No abstract

Scanning microcalorimetry has been used to study the high pressure effect on the main transition from the ripple gel P'(beta) phase to the liquid crystal (L(alpha)) phase in DPPC (dipalmitoylphosphatidylcholine). It has been demonstrated that an increase of the pressure by 200 MPa shifts the transition to higher temperatures by 36.4 degrees. The pressure increase does not affect the cooperativity of transition but reduces noticeably its enthalpy. The changes of the molar partial volume, isothermal compressibility as well as volume thermal expansibility during transition in DPPC suspension have been estimated. It has been shown that monovalent ions (Na(+), Cl(-)) in solution slightly affect the main thermodynamic parameters of the transition. Calcium ions significantly decrease distinction in compressibility and thermal expansibility between liquid-crystal and ripple gel phases of lipid suspension, which in its turn reflects less difference in their volume fluctuations.

The bilayer phase transitions from the ripple gel phase (P'(β)) to the liquid-crystal phase (L(α)) of a series of 1,2-diacylphosphatidylcholines containing a linear saturated acyl chain (C=14-19) have been studied by high-pressure scanning microcalorimetry. It has been shown that at ambient pressure, the transition temperature increases non-linearly depending on the acyl chain length. Pressure stabilizes the gel phase of lipids in a similar way; the pressure derivatives of the logarithm transition temperature as function of pressure are identical for all lipids. Based on the results obtained it has been concluded that the ratio γ of volume to enthalpy increments upon transitions in 1,2-diacylphosphatidylcholines is not dependent on the acyl chain length. When pressure grows, this ratio decreases drastically remaining identical for all lipids studied. Besides it has been demonstrated that increments of coefficients of thermal expansibility and isothermal compressibility are also rigidly bound to each other. Semi-empirical equations permitting to estimate volume parameters of the gel-to-liquid transition for 1,2-diacylphosphatidylcholines are given. The reasons for invariance of γ are discussed.