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Fluorescence detection and characterization of packing defects in lyotropic liquid crystals

Lyotropic liquid crystals belong to a class of self-assembling nanomaterials. Packing defects among molecules play an important role in the physical properties and dynamics of lyotropic liquid crystals. Time-resolved fluorescence depolarization technique has been used to detect and characterize the onset of packing defects in lyotropic liquid crystals (lipid/water mixtures). Fully hydrated binary mixtures of phosphatidylethanolamine/phosphatidylcholine (PE/PC) exhibit mesoscopic packing defect state (D) as well as 1-dimensional lamellar liquid crystalline (L(alpha )) and 2-dimensional inverted hexagonal (HII) phases. Based on previous electron microscopic investigations, the presence of this D state is characterized by the presence of interlamellar attachments and precursors of HII phase between the lipid layers. Using a rotational diffusion model for rod-shape fluorophore in a curved matrix, rotational dynamics parameters, 2nd and 4th rank order parameters, localized wobbling diffusion and curvature-dependent rotational diffusion constants of rod-shape dipyenylhexatriene (DPH) labeled PC (DPH-PC) in the host PE/PC matrix were calculated from the measured fluorescence depolarization decays of DPH fluorescence. At the onset of packing defects, abrupt increases in the rotational dynamics parameters were observed. Our results suggest that rotational dynamics parameters are very sensitive in detecting packing defects in lyotropic liquid crystals. In addition, the presence of D state can be characterized by enhanced wobbling diffusional motion and order packing of lipid molecules and the presence of localized curvatures in the lipid layers.