Measurement of the repulsive force between polyelectrolyte molecules in ionic solution: hydration forces between parallel DNA double helices.

Abstract

We have measured the repulsive force between B-form double helices in parallel packed arrays of polymer-condensed DNA in the presence of 0.005-1.0 M ionic solu-tions. Molecular repulsion is consistently exponential with a 2.5-3.5 A decay distance, when the separation between DNA surfaces is 5-15 A. Only weakly dependent on ionic strength and independent of molecular size, this intermolecular repulsion does not obey the predictions of electrostatic double-layer theory. Rather, it strongly resembles the "hydration force" first recognized and quantified between phospholipid bilayers. Only beyond 15 A separation between molecules is there evidence of electrostatic double-layer forces. The quantitative failure of electrostatic double-layer theory seen here must gravely affect accepted analyses of other polyelectrolyte systems. Because the packing of condensed DNA resembles the spacings of DNA in many bacteriophages, our results permit estimation of the "DNA pressure" in phage heads.In 1976, LeNeveu et al. (1) reported that electrically neutral phospholipid bilayer membranes in pure water exerted a mutual repulsion that decayed exponentially with separation and was detectable to separations between parallel membrane surfaces of "30 A. This repulsion was termed a "hydration force" (2) to represent the idea that it is due to the work of removing water from the vicinity of the membrane surface. Further measurements (3-9) on a variety of charged or neutral membranes revealed the existence of hydration forces in pure water or in ionic solutions. For separations of less than 20-30 A between charged membranes, these forces completely overshadow electrostatic repulsion. A qualitatively similar story has emerged from the forces measured between mica sheets in ionic solution, which show an "additional" exponentially decaying component unexpected from electrostatic double layer theory (10-13). Marcelja and coworkers (14-17) have proposed a simple but elegant theory that shows how an exponential force can arise from the spatially varying perturbation of water near a polarizing surface.We now report that similar hydration forces occur between DNA polyelectrolyte molecules. We have measured the force per unit length vs. interaxial separation of parallel DNA double helices under various ionic conditions. The strong repulsive force is detectable when the interaxial distance is about 35 A, at which point the shortest distance between DNA surfaces is ""15 A. This force between molecules grows exponentially with a 2.5-3.5 A characteristic distance as the molecules are brought together. Contrary to expectation from double-layer theory, this repulsion is insensitive to ionic strength and is of a different magnitude than predicted from electrostatic theory.We argue that hydration forces are a major factor in the interaction of particles undergoing molecular assembly. In particular, the packing of DNA in cells and in viruses must be accomplished in a way that circumvents or overcomes the effects of hydration.More generally, the domi...

Water near intracellular surfaces.

“…The ways in which we measure intermembrane or intermolecular forces are described elsewhere (6,7,15,24,25). A schematic picture of how we do it will suggest a way to connect these measurements to cellular processes.…”

“…The DNA data (25) suggest that different ions binding to a surface will confer different coefficients that reflect the combined hydration properties of the ion and the original surface . Lipids with different polar groups (specifically phosphatidylcholine and phosphatidylethanolamine show different force coefficients but similar rates of decay [10]), Hydration forces analogous to those described here have been seen between mica surfaces made polar by the adsorption of cations (17)(18)(19) .…”

“…To make the required measurements, we use the well-known polymer condensation of DNA wherein PEG (or dextran or PVP) added to a DNA solution causes the DNA double helices to form an ordered hexagonal array of parallel molecules . We measured the lattice spacing and polymer osmotic pressure (25). The distance between molecular surfaces is the interaxial distance minus the 20-A molecular diameter .…”

Probing the role of water in protein conformation and function

“…Quantitative osmotic stress, used to gently, systematically and quantitatively stress and strain systems, is combined with measuring the structural response by independent means. Such an osmotic tool, combined with X-ray diffraction for example, has been used to deform molecular arrays such as membranes (Rand & Parsegian 1989), polysaccharides (Rau & Parsegian 1990), proteins (Leikin et al 1994), DNA (Rau et al 1984 provided a detailed quantitative characterization of the forces of interaction between the structural elements.…”

Modeling and simulation of the mechanical response from nanoindentation test of DNA-filled viral capsids

“…It should be noted that all viral capsids are permeable to water and smaller ions. Thus, the measured force resisting the AFM indentation is dependent on the DNA hydration force (that is, the force required to move one molecule of water from the encapsidated DNA to the bulk) [23,24]. The decrease in packaged DNA-DNA spacing upon indentation results in an exponential increase in the repulsive hydration and electrostatic interaction [23,24].…”