9,10-Diphenylanthracene based fluorescent probes as reporters of lipid membrane phase transitions

The two fluorescent probes 2-(2-bromoethoxy)-9,10-diphenylanthracene and 9,10-diphenylanthracene were evaluated for their suitability to report temperature-induced phase transitions in phospholipid formulations. Multilamellar liposomes were formed from two different lipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine and the probes were embedded in the bilayers. The fluorescence spectra of the embedded probes were collected over the temperature range of the lipid phase transitions. The transition temperatures were determined by differential scanning calorimetry and temperature-dependent fluorescence spectroscopy. The fluorescence lifetimes of the emitting species were calculated by time-resolved fluorescent spectroscopy in gel and fluid phases. The molecular dynamics simulations were employed to describe the impact of the probe positioning within the bilayer on their emitting properties. The probes did not significantly impact the physicochemical characteristics of the vesicles. Both probes can report on main lipid phase transitions in the two types of liposomes. However, upon transition from gel to fluid phase, the emitting species show longer-lived fluorescence.