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of the inner and outer lealets of SLBs observed in temperature-controlled
AFM experiments.
In addition to the decoupling of the two lealets,
the symmetric versus asymmetric distribution of lipids within SLBs is a
question that is not elucidated yet. We and others have observed a symmetric
distribution of lipids in the inner and outer membrane lealets at least for
the mixture dioleoyl-phosphatidylcholine/dipalmitoyl-phosphatidylcholine
(DOPC/DPPC) and distearoyl-phosphatidylcholine (DSPC)/DPPC,
21,22
whereas the opposite trend has also been noted for the same mixtures.
18-20
23,24
An intermediate situation, mixed symmetry, was observed with the DSPC/
dilauroyl-phosphatidylcholine (DLPC) mixture. This was explained by a
difference in the method of SLB fabrication, i.e. the temperature used for MLV
extrusion and fusion that can inluence the symmetry of lipid distribution
into the bilayers.
24
Further systematic studies are clearly needed to better
understand the molecular mechanisms underlying this phenomenon. Finally,
it is noteworthy that SLB formation can also be inluenced by the roughness of
the supporting surface. The shape of gel domains within DOPC-DPPC bilayers
formed under the same experimental conditions is completely different when
(a)
(b)
(c)
(d)
Figure 1.2.
AFM imaging of DOPC/DPPC supported lipid bilayers. DOPC-DPPC lipid
mixtures (1:1) were used to form SLBs, either on freshly cleaved mica (a, c, d) or
on clean glass (b). The shape of the DPPC domains was completely different for the
two supports (compare a and b). This difference is probably due to the roughness
of glass (~0.2 nm) compared with mica (~ 0.04 nm). (d) is the phase image of the
SLB obtained simultaneously with the height image in c. As expected, the phase lag is
lower for a gel phase compared with a luid phase. The
z
scale is 10 nm (a, b, c), and
the phase scale is 15° (d). Images were obtained in the tapping mode. Scale bars are 1
μm (a, b) and 0.5 μm (c, d).
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