The mixing behavior of pseudobinary phosphatidylcholine-phosphatidylglycerol mixtures as a function of pH and chain length
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The miscibility of phosphatidylcholine (PC) and phosphatidylglycerol (PG) with different chain lengths (n = 14, 16) was examined by differential scanning calorimetry (DSC) at pH 2 and pH 7. The determination of the coexistence curves of the phase diagrams was performed using a new procedure, namely the direct simulation of the heat capacity curves as described recently (Johann et al. 1996, Garidel et al. 1997). From the simulations of the heat capacity curves first estimates for the nonideality parameters for nonideal mixing as a function of composition were obtained and phase diagrams were constructed using temperatures for the onset and offset of melting which were corrected for the broadening effect caused by a decrease in cooperativity of the transition. In most cases, the composition dependence of the nonideality parameters indicated nonsymmetric mixing behavior. The phase diagrams were further refined by simulations of the coexisting curves using a four-parameter model to account for nonideal and nonsymmetric mixing in the gel as well as in the liquid-crystalline phase. The mixing behavior of the systems was analyzed as a function of pH and chain length difference to elucidate the effect of these two parameters on the shape of the phase diagrams. At pH 7 the phase boundaries are much closer together and a narrower coexistence range is obtained compared to the corresponding phase diagrams at pH 2. For DPPC/DMPG at pH 2, the shape of the phase diagram and the strongly positive nonideality parameter ρ1 for the liquid-crystalline phase indicates an upper azeotropic point. This indicates an unusual behavior of the system, namely more pronounced clustering of like molecules in the liquid-crystalline phase compared to the gel phase.
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