Counterion Screening Research Articles

Charged rod-like colloidal suspensions: what can we learn from ab initio simulations?

An ab initio approach to simulate the interactions and correlations of charged rodlike colloidal particles is proposed, which combines molecular dynamics for the mesoscopic rods and classical density-functional theory for the microscopic counterions. Many-body inter-rod interactions induced by non-linear counterion screening are systematically included in this approach. It is found that these forces can satisfactorily be described by an effective Yukawa-segment model which, however, differs in general from the traditional Derjaguin-Landau-Verwey-Overbeek theory.

Charged histidine affects alpha-helix stability at all positions in the helix by interacting with the backbone charges.

To determine whether a charged histidine side chain affects alpha-helix stability only when histidine is close to one end of the helix or also when it is in the central region, we substitute a single histidine residue at many positions in two reference peptides and measure helix stability and histidine pKa. The position of a charged histidine residue has a major effect on helix stability in 0.01 M NaCl: the helix content of a 17-residue peptide is 24% when histidine is at position 3 compared to 76% when it is at position 17. This dependence of helix content on histidine position decreases sharply in 1 M NaCl, as expected for counterion screening of the charge-helix dipole interaction. Results at interior positions indicate that the position of a charged histidine residue affects helix stability at these positions. Unexpectedly high values of the helix content are found when either neutral or charged histidine is at one of the last three C-terminal positions, suggesting that either form can stabilize an isolated helix by hydrogen bonding to a main-chain CO group.

Nonlinear counterion screening in colloidal suspensions

A new ‘‘ab initio’’ method is presented which is designed to simulate highly asymmetric systems of charged particles such as micellar solutions and charge-stabilized colloidal suspensions. The hybrid description considers the macroion degrees of freedom explicitly, while the microscopic counterions are treated within the framework of density functional theory. The counterion density profile is treated as a dynamical variable which is coupled to the macroion positions; the corresponding equation of motions are derived from a Lagrangian which contains a fictitious kinetic energy term associated with the inhomogeneous counterion density, with a fictitious mass chosen so that the counterions stay as close as possible to the surface of lowest free energy (adiabatic condition). The discontinuous behavior of the counterion density profile at the macroion surfaces is suppressed by the use of a classical pseudopotential scheme without spoiling the rapid variation of the counterion density profile outside the macroion cores. The ab initio method is implemented in Molecular and Brownian Dynamics simulations of concentrated colloidal suspensions, and the results are compared to the predictions of much simpler simulations based on the pairwise additive effective Derjaguin–Landau–Verwey–Overbeek (DLVO) potential between macroions. The density profiles calculated from the DLVO model differ considerably from the predictions of the ab initio simulations, but the macroion pair structures are in reasonable agreement. Recent ‘‘improvements’’ of the standard DLVO theory are found to overestimate or underestimate the pair structure considerably. The density functional formalism may be used to derive systematic many-body corrections to the effective DLVO pair potential. The extension of the ab initio method to treat colloidal suspensions in the presence of added salt is briefly sketched.

Electrostatics of Polymorphic DNA.

The molecular electrostatic potential (MEP) and the molecular electrostatic field (MEF) are associated with significantly different patterns of distribution in the nucleic acids and their constituents. In particular, a) while the values of the minimal potentials at the reactive sites of the bases or at the phosphates increase manyfold when going from the subunits to the double helix, the values of the field undergo only very small changes under the same circumstances and b) while the deepest potentials are located in the grooves of the double helix, the greatest fields are concentrated on the phosphates of the backbone. They are also influenced differently by such environmental factors as counterion screening: while the absolute values of the potentials are profoundly reduced, the fields are increased with respect to those of the unscreened acids. MEP and MEF also govern the electrostatics of interaction of DNA with different types of species. The MEP being of particular significance in this respect for interaction with cations and the MEF for the association with neutral dipolar molecules. A number of examples are given to illustrate the significance of this situation for different conformers of DNA.

Cell-glass separation depends on salt concentration and valency: Measurements on Dictyostelium amoebae by finite aperture interferometry

ABSTRACT Using pre-aggregation amoebae of the cellular slime mould Dictyostelium discoideum we have investigated the influence of cation concentration and valency on cell–glass separation. For computing the separation we used interference reflection microscopy and converted measured image irradiance to distance by finite aperture theory. Alterations in ionic strength caused virtually instantaneous reversible changes in the inter-ference image due to changes in cell membrane–glass separation. In solutions containing monovalent cations, a change in ionic strength from 20 mM to ∼ 0·5 mM increased the separation of the plasmalemma from the glass by 60 nm. Divalent cations were better than monovalent cations at maintaining a small separation. Our results show that both divalent and trivalent cations adsorb to one or both of the interacting surfaces, in addition to acting as electrostatic double-layer counterions. The optical measurements also show that the cell membrane–glass gap is not reduced to zero by counterion screening ; this is apparently due to the presence of a cell surface glycoprotein coat.

Copolymers of adenylic and 2-aminoadenylic acids. Effect of progressive changes in hydrogen bonding and stacking on interaction with poly(uridylic acid).

Random copolymers of adenylic acid and 2-aminoadenylic acid form both double and triple helices with poly(uridylic acid) [poly(U)]. Transition temperatures of two-stranded helices increase with 2NH2A content, exhibiting a slight positive departure from linearity and indicating that the contribution to helix stability arising from introduction of the 2-amino group does not significantly depend upon base sequence. We have shown previously that poly(2NH2A) . poly(U) does not undergo a disproportionation reaction (2 leads to 3 transition). Extrapolation from melting curves of 1:1 complexes between A,2NH2A copolymers and poly(U) indicates a Tm for the 2 leads to 3 transition of poly(2NH2A) . poly(U) which is too high to be observable under normal conditions. Addition of an organic solvent (50% ethylene glycol), however, lowers Tm by promoting unstacking of single-stranded poly(2NH2A) sufficiently to permit observation of the disproportionation of poly(2NH2A) . poly(U) for the first time. Transition breadths of 1:1 complexes of A,2NH2A copolymers with poly(U) are greater than those of either of the homopolymer complexes in the middle range of composition (67 and 48% A) but not at 25% A. These results are consistent with previous calculations on the effect of heterogeneity in base-pair stability on DNA transition breadths. In the poly(A), poly(U) system, Et4N+ counterion reduces the Tm of the double and triple helices by 26 and 41 degrees C, respectively. The larger depression in the latter case arises from the higher charge density of the triple helix and less effective counterion screening by Et4N+. In the poly(2NH2A), poly(U) system Tm,2 leads to 1 is reduced by 24 degrees C, but extrapolation of the copolymer results indicates a reduction of approximately 100 degrees C for Tm,3 leads to 2, accounting for previous failure to observe a triple helix in this system. CD spectra of A,2NH2A copolymers suggest that much of the spectral region can be regarded as a contribution of the CD spectra of the parent polymers poly(A) and poly(2NH2A) but that the region from 255 to 275 nm requires that contributions made by longer range interactions be taken into account.