Atomic force microscopy studies of the adhesive properties of DPPC vesicles containing β-carotene

A role of carotenoids as modulators of physical properties of model and biological membranes has been already postulated. However, there is a lack of information on the influence of these pigments on interactions between the lipids which form such membranes. This paper applies atomic force microscopy (AFM) in to study the effects of beta-carotene on the adhesion properties of DPPC multilamellar liposomes. This allowed us to gain, for the first time, a direct insight into the interactions between the components in model systems on a molecular level. We observe that the adhesive forces in DPPC multilamellar liposomes containing 1 mol% of beta-carotene decrease exponentially with increasing temperature, and that at about 37 C they diminish. In the case of pure liposomes the decline in adhesion is of a different nature and the adhesive forces disappear at 34 C. The adhesive forces are about 5 times higher at 31 C in the presence of beta-carotene than in its absence. However, measurements using differential scanning calorimetry (DSC) showed a shift of the lamellar-to-undulled-lamellar phase transition toward lower temperatures by about 0.8 +/- 0.2 degrees C in a system containing beta-carotene. The enthalpy changes (Delta H) of this transition are similar for both systems. For the main transition, gel-to-liquid crystalline, the peak is shifted by about 0.5 +/- 0.1 degrees C, and Delta H decreases by about 30% in liposomes treated with beta-carotene in comparison to pure liposomes. Our results suggest increased cooperation between liposome components in a system with enriched beta-carotene, which cause a change in phase transition temperatures. Moreover, these interactions are very sensitive to temperature.