THEORY OF ELECTROSTATIC EFFECTS IN SOFT BIOLOGICAL INTERFACES USING ATOMIC-FORCE MICROSCOPY

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.