  The interaction of two types of vesicle systems was investigated:
micrometer-sized, giant unilamellar vesicles (GUVs) formed from
1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and
submicrometer-sized, large unilamellar vesicles (LUVs) formed from oleic acid
and oleate, both in a buffered aqueous solution (pH=8.8). Individual POPC GUVs
were transferred with a micropipette into a suspension of oleic acid/oleate
LUVs, and the shape changes of the GUVs were monitored using optical
microscopy. The behavior of POPC GUVs upon transfer into a 0.8 mM suspension of
oleic acid, in which oleic acid/oleate forms vesicular bilayer structures, was
qualitatively different from the behavior upon transfer into a 0.3 mM
suspension of oleic acid/oleate, in which oleic acid/oleate is predominantly
present in the form of monomers and possibly non-vesicular aggregates. In both
cases, changes in vesicle morphology were observed within tens of seconds after
the transfer. Vesicle initially started to evaginate. In 60% of the cases of
transfer into a 0.8 mM oleic acid suspension, the evagination process reversed
and proceeded to the point where the membrane formed invaginations. In some of
these cases, several consecutive transitions between invaginated and evaginated
shapes were observed. In the remaining 40% of the cases of transfer into the
0.8 mM oleic acid uspension and in all cases of vesicle transfer into the 0.3
mM oleic acid suspension, no invaginations nor subsequent evaginations were
observed. An interpretation of the observed vesicle shape transformation on the
basis of the bilayer-couple model is proposed, which takes into account uptake
of oleic acid/oleate molecules by the POPC vesicles, oleic acid flip-flop
processes and transient pore formation.
