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Abstracts: Lectures
LECT-27
Revealing the difference between gel and liquid ordered (raft) phases
by a hydration-sensitive fluorescent probe
Guy Duportail, Gora M’Baye, Yves Mély and Andrey S. Klymchenko
Photophysique des Interactions Biomoléculaires, UMR 7175 du CNRS, Faculté de Pharmacie,
Université Louis Pasteur, 67401 Illkirch (France). E-mail:
So far, the existing fluorescence probe techniques cannot distinguish between gel phase and liquid ordered
(raft) phases since their physicochemical properties appear quite similar. In the present study, we used a
recently developed 3-hydroxyflavone fluorescent probe (F2N8), which exhibits high sensitivity to
hydration of lipid bilayers [1]. Experiments performed at 20 °C in large unilamellar vesicles composed of
sphingomyelin or DPPC with different concentrations of cholesterol reveall the strong dehydration of the
bilayer above a critical concentration of cholesterol which results in raft formation.
For the same samples, the anisotropy of TMA-DPH remains roughly constant, indicating no changes in the
viscosity upon transition from the gel phase to the liquid ordered phase. Further transition of the liquid
ordered phase to the liquid crystalline phase by an increase in the temperature results in a dramatic increase
in the bilayer hydration, while transition from gel to liquid crystalline phase does not affect hydration
significantly. Opposite tendencies are observed by measuring the anisotropy of TMA-DPH since the gelliquid
crystalline transition changes the anisotropy to a much larger extent than the liquid ordered-liquid
crystalline phase transition does.
Thus, while viscosities of the gel and liquid ordered phases are similar, the liquid ordered phase is much
less hydrated. This is in line with the high density packing of the ordered phase due to strong interaction of
lipids with cholesterol, which results in a decrease in the void space available for water molecules and thus
in dehydration of the bilayer.
Fluorescence spectra of probe F2N8 in
large unilamellar vesicles composed of
sphingomyelin (black line) and
sphingomyelin with 35 mol% of
cholesterol (red dotted lines), at 20 °C.
Fluorescence Intensity
1.2
0.9
0.6
0.3
0.0
450 500 550 600 650 700
Wavelength, nm
Sphingomyelin (gel)
Sphingomyelin (raft)
Reference: [1] A. S. Klymchenko et al., Biochim. Biophys. Acta 1665 (2004) 6-19.
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