AJP - Cell Physiology, Vol 235, Issue 3 109-C117, Copyright © 1978 by American Physiological Society
Screening of membrane surface charges by divalent cations: an atomic representation
J. S. D'Arrigo
To arrive at a clear and atomically realistic representation of the process
of ionic screening, a model with the following necessary and justifiable
constraints was devised. 1) The minimum internuclear distance (IND) between
a negative site on the membrane and a cation screening the site is equal to
the sum of the site's "equivalent" radius (rs) + the diameter of a water
molecule (approximately 2.8 A) + crystal radius of the cation (rc). 2) The
average value for the dielectric constant (D) over IND is given by D
approximately 80 ((IND - rs - rc)/IND). When this simple atomic model for
ionic screening is employed in conjunction with equilibrium ion-selectivity
theory, it is possible to predict quantitatively, from coulombic energy
calculations, the secondary stereospecific actions of certain
alkaline-earth cations as well as the predominant screening effect of these
divalent cations at the surfaces of different types of membranes. The model
also successfully predicts the transition from a predominantly screening
situation to a predominantly binding situation, which was observed
experimentally when negative surface charge density was decreased in nerve.
