Biomedical subjects

R R Netz

Publications and source records attributed to R R Netz.

16 recordsLinked to original sources

PubMed2007

Density depletion at solid-liquid interfaces: a neutron reflectivity study.

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.

Journal Article↗

PubMed2006

Shear-flow-induced unfolding of polymeric globules.

The behavior of a single collapsed polymer under shear flow is examined using hydrodynamic simulations and scaling arguments. Below a threshold shear rate gamma[.]{*}, the chain remains collapsed and only deforms slightly, while above gamma[.]{*} the globule exhibits unfolding/refolding cycles. Hydrodynamics are crucial: In the free draining case, gamma[.]{*} scales with the globule radius R as gamma[.]{*} approximately R{-1}, while in the presence of hydrodynamic interactions gamma[.]{*} approximately R. Experiments on the globular von Willebrand protein confirm the presence of an unfolding transition at a well-defined critical shear rate.

Humans↗

PubMed2005

Structural investigations of DNA-polycation complexes.

The internal structure of DNA-polycation complexes is investigated by synchrotron small-angle X-ray scattering (SAXS). Hexagonal packing of DNA is observed for DNA complexed with poly-L-lysine (PL), poly-L-arginine (PA), spermine (Sp), and linear and branched polyethyleneimine (lPEI and bPEI, respectively). Variations in the internal spacings and degree of long-range ordering are dependent on both polycation type and concentration of added salt. With increasing concentration of monovalent salt, a discontinuous phase transition is observed from compact to loose bundles and finally to an isotropic network phase. This salt-induced melting transition was found to be universal for all polyplexes studied and is in quantitative agreement with a simple free energy model based solely on electrostatic and entropic contributions. Using the osmotic stress method, bulk modulus (K) is measured for PL-DNA and PA-DNA polyplexes at various salt concentrations. With increasing osmotic force, we show that the salt-induced melting transition is shifted and compression in the loose bundle regime is in qualitative agreement with our model.

Computer Simulation↗

PubMed2004

Attraction of like-charged macroions in the strong-coupling limit.

Like-charged macroions attract each other as a result of strong electrostatic correlations in the presence of multivalent counterions or at low temperatures. We investigate the effective electrostatic interaction between i) two like-charged rods and ii) two like-charged spheres using the recently introduced strong-coupling theory, which becomes asymptotically exact in the limit of large coupling parameter (i.e. for large counterion valency, low temperature, or high surface charge density on macroions). In contrast to previous applications of the strong-coupling theory, we deal with curved surfaces and an additional parameter, referred to as Manning parameter, is introduced, which measures the ratio between the radius of curvature of macroions to the Gouy-Chapman length. This parameter, together with the size of the confining box enclosing the two macroions and their neutralizing counterions, controls the counterion-condensation process that directly affects the effective interactions. For sufficiently large Manning parameters (weakly-curved surfaces), we find a strong long-ranged attraction between two macroions that form a closely-packed bound state with small surface-to-surface separation of the order of the counterion diameter in agreement with recent simulations results. For small Manning parameters (highly-curved surfaces), on the other hand, the equilibrium separation increases and the macroions unbind from each other as the confinement volume increases to infinity. This occurs via a continuous universal unbinding transition for two charged rods at a threshold Manning parameter of Epsilon c = 2/3, while the transition is strongly discontinuous for spheres because of a pronounced potential barrier at intermediate distances. Unlike the cylindrical case, the attractive forces between spheres disappear slowly for increasing confinement volume due to the complete de-condensation of counterions. Scaling arguments suggest that for moderate values of coupling parameter, strong-coupling predictions remain valid for sufficiently small surface-to-surface separations.

Biophysics↗

PubMed2004

Variational theory for a single polyelectrolyte chain revisited.

We reconsider the electrostatic contribution to the persistence length, le, of a single, infinitely long-charged polymer in the presence of screening. A Gaussian variational method is employed, taking le as the only variational parameter. For weakly charged and flexible chains, crumpling occurs at small length scales because conformational fluctuations overcome electrostatic repulsion. The electrostatic persistence length depends on the square of the screening length, le approximately kappa(-2), as first argued by Khokhlov and Khachaturian by applying the Odijk-Skolnick-Fixman (OSF) theory to a string of crumpled blobs. We compare our approach to previous theoretical works (including variational formulations) and show that the result le approximately kappa(-2), found by several authors comes from the improper use of a cutoff at small length scales. For highly charged and stiff chains, crumpling does not occur; here we recover the OSF result and validate the perturbative calculation for slightly bent rods.

Journal Article↗

PubMed2003

Electrofriction and dynamic stern layers at planar charged surfaces.

Using dynamic simulations, the electrophoretic mobility of counterions at a substrate with fixed or mobile surface charges under the action of a lateral electric field is studied. The lateral charge inhomogeneity and corrugation of the substrate is taken into account. Because of the pronounced electrofriction between counterions and surface ions, a large fraction of counterions is practically immobilized for highly charged substrates. This explains the experimentally observed saturation of the electrophoretic mobility of charged particles in the limit of high surface charge density.

Journal Article↗

PubMed2003

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.

Computer Simulation↗

PubMed2003

Variational charge renormalization in charged systems.

We apply general variational techniques to the problem of the counterion distribution around highly charged objects where strong condensation of counterions takes place. Within a field-theoretic formulation using a fluctuating electrostatic potential, the concept of surface-charge renormalization is recovered within a simple one-parameter variational procedure. As a test, we reproduce the Poisson-Boltzmann surface potential for a single-charged planar surface both in the weak-charge and strong-charge regime. We then apply our techniques to non-planar geometries where closed-form solutions of the non-linear Poisson-Boltzmann equation are not available. In the cylindrical case, the Manning charge renormalization result is obtained in the limit of vanishing salt concentration. However, for intermediate salt concentrations a slow crossover to the non-charged-renormalized regime (at high salt) is found with a quasi-power-law behavior which helps to understand conflicting experimental and theoretical results for the electrostatic persistence length of polyelectrolytes. In the spherical geometry charge renormalization is only found at intermediate salt concentrations, in agreement with previous numerical results.

Biophysical Phenomena↗

PubMed2003

Non-linear osmotic brush regime: simulations and mean-field theory.

We investigate polyelectrolyte brushes in the osmotic regime using both theoretical analysis and molecular dynamics simulation techniques. In the simulations at moderate Bjerrum length, we observe that the brush height varies weakly with grafting density, in contrast to the accepted scaling law, which predicts a brush thickness independent of the grafting density. We show that such behavior can be explained by considering lateral electrostatic effects (within the non-linear Poisson-Boltzmann theory) combined with the coupling between lateral and longitudinal degrees of freedom due to the conserved polymer volume (which are neglected in scaling arguments). We also take the non-linear elasticity of polyelectrolyte chains into consideration, which makes significant effects as chains are almost fully stretched in the osmotic regime. It is shown that all these factors lead to a non-monotonic behavior for the brush height as a function of the grafting density. At large grafting densities, the brush height increases with increasing the grafting density due to the volume constraint. At small grafting densities, we obtain a re-stretching of the chains for decreasing grafting density, which is caused by lateral electrostatic contributions and is controlled by the counterion-condensation process around polyelectrolyte chains. These results are obtained assuming all counterions to be trapped within the brush, which is valid for sufficiently long chains of large charge fraction.

Journal Article↗

PubMed2002

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.

Biophysical Phenomena↗

PubMed2002

Simulations of counterions at charged plates.

Using Monte Carlo simulations, we study the counterion distribution close to planar charged walls in two geometries: i) when only one charged wall is present and the counterions are confined to one half-space, and ii) when the counterions are confined between two equally charged walls. In both cases the surface charge is smeared out and the dielectric constant is the same everywhere. We obtain the counterion density profile and compare it with both the Poisson-Boltzmann theory (asymptotically exact in the limit of weak coupling, i.e. low surface charge, high temperature and low counterion valence) and the strong-coupling theory (valid in the opposite limit of high surface charge, low temperature and high counterion valence) and with previously calculated correction terms to both theories for different values of the coupling parameter, thereby establishing the domain of validity of the asymptotic limits. Gaussian corrections to the leading Poisson-Boltzmann behavior (obtained via a systematic loop expansion) in general perform quite poorly: At coupling strengths low enough so that the Gaussian (or one-loop) correction does describe the numerical deviations from the Poisson-Boltzmann result correctly, the leading Poisson-Boltzmann term by itself matches the data within high accuracy. This reflects the slow convergence of the loop expansion. For a single charged plane, the counterion pair correlation function indicates a behavioral change from a three-dimensional, weakly correlated counterion distribution (at low coupling) to a two-dimensional, strongly correlated counterion distribution (at high coupling), which is paralleled by the specific-heat capacity which displays a rounded hump at intermediate coupling strengths. For the case of counterions confined between two equally charged walls, we analyze the inter-wall pressure and establish the complete phase diagram, featuring attraction between the walls for large enough coupling strength and at intermediate wall separation. Depending on the thermodynamic ensemble, the phase diagram exhibits a discontinuous transition where the inter-wall distance jumps to infinity (in the absence of a chemical potential coupling to the inter-wall distance, as for charged lamellae in excess solvent) or a critical point where two coexisting states with different inter-wall distance become indistinguishable (in the presence of a chemical potential, as for charged lamellae with a finite fixed solvent fraction). The attractive pressure decays with the inter-wall distance as an inverse cube, similar to analytic predictions, although the amplitude differs by an order of magnitude from previous theoretical results. Finally, we discuss in detail our simulation methods and compare the finite-size scaling behavior of different boundary conditions (periodic, minimal image and open).

Journal Article↗

PubMed2001

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. In the second part, the pair interaction between segments of an (on average) neutral membrane consisting of mobile positive and negative charges is derived both field theoretically in the Gaussian approximation and using a simple ion-pairing approximation. This model might also apply to strongly charged membranes with strongly condensed counter ions. The resulting contribution to the elastic energy is negative and thus favors undulations of the membrane. The bending modulus is extracted from the large-scale or small-momentum behavior of the elastic kernel and found to be comparable to k(B)T for the case where ion pairing is dominant. The large-momentum elastic response exhibits a markedly different scaling than the small-momentum regime and sensitively depends on the small-distance cutoff and thus on molecular details.

Journal Article↗

PubMed2001

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. A systematic and asymptotically exact strong-coupling field theory yields the bound state from a competition between counterion entropy and electrostatic attraction, in agreement with simple scaling arguments and simulations.

Journal Article↗

PubMed2000

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. For low salt concentrations, we find multiple conformational transitions, leading to an extended DNA configuration. The wrapped states are characterized by spontaneously broken rotational and mirror symmetries, giving rise to four distinct structures.

DNA↗

PubMed2000

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. This process is driven by an effective line tension which depends on the size of the stickers and arises from the interplay of shape fluctuations and sticker clusters.

Cell Adhesion Molecules↗

PubMed1999

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. Likewise, the effective interactions between charged particles are strongly modified in the neighborhood of such a salty interface. On the other hand, charged particles which are immersed in a salt solution are repelled from the air (or a substrate) interface, and the interaction between two charges decays algebraically close to such an interface. These general results have experimentally measurable consequences for the adsorption of charged colloids or charged polymers at monolayers, solid substrates, and interfaces.

Journal Article↗