Vg. Levadny et al., THEORY OF ELECTROSTATIC EFFECTS IN SOFT BIOLOGICAL INTERFACES USING ATOMIC-FORCE MICROSCOPY, Biophysical journal, 70(4), 1996, pp. 1745-1752
We calculated the electrostatic force between a planar interface, such
as a planar-supported lipid bilayer membrane, and the tip of a stylus
on which another lipid bilayer or some other biomacromolecular system
might be deposited. We considered styli with rounded tips as well as
conical tips. To take into account the effect of dynamical hydrogen-bo
nded structures in the aqueous phase, we used a theory of nonlocal ele
ctrostatics. We used the Derjaguin approximation and identified the sy
stems for which its use is valid. We pointed out where our approach di
ffers from previous calculations and to what extent the latter are ina
dequate. We found that 1) the nonlocal interactions have significant e
ffects over distances of 10-15 Angstrom from the polar zone and that,
at the surface of this zone, the effect on the calculated force can be
some orders of magnitude; 2) the lipid dipoles and charges are locate
d a distance L from the hydrophobic layer in the aqueous medium and th
is can have consequences that may not be appreciated if it is ignored;
3) dipoles, located in the aqueous region, can give rise to forces ev
en though the polar layer is uncharged, and if this is ignored the int
erpretation of force data can be erroneous if an attempt is made to ra
tionalize an observed force with a knowledge of an uncharged surface;
4) the shape of the stylus tip can be very important, and a failure to
take this into account can result in incorrect conclusions, a point m
ade by other workers; and 5) when L is nonzero, the presence of charge
s and dipoles can yield a force that can be nonmonotonic as a function
of ionic concentration.