Frequency-resolved Intramolecular Excimer Fluorescence Study of Lipid Bilayer and Nonbilayer Phases

Frequency-domain fluorescence intensity decays of the intramolecular excimer forming (DipyPE) in a fully hydrated dioleoyl-phosphatidylethanolamine (DOPE) suspension have been measured at the monomer (395 nm) and excimer (475 nm) emissions and at different temperatures (0-30 degrees C). A classical Birks (two-state) and a new three-state kinetics models were used to analyze the frequency-domain data. The three-state model allowed us to resolve various intramolecular dynamics parameters of DipyPE in the host DOPE suspension. Those parameters are the excimer association (K(dm)) and dissociation (K(md)) rate constants, effective concentration (C), and lateral diffusion rate (f) of the pyrene moieties in the DipyPE. In contrast, only CK(dm) and K(md) were determined based on the two-state model. We observed that K(dm) declined while C increased abruptly at approximately 12 degrees C, the known thermotropic lamellar liquid crystalline-to-inverted hexagonal (L(alpha)-H(II)) phase transition temperature of DOPE. No abrupt changes in K(md) and f were observed at all temperatures. We concluded that the rotation of the lipid acyl chains is hindered and the free volume available for the lipid terminal methyl ends is reduced as the lipid membrane enters the highly curved H(II) phase from the planar L(alpha) phase.