Temperature-dependent bifurcation of cooperative interactions in pure and enriched in β-carotene DPPC liposomes

We examined the influence of temperature on lipid intermolecular interactions and the organization of bilayers within multilamellar dipalmitoylphosphatidylcholine (DPPC) liposomes. We also investigated the effect of 0.5 mol% β-carotene, a non-polar carotenoid, on the adhesive properties of these liposomes. Atomic force microscopy (AFM) and differential scanning calorimetry (DSC) were used to correlate the changes in the physical properties of the liposomal systems with their thermotropic behaviour. Using DSC we detected two transitions in pure DPPC vesicles and in those containing 0.5 mol% β-carotene. In both systems the pretransition occurred at 34.5(1)°C and the main phase transition at 41.4 °C during heating. Upon cooling, the temperatures of the pretransition and the main transition decreased by about 6 °C and 1 °C, respectively. Changes in enthalpy and entropy were also similar in the two investigated systems. Data obtained in parallel AFM force experiments show that the adhesive forces between the liposomal systems and AFM probe strongly depend on the loading rate. Moreover, their characteristic monotonic changes and discontinuities are sensitive to temperature. In the range of temperatures from 27 °C to 31 °C, i.e. below the temperature of phase transition from gel to ripple phase, the adhesive forces measured in a water environment are about an order of magnitude higher in the presence of β-carotene than in pure DPPC liposomes. The observed variable dependence of adhesion on the loading rate suggests that there are changes in the long- and short-range interactions between lipids, and that these may be related to the occurrence of some clustering effects. In addition, the simultaneous existence of different subphases was found in the gel phase of DPPC liposomes. The presence of β-carotene at a level of 0.5 mol% stimulates the structural reorganization of DPPC multilamellar vesicles and enhances the bifurcation phenomenon detected in these systems.