Electrical double layer, zeta potential, and electrophoretic charge of double-stranded DNA.

Abstract

AbstractThe principal aim of this work is the proper interpretation of electrophoresis as an important tool for determining the immediate ionic environment of polyelectrolyte molecules like DNA. We also explore the relation with other experiments. We define an electrostatic model of double‐stranded DNA in monovalent salt solutions. The electrical double layer is divided in an inner Stern layer, inside the surface of hydrodynamic shear of DNA, and a Gouy‐Chapman type diffuse ionic atmosphere with cylindrical symmetry outside the shear surface. We discuss the assumptions made in applying to DNA the charge‐potential relation and the electrical free‐energy expression of the charged cylinder with a Gouy double layer. The conclusion is that reliable free‐energy data, better than 1kT per phosphate group, require a detailed model of the Stern layer. On the other hand, the Gouy model of a charged cylinder should be fairly good for the interpretation of the electrophoretic mobility of DNA. We introduce several modifications of the linear electrophoresis theory of the cylinder (Henry‐Gorin), related to the averaging over the orientations of DNA in the external electrical field, the relaxation effect, the nonlinear charge‐potential relation, and to geometrical aspects of the DNA model. Applied to published mobility data the modified theory yields roughly the same ζ potentials as before, but DNA charges which are about twice as high as derived earlier from the linear Henry‐Gorin theory. The charge calculation is quite sensitive to the assumed geometrical model of DNA. Our electrophoresis results are somewhat higher than the charges derived from Donnan equilibria of DNA in monovalent salt solutions measured by Strauss et al. The required self‐consistency of electrophoretic charge and ζ potential of DNA as a function of the salt concentration is treated on the basis of adsorption of counterions to the DNA phosphate groups (site binding). It is predicted that at low ionic strength the electrophoretic charge of DNA should be independent of the salt concentration.