title: Membrane order parameters for interdigitated lipid bilayers measured via polarized total-internal-reflection fluorescence microscopy.
abstract: Incorporating ethanol in lipid membranes leads to changes in bilayer structure, including the formation of an interdigitated phase. We have used polarized total-internal-reflection fluorescence microscopy (pTIRFM) to measure the order parameter for Texas Red DHPE incorporated in the ethanol-induced interdigitated phase (LβI) formed from ternary lipid mixtures comprising dioleoylphosphatidylcholine, cholesterol and egg sphingomyelin or dipalmitoylphosphatidylcholine. These lipid mixtures have 3 co-existing phases in the presence of ethanol: liquid-ordered, liquid-disordered and LβI. pTIRFM using Texas Red DHPE shows a reversal in fluorescence contrast between the LβI phase and the surrounding disordered phase with changes in the polarization angle. The contrast reversal is due to changes in the orientation of the dye, and provides a rapid method to identify the LβI phase. The measured order parameters for the LβI phase are consistent with a highly ordered membrane environment, similar to a gel phase. An acyl-chain labeled BODIPY-FL-PC was also tested for pTIRFM studies of ethanol-treated bilayers; however, this probe is less useful since the order parameters of the interdigitated phase are consistent with orientations that are close to random, either due to local membrane disorder or to a mixture of extended and looping conformations in which the fluorophore is localized in the polar headgroup region of the bilayer. In summary, we demonstrate that order parameter measurements via pTIRFM using Texas Red-DHPE can rapidly identify the interdigitated phase in supported bilayers. We anticipate that this technique will aid further research in the effects of alcohols and other additives on membranes.
authors: Measurement Science and Standards, National Research Council Canada, Ottawa, ON K1A 0R6, Canada; Department of Physics, University of Ottawa Ottawa, ON K1N 6N5, Canada.
authors: An T
authors: AT
authors: Ngo
authors: Measurement Science and Standards, National Research Council Canada, Ottawa, ON K1A 0R6, Canada.
authors: Zygmunt J
authors: ZJ
authors: Jakubek
authors: Measurement Science and Standards, National Research Council Canada, Ottawa, ON K1A 0R6, Canada.
authors: Zhengfang
authors: Z
authors: Lu
authors: Department of Physics, University of Ottawa Ottawa, ON K1N 6N5, Canada.
authors: Béla
authors: B
authors: Joós
authors: Ottawa Hospital Research Institute, Ottawa, ON K1H 8M5, Canada.
authors: Catherine E
authors: CE
authors: Morris
authors: Measurement Science and Standards, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. Electronic address: Linda.Johnston@nrc-cnrc.gc.ca.
authors: Linda J
authors: LJ
authors: Johnston
keywords: Ethanol; Interdigitated phase; Lipid bilayers; Order parameters; Polarized total internal reflection fluorescence microscopy