Temperature‐Induced Changes in Fluorescence Properties as a Probe of Porphyrin Microenvironment in Lipid Membranes

Temperature‐induced fluorescence changes were studied for hematoporphyrin and protoporphyrin, incorporated into liposomes of dipalmitoylphosphoglycerocholine (Pam 2 Gro P Cho) or dimiristoylphosphoglycerocholine (Myr 2 Gro P Cho). In some cases, cholesterol or cardiolipin were added to the vesicles for better mimicking the lipid composition of biological membranes. The experimental conditions were appropriately chosen in order to reproduce different possible configurations of the porphyrin molecule in lipid membranes: namely, at the polar water/headgroups, headgroups/lipid and lipid/lipid interfaces. A peculiar feature observed in some of the above liposomal systems was the appearence of discontinuities in the Arrhenius plots of the fluorescence quantum yields, with relevant changes of the values of activation energies. These discontinuities were due to an increase of the fluorescence signal in a temperature range corresponding to the transition of the different lipids from the gel‐to‐liquid crystal state. The observed phenomena are consistent with the formation of non‐covalent linear dimers or linear higher aggregates of the porphyrin molecules. The intermolecular contacts required for the formation of these species are favored by at least three situations: disruption of the ordered lipid structure during the gel‐to‐liquid crystal phase transition; competition of other polar groups (e.g., the ‐OH group of cholesterol) with the porphyrin carboxylate groups for the polar phospholipid headgroups; and steric constraints due to overcrowding of porphyrin molecules in a restricted space.