Perturbations of membrane structure by optical probes: II. Differential scanning calorimetry of dipalmitoyllecithin and its analogs interacting with merocyanine 540

Differential scanning calorimetry of multilamellar liposomes, interacting with the optical probe Merocyanine 540, yields quantitative information about perturbances of the bilayer structure induced by this dye. At low dye: lipid ratios, the dye perturbs primarily its own microenvironment, which is laterally separated from the unmodified lipid domain and exhibits modified thermotropic properties. A further increase in the dye concentration results in a perturbance of the whole lipid bilayer. The degree of perturbance is sensitive to structural modifications in the head-group region of the lipids. It is concluded that Merocyanine 540 reports in every case, even at infinite dilution, on localized events originating from a perturbed microenvironment.