Rudi Podgornik

DNA-DNA interactions

The forces that govern DNA double helix organization are being finally systematically measured. The non-specific longer-range interactions--such as electrostatic interactions, hydration, and fluctuation forces--that treat DNA as a featureless rod are reasonably well recognized. Recently, specific interactions--such as those controlled by condensing agents or those consequent to helical structure-are beginning to be recognized, quantified and tested.

Electrostatic correlation forces between surfaces with surface specific ionic interactions

We have studied some consequences of a generalized theory of electrostatic interactions between two macroscopic surfaces immersed in a dilute electrolyte. The ionic interactions close to the two bounding surfaces were supposed to be specifically affected by the presence of the surfaces, leading to a surface contribution to the total free energy density of the system. A functional integral (sine‐Gordon) representation of the grand canonical partition function was derived and evaluated in the saddle‐point approximation, leading to the repulsive mean‐field and an attractive first‐order correlation correction to the interaction free energy. The magnitude of the correlation term was investigated in detail and general conditions on the form of the surface‐specific free energy density were derived that would lead to an anomalously large exponentially decaying attractive interactions. We present some arguments in favor of the view that this anomalous long‐range attraction could well represent a case for the elect...

Beyond standard Poisson-Boltzmann theory: ion-specific interactions in aqueous solutions

The forces between charged macromolecules, usually given in terms of osmotic pressure, are highly affected by the intervening ionic solution. While in most theoretical studies the solution is treated as a homogeneous structureless dielectric medium, recent experimental studies concluded that, for a bathing solution composed of two solvents (binary mixture), the osmotic pressure between charged macromolecules is affected by the binary solvent composition. By adding local solvent composition terms to the free energy, we obtain a general expression for the osmotic pressure, in planar geometry and within the mean-field framework. The added effect is due to the permeability inhomogeneity and nonelectrostatic short-range interactions between the ions and solvents (preferential solvation). This effect is mostly pronounced at small distances and leads to a reduction in the osmotic pressure for macromolecular separations of the order 1--2 nm. Furthermore, it leads to a depletion of one of the two solvents from the charged macromolecules (modeled as planar interfaces). Lastly, by comparing the theoretical results with experimental ones, an explanation based on preferential solvation is offered for recent experiments on the osmotic pressure of DNA solutions.The Poisson-Boltzmann mean-field description of ionic solutions has been successfully used in predicting charge distributions and interactions between charged macromolecules. While the electrostatic model of charged fluids, on which the Poisson-Boltzmann description rests, and its statistical mechanical consequences have been scrutinized in great detail, much less is understood about its probable shortcomings when dealing with various aspects of real physical, chemical and biological systems. These shortcomings are not only a consequence of the limitations of the mean-field approximation per se, but perhaps are primarily due to the fact that the purely Coulombic model Hamiltonian does not take into account various additional interactions that are not electrostatic in their origin. We explore several possible non-electrostatic contributions to the free energy of ions in confined aqueous solutions and investigate their ramifications and consequences on ionic profiles and interactions between charged surfaces and macromolecules.

Ion-specific hydration effects: Extending the Poisson-Boltzmann theory

Abstract In aqueous solutions, dissolved ions interact strongly with the surrounding water and surfaces, thereby modifying solution properties in an ion-specific manner. These ion-hydration interactions can be accounted for theoretically on a mean-field level by including phenomenological terms in the free energy that correspond to the most dominant ion-specific interactions. Minimizing this free energy leads to modified Poisson-Boltzmann equations with appropriate boundary conditions. Here, we review how this strategy has been used to predict some of the ways ion-specific effects can modify the forces acting within and between charged interfaces immersed in salt solutions.

Dielectric decrement as a source of ion specific effects

Many theoretical studies were devoted in the past to ion-specific effects trying to interpret a large body of experimental evidence, such as surface tension at air/water interfaces and force measurements between charged objects. Although several mechanisms were suggested to explain the results, such as dispersion forces and specific surface-ion interactions, we would like to suggest another source of ion-specificity originating from the local variations of the dielectric constant due to the presence of ions in the solution. We present a mean-field model to account for the heterogeneity of the dielectric constant caused by the ions. In particular, for ions that decrease the dielectric constant we find a depletion of ions from the vicinity of charged surfaces. For a two-plate system, the same effect leads to an increase of the pressure in between two surfaces. Our results suggest that the effect of ions on the local dielectric constant should be taken into account when interpreting experiments that address ion-specific effects.

Elastic moduli renormalization in self-interacting stretchable polyelectrolytes

We study the effect of intersegment interactions on the effective bending and stretching moduli of a semiflexible polymer chain with a finite stretching modulus. For an interaction potential of a screened Debye–Huckel type, renormalization of the stretching modulus is derived on the same level of approximation as the celebrated Odijk–Skolnick–Fixman result for the bending modulus. The presence of mesoscopic intersegment interaction potentials couples the bending and stretching moduli in a manner different from that predicted by the macroscopic elasticity theory. We thus advocate a fundamental change in the perspective regarding the dependence of elastic moduli of a flexible polyelectrolyte on the ionic conditions: stretchability. The theory presented here and its consequences compare favorably with recent experiments on DNA bending and stretching at not too low salt conditions.

Ions in mixed dielectric solvents: density profiles and osmotic pressure between charged interfaces.

The forces between charged macromolecules, usually given in terms of osmotic pressure, are highly affected by the intervening ionic solution. While in most theoretical studies the solution is treated as a homogeneous structureless dielectric medium, recent experimental studies concluded that, for a bathing solution composed of two solvents (binary mixture), the osmotic pressure between charged macromolecules is affected by the binary solvent composition. By adding local solvent composition terms to the free energy, we obtain a general expression for the osmotic pressure, in planar geometry and within the mean-field framework. The added effect is due to the permeability inhomogeneity and nonelectrostatic short-range interactions between the ions and solvents (preferential solvation). This effect is mostly pronounced at small distances and leads to a reduction in the osmotic pressure for macromolecular separations of the order 1-2 nm. Furthermore, it leads to a depletion of one of the two solvents from the charged macromolecules (modeled as planar interfaces). Lastly, by comparing the theoretical results with experimental ones, an explanation based on preferential solvation is offered for recent experiments on the osmotic pressure of DNA solutions.

Two-body polyelectrolyte-mediated bridging interactions

We investigate theoretically polyelectrolyte bridging interactions on the two-body level. The model system is composed of two macroions with two oppositely charged flexible chains. The electrostatic interactions are treated on the Debye–Huckel level. The formal level of the theory is provided by the Feynman–Kleinert variational method generalized to include also self-interactions between polyelectrolyte segments. The variational equations are shown to exhibit two solution branches corresponding to strong and weak coupling, whereas conformations of the chain can be described as weakly or strongly paired. We investigate the effective pair interaction between the macroions in the parameter space and comment on the relevance of the calculation for bridging interactions in experimental context.

Inhomogeneous coulomb fluid. A functional integral approach

A functional integral representation of the grand canonical classical statistical integral for the inhomogeneous Coulomb fluid is derived. The charged species are confined between two interfaces, also defining the dielectric inhomogeneity in the system, bearing constant surface charges. The thermodynamic potential is obtained in a closed form if the Gaussian approximation for the fluctuations around the mean electrostatic potential is used. The formalism embodies the mean field (Poisson–Boltzmann) terms generalized by the presence of image interactions plus the correlation (fluctuation) terms, which give significant correction to the classical expressions for the force between charged interfaces. The numerical results for a counterion-only system with charged surfaces are treated in detail and compared with simulation data.

The free energy,enthalpy and entropy of hydration of phospholipid bilayer membranes and their difference on the interfacial separation

Abstract The phenorncnological theory of water binding to non-charged phospholipid bilayer membranes is presented and the free energy, enthalpy, and entropy of hydration are calculated as a function ofinterfacial separation. Tlicsc estimated hydration quantities agree with our experimental data provided the interfacial effects arc described in terms of the sur- face water -orienting field of constant strength.