Effects of unsaturation and curvature on the transverse distribution of intramolecular dynamics of dipyrenyl lipids.

The roles of acyl chain unsaturation and curvature in the excimer formation efficiency (EFE) of site-specific conjugated pyrene molecules in lipid membranes have been investigated by steady-state and time-resolved fluorescence spectroscopy. Six 1-2-(pyrenyl-n-acyl)-phosphatidylcholine (dipy(n)PC) probes, with pyrenyl chains of varying methylene units n from 4 to 14 carbons, were incorporated separately into dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylethanolamine (DOPE) lipid membranes at 0.1 mol%. Both the excimer-to-monomer fluorescence intensity ratio and association-to-dissociation rate constant ratio of conjugated pyrenes were used to quantify EFE. At all temperatures (T = 0-30 degrees C) and for n = 4 and 6, the EFE for DOPE was always smaller than EFE for DOPC. At T < 10 degrees C (where DOPE and DOPC are in the liquid crystalline L alpha phase) and for n > 8, the EFE for curvature frustrated DOPE was significantly greater than EFE for nonfrustrated DOPC (control), and the difference increased gradually with n. At T> 18 degrees C (where DOPE is in the inverted hexagonal H(II) phase and DOPC is in the L alpha phase) and for n > 8, EFE for the curvature-relaxed DOPE was again smaller than the EFE for DOPC control. The contributions of splay conformation and internal dynamics of pyrenyl chains to EFE were examined separately using a lattice model. Our results suggest that i) the cis double bonds of the host lipid matrix strongly perturb both the conformation and dynamics of conjugated pyrenes at the specific location around n = 8, and ii) the lateral stress at the upper part (n < 8) of the curvature frustrated bilayer membranes (DOPE) may be significantly relaxed once the membrane surface adopts a favorable negative interfacial curvature.