NDLI: Nonideal mixing and phase separation in phosphatidylcholine-phosphatidic acid mixtures as a function of acyl chain length and pH.

Content Provider	PubMed Central
Author	Garidel, P. Johann, C. Blume, A.
Abstract	The miscibilities of phosphatidic acids (PAs) and phosphatidylcholines (PCs) with different chain lengths (n = 14, 16) at pH 4, pH 7, and pH 12 were examined by differential scanning calorimetry. Simulation of heat capacity curves was performed using a new approach that incorporates changes of cooperativity of the transition in addition to nonideal mixing in the gel and the liquid-crystalline phase as a function of composition. 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 onset and end of melting, which were corrected for the broadening effect caused by a decrease in cooperativity. In all cases the composition dependence of the nonideality parameters indicated nonsymmetrical mixing behavior. The phase diagrams were therefore further refined by simulations of the coexistence curves using a four-parameter approximation to account for nonideal and nonsymmetrical mixing in the gel and the liquid-crystalline phase. The mixing behavior was studied at three different pH values to investigate how changes in headgroup charge of the PA influences the miscibility. The experiments showed that at pH 7, where the PA component is negatively charged, the nonideality parameters are in most cases negative, indicating that electrostatic effects favor a mixing of the two components. Partial protonation of the PA component at pH 4 leads to strong changes in miscibility; the nonideality parameters for the liquid-crystalline phase are now in most cases positive, indicating clustering of like molecules. The phase diagram for 1,2-dimyristoyl-sn-glycero-3-phosphatidic acid:1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine mixtures at pH 4 indicates that a fluid-fluid immiscibility is likely. The results show that a decrease in ionization of PAs can induce large changes in mixing behavior. This occurs because of a reduction in electrostatic repulsion between PA headgroups and a concomitant increase in attractive hydrogen bonding interactions.
Related Links	http://dx.doi.org/10.1016/s0006-3495(97)78863-5
Ending Page	2210
Page Count	15
Starting Page	2196
File Format	PDF
ISSN	00063495
e-ISSN	15420086
Journal	Biophysical Journal
Issue Number	5
Volume Number	72
Language	English
Publisher Date	1997-05-01
Access Restriction	Open
Subject Keyword	Biophysics Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Biophysics

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