INTRAMOLECULAR EXCIMER KINETICS OF FLUORESCENT DIPYRENYL LIPIDS .2. DOPE/DOPC MEMBRANES

The intramolecular dynamics of the excimer-forming dipyrenyl lipids (Dipy(n)PE) of different chain lengths (n) in fully hydrated dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidyl-choline (DOPC) binary mixtures was investigated by the use of frequency-domain fluorescence intensity decay technique. Using a 3-state model (see companion paper), the extent of aggregation and rotational rate of the two covalently attached pyrene moieties in Dipy(n)PE were estimated from the frequency-domain data. At 1 degrees C, the rotational rate and aggregation for Dipy(4)PE and Dipy(10)PE were insensitive to DOPE% of the lipid bilayer. At 27 degrees C, the rotational rate decreased, whereas the aggregation increased steadily for Dipy(10)PE as the DOPE% of the bilayer increased from 0 to 80. However, an abrupt increase in the rotational rate and a decrease in the aggregation for Dipy(10)PE were detected as the DOPE% reached 100, at which point the membranes are in the inverted hexagonal (H-parallel to) phase. No similar changes were found for Dipy(4)PE. These results indicate that the presence of PE with large intrinsic-curvature increases the lateral stress at the region near the center of the bilayer, and that this stress can be relieved as the membranes enter the highly curved H-parallel to, phase.