IR spectroscopic determination of gel state miscibility in long-chain phosphatidylcholine mixtures

We report on the gel-state microaggregation in binary mixtures of diacylphosphatidylcholines over temperatures ranging from -19 degrees C to near the gel-to-liquid crystal transition. Microaggregates with lateral dimensions in the range 1-100 chains were detected and measured with an isotope infrared method that relates the splitting or the shape of the methylene scissors band to aggregate size. Measurements were made on fully hydrated dispersions of diC(18)(D)PC/diC(20)(H)PC, diC(18)(D)PC/diC(22)(H)PC, and diC(18)(D)PC/diC(24)(H)PC at molar ratios of 4:1. Low levels of aggregation were determined with reference to the spectrum of the random mixture diC(18)(D)PC/diC(18)(H)PC. For diC(18)(D)PC/diC(20)(H)PC at -19 degrees C, which previous calorimetric measurements have indicated is a nearly ideal, we found about 4% of the minority component chains to be involved in aggregates. For diC(18)(D)PC/diC(22)(H)PC, the value increased to about 11%. DiC(18)(D)PC/diC(24)(H)PC was found to be highly fractionated, in agreement with the earlier studies. The unit subcell, which defines the type of acyl-chain packing, was determined for the components of the mixtures. The temperature behavior of the phases and the temperatures at which the minority component domains undergo dissolution were determined.