DETECTION AND CHARACTERIZATION OF THE ONSET OF BILAYER PACKING DEFECTS BY NANOSECOND-RESOLVED INTRAMOLECULAR EXCIMER FLUORESCENCE SPECTROSCOPY

Bilayer packing defects in binary dilinoleoylphosphatidylethanolamine and 1-palmitoyl-2-oleoylphosphatidylcholine (DLPE/POPC) lipid mixtures have been studied by the use of nanosecond-resolved intramolecular ex cimer fluorescence spectroscopy. Frequency-domain fluorescence intensi ty decays of dual-chain labelled dipyrenyl lipids of different chain l engths in DLPE/POPC mixtures were acquired at both the monomer (392 nm ) and excimer (475 nm) emission channels and at 20 degrees C. On the b asis of a new intramolecular excimer formation kinetic model, the exte nt of aggregation and the rotational mobility, in terms of the equilib rium constant of the monomer to aggregated state and the excimer assoc iation rate constant, respectively, of the intralipid pyrenes were cal culated from the frequency-domain data. Within the range of 60-100% DL PE where bilayer defects are known to coexist with bilayer and non-bil ayer states, a prominent peak in the equilibrium constant and a concom itant dip in the excimer association constant at similar to 80% DLPE w ere observed. Our nanosecond-resolved fluorescence results suggest tha t the intramolecular excimer kinetic parameters of dipyrenyl lipids ar e very sensitive to the onset of bilayer packing defects in lipid memb ranes. Moreover, the onset of bilayer defect state is characterized by the greater extent of aggregation and more hindered rotational mobili ty of the acyl chains as compared with the bilayer (0% DLPE) and non-b ilayer inverted hexagonal (100% DLPE) states of the lipid membranes.