The results of a calorimetric and fluorescent probe study of the thermotropic behavior of various types of dispersions of dipalmitolphosphatidylcholine bilayer vesicles are reported. Bangham-type, multilamellar vesicles exhibit tow distinct phase transitions at 34.6 and 41.2 degrees C. On the other hand, single-lamellar spherical vesicles appear to exhibit a single transition at 37 degrees C. The single-lamellar vesicles are thermodynamically unstable below 27 degrees C and slowly transform into a multilamellar structure with a single phase transition of 41.2 degrees C. These transformed structures resemble, but are not identical with, Bangham-type vesicles. An experimentally testable thermodynamic and kinetic model based upon these results is developed.
The fluorescence depolarization associated with the hydrophobic fluorescent probe 1,6-diphenyl-1,3,5-hexatriene is used to monitor the changes in fluidity accompanying the gel-liquid crystalline phase transition in synthetic phosphatidycholine dispersions. The parameters of the phase transition are determined for both large, multilamellar liposomes and small, single-lamellar vesicles. These parameters are compared with those obtained using other techniques. In addition, the data are interpreted in terms of two limiting molecular models, which in turn offer insight into the structural differences between multilammelar liposomes and small vesicles.
Vibrational Raman spectroscopic experiments have been performed as a function of temperature on aqueous dispersions of synthetic DL-erythro-N-lignoceroylsphingosylphosphocholine [C(24):SPM], a racemic mixture of two highly asymmetric hydrocarbon chain length sphingomyelins. Raman spectral peak-height intensity ratios of vibrational transitions in the C-H stretching-mode region show that the C(24):SPM-H2O system undergoes two thermal phase transitions centered at 48.5 and 54.5 degrees C. Vibrational data for fully hydrated C(24):SPM are compared to those of highly asymmetric phosphatidylcholine dispersions. The Raman data are consistent with the plausible model that the lower temperature transition can be ascribed to the conversion of a mixed interdigitated gel state (gel II) to a partially interdigitated gel state (gel I) and that the higher temperature transition corresponds to a gel I----liquid-crystalline phase transition. The observation of a mixed interdigitated gel state (gel II) at temperatures below 48.5 degrees C implies that biological membranes may have lipid domains in which some of the lipid hydrocarbon chains penetrate completely across the entire hydrocarbon width of the lipid bilayer.
A recently developed differential scanning calorimeter has been used to characterize the thermotropic behavior of aqueous dispersions of liposomes containing sphingomyelin. Liposomes derived from sheep brain sphingomyelin exhibit a broad gel-liquid crystalline phase transition in the temperature range of 20-45 degrees C. The transition is characterized by maxima in the heat capacity function at 31.2 and 37.1 degrees C and a total enthalpy change of 7.2 +/-0.4 kcal/mol. Beef brain sphingomyelin liposomes behave similarly but exhibit heat capacity maxima at 30, 32, and 38 degrees C and a total enthalpy change of 6.9 kcal/mol. The thermotropic behavior of four pure synthetic sphingomyelins is reminiscent of multilamellar lecithin liposomes in that a single, sharp, main transition is observed. Results obtained for liposomes containing mixtures of different sphingomyelins are complex. A colyophilized mixture of N-palmitoylsphingosinephosphorylcholine, N-stearoylsphingosinephosphorylcholine, and N-lignocerylsphingosinephosphorylcholine in a 1 : 1 : 1 mol ratio exhibits a single transition with a Tm below that observed for the individual components. On the other hand a 1 : 1 mixture of N-stearoylsphingosinephosphorylcholine and 1-palmitoyl-2-oleylphosphatidylcholine exhibits three maxima in the heat capacity function. It is clear from these results that the thermotropic behavior of sphingomyelin-containing liposomes is a complex function of the exact composition. Furthermore, it appears that the behavior of the liposomes derived from natural sphingomyelins cannot be explained in terms of phase separation of the individual components.
The size and surface curvature dependence of the properties and stability of single lamellar vesicles have been investigated by using a variety of physicochemical techniques. Dipalmitoylphosphatidylcholine single lamellar vesicles of sizes ranging between 200 and 900 A in diameter have been prepared by the French press method and characterized with respect to their size distribution, stability, and thermotropic behavior by negative stain electron microscopy, molecular sieve chromatography, nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. Vesicles with a diameter smaller than 400 A are unstable below their transition temperature and fuse spontaneously to form larger single lamellar vesicles. Correlation analysis of experimentally obtained size distributions and calorimetric phase transitions profiles allowed estimation of the size dependence of the transition temperature. The phase transition temperature depends on the vesicle size in a sigmoidal fashion. Throughout the entire 200-700 A diamter range, the phase transition parameters are sensitive to size; however, the size dependence is especially pronounced around 400 A in diameter. The anomalous size dependence of the transition temperature for vesicles smaller than 400 A in diameter has been attributed to a decrease in the effective bilayer curvature due to packing rearrangements of the lipid molecules. Changes in the fractional degree of self-quenching of trapped 6-carboxyfluorescein induced by osmotic stress indicate that large single lamellar vesicles are not spherical under isoosmotic conditions. These vesicles are relatively flexible and can sustain almost a 2-fold increase in their internal aqueous volume without any leakage of the internal content.
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