Thermodynamics of the gel to liquid crystal 1,2-diacylphosphatidylcholines transition. High-pressure microcalorimetry

The bilayer phase transitions from the ripple gel phase (image) to the liquid crystal phase (Lα) of a series of 1,2-diacylphosphatidylcholines containing a linear saturated acyl chain (C = 14–19) have been studied by high-pressure scanning microcalorimetry. The effect of pressure on the temperature and heat effect of transitions has been analyzed. Based on the data obtained, the increment of main thermodynamic functions of transition including volume increments of lipid suspensions, increments of isothermal compressibility and thermal expansion during transition have been estimated. The character of the dependence of thermodynamic functions of transition on pressure and acyl chain lengths has been analyzed. The contribution of one methylene group to the basic thermodynamic functions of transition has been estimated. At ambient pressure, each CH2 group adds ~3.5 kJ mol−1 to the enthalpy increment, ~2.5 cm3 mol−1 to the volume increment, ~1.6 × 10−3 cm3 mol−1 MPa−1 and ~2.6 × 10−2 cm3 mol−1 K−1 to the partial pressure and temperature derivatives of the volume increment, respectively.