Effect of mechanical, electrical and osmotic pressure on the phase transition properties and stability of phospholipid bilayers : Landau theory of one component systems

A theoretical model is developed to describe not only the temperature-induced phase transition of phospholipid bilayers but the phase transitions induced by different types of pressure as well. The calculated pressure-temperature phase diagrams show that the first order gel-liquid crystalline phase transition becomes a second order one for dipalmitoylphosphatidylcholine (DPPC) bilayers at the following critical parameters: Tc = 38.8 °C, Ác = 5*10-13 N/chain (transmembrane compression) or Tc = 38.2 °C, sc = 3*10-3 N/m chain (lateral tension). The boundary of liquid crystalline and isotropic fluid phase and the limit of membrane stability are shown on the phase diagrams too. From the limit of membrane stability the calculated pore edge energy is about 4*10-12 N. By means of the transmembrane pressure-temperature phase diagram the diagram of transmembrane potential-temperature is calculated too. According to this latter diagram the reversible electrical breakdown phenomenon is connected with a sudden change of the membrane structure from liquid crystalline phase into fluid phase and vice versa. It is shown that the transition into the fluid phase results in the formation of hydrophilic pores and as a consequence a drastic increase of membrane conductivity. These pores are distributed homogeneously at the surface of the flat membrane and their size is independent of the transmembrane potential. But at a given transmembrane potential we reach the limit of membrane stability and the complete mechanical breakdown of flat membrane takes place. However concerning the spherical vesicles this critical transmembrane potential results in only local instabilities or local mechanical breakdown, i.e. formation of holes at the “pole caps” where the largest values of the field strength vector exist. The size of these holes depends on the transmembrane potential and after removing the field a resealing process of vesicles takes place. In the phase diagrams between pure gel and pure liquid crystalline phase two mixed phases are shown i.e. gel phase with liquid crystalline clusters and liquid crystalline phase with gel clusters. It is pointed out that the width of this mixed phase region depends on the strength of different external fields and it is proportional to the width of gel-liquid crystalline phase transition at a constant rate of impurities. The calculated results of the model are in agreement with the following experimental data of phospholipid membranes: - phase transition temperature; - phase transition enthalpy changes; - phase transition volume changes; - Clapeyron slopes; - linear relationship between transition temperature and transition pressure; - reversibility of electrical breakdown; - electrical breakdown potential; - temperature dependence of electrical breakdown potential; - membrane conductivity during electrical breakdown; - pore edge energy; - critical pore radius; - critical salt concentration resulting in the lysis of the vesicles.