Abstract:
Electrostatic forces between membranes containing
charged lipids were assumed to play an important role in
influencing interactions between membranes long before
quantitative measurements of such forces were available.
~ur measurements were designed to measure electrostatic
forces between layers of lecithin charged with lipi~s carrying
ionizable head groups. These experiments have shown
that the interactions between charged lipid bila.yere are
dominated by electrostatic forces only at separations
greater than 30 A. At smaller separations the repulsion
between charged bilayers is dominated by strong hydration
forces.
The net repulsive force between egg lecithin bilayers
containing various amounts of cherged lipids
(phosphatidylglycerol (PG) 5,10 ano 50 mole%, phosphatidyli.
nosi tol (PI) 10 mole% and sodium oleate (Na-Ol)
3,5 and 10 mole%, where mole% gives the ratio of the number
of moles' of .charged lipid to the total number of moles
of all lipids present in the sample) was stuoied with the
help ('If the osmotic streas technique described by LeNeveu
et aI, (1977). Also, the forces between pure PG were j_nvestigated
in the same manner. The results have been
plotted showing variation of force as a function of bilay-
_ er separation dw• All curVes 90 obtained called force
curves, were found to be similar in sha.pe, showing two distinct
regions, one when dw<.30 A is a region cf very rapid iiivariation
of force with separation ( it is the region dominated
by hydre,tion force) and second when dw> 40 A is a
region of very slow variation of force with separB.tion
( it is the region dominated by the electrostatic force).
Between these two regions there exists a transition area
in which, in most systems studied, a phase separation of lipids
into fractions containing different amounts of charged
groups, was observed.
A qualitative analysis showed that our results were
v/ell described by the simple electrostatic double -le.yer
theory. For quantitative agreement between measured and
calculated force curves however, the charge density for
the calculations had to be taken as half of that given by
the number density of charged lipids present in the lecithin
bilayers. It is not clear at the moment what causes
such low apparent degree of ionization among the charged
head groups, and further study is needed in this area.
