Kinetics of the gel-to-liquid phase transition of binary lipid bilayers using volume perturbation calorimetry
The relaxation kinetics of the gel-liquid crystalline transition of multilamellar vesicles (MLV) made of binary mixtures of dimyristoylphosphatidylcholine (DMPC), diapalmitoylphosphatidylcholine (DPPC) and distearoyl-phosphatidylcholine (DSPC) have been studied with volume perturbation calorimetry. The temperature and pressure relaxations following a volume perturbation were used to monitor the transition time-course. Data collected in the time domain were converted into and analyzed in the frequency domain using fourier series representations of the perturbation and response functions. In binary phosphatidylcholine mixtures, the relaxation process consists of more than one exponential decay. The overall relaxation rate of a binary lipid system is increased relative to a single component lipid system. The mean relaxation rate of DMPC-DSPC MLV containing 6 mole% DSPC is nearly three orders of magnitude greater than that of DMPC MLV. But addition of 6 mole% DMPC into DSPC had an effect on the mean relaxation rate at least one order of magnitude smaller. Our data for DMPC-DSPC, DMPC-DPPC and DPPC-DSPC systems show that mixtures with a small amount of lipid with higher melting temperature, T m , in a dominant amount of lipid with lower T m have faster mean relaxation rates than those with a small amount of lipid with lower T m in a dominant amount of lipid with higher T m . Two relaxation time maxima were observed in the phase transition region of al lthree 1:1 phosphatidylcholine mixtures studied, even though the corresponding heat capacity function may have only one maximum. These two relaxation time maxima correspond to temperatures where the two maxima of the equilibrium heat capacity function occur for 1:1 DMPC-DSPC mixture, or the onset and completion edges of the phase transition for 1:1 DMPC-DPPC and DPPC-DSPC mixture. These results suggest that the long relaxation times (∼ sec.) for MLV of pure lipid is not the result of bilayer-bilayer interactions; that binary lipid systems may exhibit dynamic lateral phase separation; and that diffusion may play a role in the dynamics of the relaxation process.
Key wordsLipid volume perturbation kinetics calorimetry phase transition, temperature phase transition, pressure
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