Fluidity of bacterial membrane lipids monitored by intramolecular excimerization of 1,3-di(2-pyrenyl)propane

Abstract

Intramolecular excimer formation of 1,3-di(2-pyrenyl)propane was used to study the fluidity of liposomes prepared from membrane polar lipids of Bacillus stearothermophilus. On the basis of spectral data, local polarity and polarizability parameters were established suggesting that the probe molecules are located well inside the membranes, but displaced towards the polar head groups of the phospholipid molecules. The excimerization rate is very sensitive to lipid phase transitions and pretransitions of synthetic pure lipid bilayers. In bacterial lipids from cultures grown at 55 and 68°C, thermal profiles of excimer to monomer intensity ratios ( I′ I ) show a broad transition which is displaced to higher temperatures in response to the increase of the growth temperature; these results correlate well with differential scanning calorimetry data and fluorescence polarization of diphenylhexatriene. Additionally, lipid bilayers of bacteria grown at 68°C exhibit a decreased membrane fluidity 1 1 The term fluidity is used here in an operational sense and defined as being directly proportional to the rate of excimer formation by 2Py(3)2Py or to the fluorescence depolarization of DPH. This fluidity is related but not identical with the physical definition of fluidity , as monitored by both fluorescent probes.