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Biophys J, July 2000, p. 416-425, Vol. 79, No. 1
and
*Laboratory for Fluorescence Dynamics, Department of Physics,
University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, and
Department of Biochemistry, Temple University School of
Medicine, Philadelphia, Pennsylvania 19140 USA
The effects of temperature and pH on Laurdan
(6-lauroyl-2-(dimethylamino)naphthalene) fluorescence intensity images
of giant unilamellar vesicles (GUVs) (~20-150 µm in diameter)
composed of the polar lipid fraction E (PLFE) from the
thermoacidophilic archaebacteria Sulfolobus
acidocaldarius have been studied using two-photon excitation.
PLFE GUVs made by the electroformation method were stable and well
suited for microscopy studies. The generalized polarization (GP) of
Laurdan fluorescence in the center cross section of the vesicles has
been determined as a function of temperature at pH 7.23 and pH 2.68. At
all of the temperatures and pHs examined, the GP values are low (below
or close to 0), and the GP histograms show a broad distribution width
(> 0.3). When excited with light polarized in the y
direction, Laurdan fluorescence in the center cross section of the PLFE
GUVs exhibits a photoselection effect showing much higher intensities
in the x direction of the vesicles, a result opposite
that previously obtained on monopolar diester phospholipids. This
result indicates that the chromophore of Laurdan in PLFE GUVs is
aligned parallel to the membrane surface. The x
direction photoselection effect and the low GP values lead us to
further propose that the Laurdan chromophore resides in the polar
headgroup region of the PLFE liposomes, while the lauroyl tail inserts
into the hydrocarbon core of the membrane. This unusual L-shaped
disposition is presumably caused by the unique lipid structures and by
the rigid and tight membrane packing in PLFE liposomes. The GP
exhibited, at both pH values, a small but abrupt decrease near 50°C,
suggesting a conformational change in the polar headgroups of PLFE.
This transition temperature fully agrees with the
d-spacing data recently measured by small-angle x-ray
diffraction and with the pyrene-labeled phosphatidylcholine and
perylene fluorescence data previously obtained from PLFE multilamellar vesicles. Interestingly, the two-photon Laurdan fluorescence images showed snowflake-like lipid domains in PLFE GUVs at pH 7.23 and low
temperatures (<20°C in the cooling scan and <24°C in the heating scan). These domains, attributable to lipid lateral separation, were
stable and laterally immobile at low temperatures (<23°C), again
suggesting tight membrane packing in the PLFE GUVs.
Biophys J, July 2000, p. 416-425, Vol. 79, No. 1
© 2000 by the Biophysical Society 0006-3495/00/07/416/10 $2.00
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