Intramolecular dynamics of lyotropic liquid crystals: Theory and experiment

The intramolecular dynamics of lyotropic liquid crystals have been investigated by the use of nanosecond resolved fluorescence spectroscopy. Frequency-domain fluorescence intensity decays of the intramolecular excimer forming dipyrenylphosphatidylethanolamine (DipyPE) in fully hydrated dioleoylphosphatidylethanolamine (DOPE) and dipyrenylphosphatidylcholine (DipyPC) in dimyristoylphosphatidylcholine (DMPC) suspensions have been measured at the monomer (395 nm) and excimer (475 nm) emissions and at different temperatures (0-40°C). A classical Birks (2-state) and a new 3-state kinetic models were employed to analyze the frequencydomain data. The 3-state model allowed one to resolve various intramolecular dynamics parameters of DipyPE or DipyPC in the host DOPE and DMPC suspensions. Those parameters are the excimer association (Keirn) and dissociation (Kmd) rate constants, effective concentration (C) and lateral diffusion rate (f) of the pyrene moieties in the DipyPE or DipyPC. In contrast, only CKeirn and Kmd were determined based on the 2- state model. It was observed that Keirn declined while C increased abruptly at -l2°C, the known thermotropic lamellar liquid crystalline-to-inverted hexagonal (La-HII) phase transition temperature of DOPE, for DipyPE. On the other hand, it was observed that Keirn increased while C declined abruptly at -22 °C, the known thermotropic lamellar gel-to-Lamellar liquid crystalline (Lp-La) phase transition temperature of DMPC, for DipyPC. No abrupt changes in Kmd and f were observed at all temperatures for either DipyPE or DipyPC. 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 either the HII or Lp phase from the La phase.