It is generally known that unilamellar vesicles prepared from
zwitterionic phosphatidylcholines (PCs)
do not aggregate in the presence of multivalent cations, as opposed to
the case of electrostatically stabilized
vesicles of acidic phospholipids. However, we have found that
Be2+ can induce the aggregation of PC
vesicles. The aggregation exhibited characteristic features
concerning the Be2+ concentration and
temperature, as follows: (i) there exists an optimum concentration
range of Be2+ to induce the aggregation,
and (ii) the aggregation becomes most pronounced at the temperature
corresponding to the bilayer phase
transition of the vesicle membranes; it is suppressed completely at
higher temperatures. In addition, the
Be2+-induced aggregation of PC vesicles was found to be
reversible with respect to the Be2+
concentration
and temperature. The reversibility of the aggregation suggests
that vesicles are trapped in a secondary
minimum of intervesicular potential in the aggregating entities.
This in turn leads to the speculation that
the repulsive hydration force characteristic of PC vesicles is
partially destroyed by the action of Be2+.
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