Nonequilibrium unfolding of polyelectrolyte condensates in electric fields.

Using simulations and scaling methods, the effect of an electric field on a collapsed polyelectrolyte globule is investigated, where conduction by counterions and the polyelectrolyte itself is taken into account. At a critical field E(*), a nonequilibrium transition occurs at which the polyelectrolyte unfolds and aligns parallel to the external field. E(*) is determined using scaling results for the polarizability of a polyelectrolyte globule and exhibits a dependence on the chain length N, E(*) approximately N(-1/2), which might be useful for electrophoretic separation of charged biopolymers.

Complexes of semiflexible polyelectrolytes and charged spheres as models for salt-modulated nucleosomal structures.

We investigate the complexation behavior between a semiflexible charged polymer and an oppositely charged sphere with parameters appropriate for the DNA-histone system. We determine the ground state of a simple free energy expression (which includes electrostatic interactions on a linear level) numerically and use symmetry arguments to divide the obtained DNA configuration into broad classes, thereby obtaining global phase diagrams. We pay specific attention to the effects of salt concentration, DNA length variation, DNA charge renormalization, and externally applied force on the obtained complex structures.

Debye-Hückel theory for interfacial geometries.

The Debye-Hückel theory for bulk electrolyte solutions is generalized to planar interfacial geometries, including screening effects due to mobile salt ions which are confined to the interface and solutions with in general different salt concentrations and dielectric constants on the two sides of the interface. We calculate the general Debye-Hückel interaction between fixed test charges, and analyze a number of relevant special cases as applicable to charged colloids and charged polymers. Salty interfaces, which are experimentally realized by monolayers or bilayers made of cationic and anionic surfactants or lipids, exert a strong attraction on charged particles of either sign at large separations from the interface; at short distances image-charge repulsion sets in.

Poisson-Boltzmann theory for membranes with mobile charged lipids and the pH-dependent interaction of a DNA molecule with a membrane.

We consider a planar stiff model membrane consisting of mobile surface groups whose state of charge depends on the pH and the ionic composition of the adjacent electrolyte solution. To calculate the mean-field interaction potential between a charged object and such a model membrane, one needs to solve a Poisson-Boltzmann boundary value problem. We here derive and discuss the boundary condition at the membrane surface, a condition that is generally appropriate for biological membranes where two charge-regulating mechanisms are present at the same time: the pH-dependent chemical charge regulation and a regulation through the in-plane mobility of the surface groups.

Buckling and nonlocal elasticity of charged membranes.

The elastic behavior of an interacting, and, in specific, of a charged flexible membrane is considered. In the first part of this paper the effective nonlocal elastic energy of a membrane due to a pairwise and arbitrary intra-membrane interaction is derived. Nonlocal elasticity is included to all orders, this description, therefore, corresponds to an infinite resummation of the standard gradient expansion.

Binding of similarly charged plates with counterions only.

Similarly and highly charged plates in the presence of multivalent counterions attract each other and form electrostatically bound states. Using Monte-Carlo simulations, we obtain the interplate pressure in the global parameter space. The equilibrium plate separation, where the pressure changes from attractive to repulsive, exhibits a novel unbinding transition.

Unbinding transitions and phase separation of multicomponent membranes.

Multicomponent membranes in contact with another surface or wall are studied by a variety of theoretical methods and Monte Carlo simulations. The membranes contain adhesion molecules which are attracted to the wall and, thus, act as local stickers. It is shown that this system undergoes lateral phase separation leading to discontinuous unbinding transitions if the adhesion molecules are larger than the nonadhesive membrane components.

Salt-induced DNA-histone complexation.

We study numerically the binding of one semiflexible charged polymer onto an oppositely charged sphere. Using parameters appropriate for DNA-histone complexes, we find complete wrapping for intermediate salt concentrations only, in agreement with experiments. For high salt concentrations, a strongly discontinuous dewrapping occurs.

Complexation Behavior of Polyampholytes and Charged Objects.

We study theoretically the interaction of a polyampholyte chain with charged planes, cylinders, and spheres. Due to the random character of the charge distribution along the chain, a polyampholyte possesses a spontaneous dipole moment, which can interact favorably with charged objects. Depending on the charge strength of the object and the polyampholyte length and fraction of charged monomers, this attractive interaction can be strong enough to induce adsorption.

Polymer Brushes: From Self-Consistent Field Theory to Classical Theory.

We consider planar brushes formed by end-grafted polymers with moderate to strong excluded-volume interactions. We first rederive the mean-field theory and solve the resulting self-consistent equations numerically. In the continuum limit, the results depend sensitively on a single parameter, beta, whose square is the ratio of the scaling prediction for the brush height to the unperturbed polymer radius of gyration, and which measures therefore the degree to which the polymers are stretched.