Interdigitated bilayer packing motifs: Raman spectroscopic studies of the eutectic phase behavior of the 1-stearoyl-2-caprylphosphatidylcholine/dimyristoylphosphatidylcholine binary mixture.

The thermotropic properties and acyl chain packing characteristics of multilamellar dispersions of binary mixtures of 1-stearoyl-2-caprylphosphatidylcholine (C(18):C(10)PC), an asymmetric chain species, and dimyristoylphosphatidylcholine (C(14):C(14)PC), a symmetric chain lipid, were monitored by vibrational Raman spectroscopy. In order to examine each component of the binary mixture separately, the acyl chains of the symmetric chain species were perdeuterated. As shown by differential scanning calorimetry, the mismatch in the gel phase bilayer thickness between the two lipid components generates a lateral phase separation resulting in two distinct gel phases, G(I) and G(II), which coexist over much of the composition range. The Raman data demonstrate that the mixed interdigitated phase (three chains per headgroup), analogous to single component phase behavior, is retained when the C(18):C(10)PC component act as a host for the G(I) gel phase. In contrast, the C(18):C(10)PC molecules exhibit partial interdigitation (two chains per headgroup) when they are included as guests within the C(14):C(14)PC host matrix to form the G(II) gel phase. Compared to pure C(14):C(14)PC bilayers at equivalent reduced temperatures, the host G(II) gel phase C(14):C(14)PC molecules exhibit an increased acyl chain order, while for the host G(I) gel phase the C(14):C(14)PC lipid species show increased intrachain disorder.