The thermotropic phase behavior of binary mixtures of dimyristoylphosphatidylcholine with dimyristoyl glycerol (DMPC-DMG) has been studied in aqueous dispersion by using differential scanning calorimetry and spin label electron spin resonance spectroscopy. Phase identifications have been made by means of (31)P nuclear magnetic resonance spectroscopy and x-ray diffraction. The binary phase diagram of DMPC-DMG mixtures displays three regions corresponding to the existence of compounds (C1 and C2, respectively) with approximately 1:1 and 1:2 mol/mol DMPC:DMG stoichiometries. The first region displays immiscibility between DMPC and C1 in the low temperature lamellar phase and miscibility of the components in the fluid phase that is lamellar. The second region displays immiscibility between C1 and C2 in the low temperature phase that is lamellar, whereas the fluid phase is of the inverted hexagonal type (H(II)). The third region displays immiscibility between C2 and DMG in the low temperature phase that is lamellar, whereas the fluid phase is isotropic. The presence of immiscible DMG in the low temperature phase of the third region is indicated by hysteresis in the temperature scans corresponding to conversion between the stable and metastable crystalline polymorphs. Analysis of the first region of the phase diagram using regular solution theory further demonstrates the existence of a DMPC:DMG complex with approximately 1:1 stoichiometry and provides parameters for the nonideality of mixing in the fluid phase.

Static and dynamic light scattering experiments are reported for the system 2,6‐dimethyl pyridine/water to determine the critical amplitude ζo of local concentration fluctuations and the critical amplitude Ωo of the characteristic frequency ΩD to scale ultrasound absorption data. It is found: ζo = (2.7 ± 0.2) · 10−8 cm and Ωo/π = 2.9 GHz. Measurements of the temperature dependence of specific heat capacity at constant pressure lead to a critical amplitude A = (0.183 ± 0.002) J g−1 K−1. The two scale factor universality is tested by combining the values of ζ0 and A as well as the values of ζ0 and τ*0 and B* (B*: critical amplitude of the coexistence curve, τ*0 critical amplitude of turbidity). The two ratios of the critical amplitudes calculated from the experimental data are higher by about 15% than the highest theoretical predicted value.

Aqueous dispersions (pH 4.0) of a 2:1 (mol/mol) mixture of myristic acid with dimyristoylphosphatidylcholine undergo a sharp transition at 45-47 degrees C from a lamellar gel phase to a fluid phase which is optically isotropic. This fluid phase gives rise to 31P-NMR spectra, and 2H-NMR spectra of the chain-deuterated components, which are also isotropic. X-ray diffraction studies of the fluid phase at 49 degrees C, reveal reflections with spacings in the ratio square root of 2: (square root of 3): square root of 4: square root of 6: square root of 8, accompanied by a strong diffuse scatter. These reflections index on a cubic lattice of primitive space group Pn3 or Pn3m, or possibly the body-centered group Im3m, with a lattice constant of 21.2 nm. The dimensions of the phase are consistent with a structure composed of two systems of tetrahedrally (octahedrally) oriented inverted lipid cylinders, found for other cubic lipid phases with Pn3m (Im3m) symmetry. At higher temperatures the cubic phase gradually converts, with increasing temperature, to a coexisting inverted hexagonal phase.

The content of N-acylethanolamines (NAEs) increases dramatically in cell membranes when the parent organism is subjected to injury or stress. This increase has been attributed to stress-combating mechanisms of the organism. In this study, a binary phase diagram of hydrated mixtures of N-palmitoylethanolamine (NP-E)--an endogenous ligand for the peripheral cannabinoid receptor (CB-2)--with dipalmitoylphosphatidylcholine (DPPC) is established by high-sensitivity differential scanning calorimetry (DSC). The structures of the phases involved were determined by using 31P-NMR spectroscopy and low-angle X-ray scattering. DSC studies show that NP-E and DPPC mix well in the composition range DPPC/NP-E=100:0 to 40:60 (mol/mol). At higher contents of NP-E, phase separation is indicated by the presence of additional transitions in the thermograms. Characterization of the structures formed by the mixtures with 31P-NMR shows that, up to 80 mol% NP-E, DPPC remains in the lamellar phase. The low-angle X-ray diffraction data are also consistent with a lamellar gel-phase structure for DPPC/NP-E mixtures up to 60 mol% NP-E. Above 70 mol% NP-E, NP-E phase separates in the gel-phase region, while complete miscibility is observed in the fluid phase. These results provide a structural basis for understanding the membrane interactions of NAEs, which is necessary for understanding the mechanism of their putative stress-combating role in the parent organisms.

No abstract