Moscow, Russia Received October 20, 2004 Abstract—The possibility to describe the principal phase transition in lipid systems as a weak orientational first-order phase transition with characteristics close to those of a second-order phase transition characterized by pretransition phenomena and rather strong fluctuations has been considered. A first order transition is explained by interactions between fluctuations of molecule orientations and density fluctuations at a low shear modulus. The jumpwise behavior of enthalpy, volume, heat capacity, compressibility, sound velocity and absorption, and the order parameter are analyzed. It is shown that because of molecular van der Waals interac- tions the shear modulus should give rise to more pronounced jumps of the above quantities with an increase in molecule lengths. © 2005 Pleiades Publishing, Inc.