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Biophys J, February 2001, p. 765-775, Vol. 80, No. 2

and
*Helsinki Biophysics and Biomembrane Group, Department of Medical
Chemistry, Institute of Biomedicine, University of Helsinki,
FIN-00014 Helsinki, Finland; and
The Hormel
Institute, University of Minnesota, Austin, Minnesota 55912 USA
The mixing behavior of dimyristoylphosphatidylcholine
(DMPC) with either N-palmitoyl-sphingosine (C16:0-ceramide) or
N-nervonoyl-sphingosine (C24:1-ceramide) was examined using
monomolecular films. While DMPC forms highly elastic liquid-expanded
monolayers, both neat C16:0-ceramide and C24:1-ceramide yield stable
solid condensed monomolecular films with small areas and low
interfacial elasticity. Compression isotherms of mixed
C16:0-ceramide/DMPC films exhibit an apparent condensation upon
increasing Xcer16:0 at all surface pressures. The average area isobars, coupled with the lack of a
liquid-expanded to condensed phase transition as
Xcer16:0 is increased, are indicative of
immiscibility of the lipids at all surface pressures. In contrast,
isobars for C24:1-ceramide/DMPC mixtures show surface
pressure-dependent apparent condensation or expansion and surface
pressure-area isotherms show a composition and surface
pressure-dependent phase transition. This suggests miscibility, albeit
non-ideal, of C24:1-ceramide and DMPC in both liquid and condensed
surface phases. The above could be verified by fluorescence microscopy
of the monolayers and measurements of surface potential, which revealed
distinctly different domain morphologies and surface potential values
for the DMPC/C16:0- and DMPC/C24:1-ceramide monolayers. Taken together,
whereas C16:0-ceramide and DMPC form immiscible pseudo-compounds,
C24:1-ceramide and DMPC are partially miscible in both the
liquid-expanded and condensed phases, and a composition and lateral
pressure-dependent two-phase region is evident between the
liquid-expanded and condensed regimes. Our results provide novel
understanding of the regulation of membrane properties by ceramides and
raise the possibility that ceramides with different acyl groups could
serve very different functions in cells, relating to their different
physicochemical properties.
Biophys J, February 2001, p. 765-775, Vol. 80, No. 2
© 2001 by the Biophysical Society 0006-3495/01/02/765/11 $2.00
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