Biomedical subjects

B W Ninham

Publications and source records attributed to B W Ninham.

At least 19 recordsLinked to original sources

PubMed2006

Extended DLVO theory: electrostatic and non-electrostatic forces in oxide suspensions.

According to classical DLVO theory all ions of background salt solution with the same ionic charge should result in the same effective force between colloidal particles. However, the relative effectiveness of different ions in influencing forces between ceramic oxide surfaces follows either a reversed Hofmeister sequence or a direct Hofmeister sequence depending on the type of oxide and if the pH is above or below the isoelectric point (iep). This ion specificity is inexplicable in classical double layer theory that deals only with pure electrostatic forces acting between the ions and the colloidal particles. A theoretical explanation is given here. At, and above, biological salt concentrations other, non-electrostatic (NES) ion specific forces act that are ignored in such modeling. In this overview we present the basic theory for the double layer near a single oxide surface and for the extended DLVO forces between oxide colloidal particles that accounts for these NES forces. We will demonstrate that ion specificity can be understood to a large degree once NES forces are included consistently in the non-linear theory.

Algorithms↗

PubMed2006

Ion specific surface forces between membrane surfaces.

Entities such as ion distributions and forces between lipid membranes depend on effects due to the intervening salt solution that have not been recognized previously. These specific ion or Hofmeister effects influence membrane fusion. A typical illustrative example is this: measurements of forces between double-chained cationic bilayers adsorbed onto molecularly smooth mica surfaces across different 0.6-2 mM salt solutions have revealed a large degree of ion specificity [Pashley et al. J. Phys. Chem. 1986, 90, 1637]. This has been interpreted in terms of very specific anion "binding" to the adsorbed bilayers, as it would too for micelles and other self-assembled systems. However, we show here that inclusion of nonelectrostatic (NES) or ionic dispersion potentials acting between ions and the two surfaces explains such "ion binding". The observed Hofmeister sequence for the calculated pressure without any direct ion binding is given correctly. This demonstrates the importance of a source of ion specificity that has been ignored. It is due to ionic physisorption caused by attractive NES ionic dispersion potentials. There appear to be some far reaching consequences for interpretations of membrane intermolecular interactions in salt solutions.

Hydrogen-Ion Concentration↗

PubMed2006

Why pH titration in protein solutions follows a Hofmeister series.

Measurements of pH in single-phase cytochrome c suspensions are reported. The pH, as determined by a glass electrode, has a fixed value. With the addition of salt, the supposedly fixed pH changes strongly. The pH depends on salt type and concentration and follows a Hofmeister series. A theoretical interpretation is given that provides insights into such Hofmeister effects. These occur generally in protein solutions. While classical electrostatic models provide partial understanding of such trends in protein solutions, they fail to explain the observed ion specificity. Such models neglect electrodynamic fluctuation (dispersion) forces acting between ions and proteins. We use a Poisson-Boltzmann cell model that takes these ionic dispersion potentials between ions and proteins into account. The observed ion specificity can then be accounted for. Proteins act as buffers that display similar salt-dependent pH trends not previously explained.

Algorithms↗

PubMed2005

Specific ion effects in solutions of globular proteins: comparison between analytical models and simulation.

Monte Carlo simulations have been performed for ion distributions outside a single globular macroion and for a pair of macroions, in different salt solutions. The model that we use includes both electrostatic and van der Waals interactions between ions and between ions and macroions. Simulation results are compared with the predictions of the Ornstein-Zernike equation with the hypernetted chain closure approximation and the nonlinear Poisson-Boltzmann equation, both augmented by pertinent van der Waals terms. Ion distributions from analytical approximations are generally very close to the simulation results. This demonstrates that properties that are related to ion distributions in the double layer outside a single interface can to a good approximation be obtained from the Poisson-Boltzmann equation. We also present simulation and integral equation results for the mean force between two globular macroions (with properties corresponding to those of hen-egg-white lysozyme protein at pH 4.3) in different salt solutions. The mean force and potential of mean force between the macroions become more attractive upon increasing the polarizability of the counterions (anions), in qualitative agreement with experiments. We finally show that the deduced second virial coefficients agree quite well with experimental results.

Animals↗

PubMed2005

Building bridges between the physical and biological sciences.

This paper attempts to identify major conceptual issues that have inhibited the application of physical chemistry to problems in the biological sciences. We will trace out where theories went wrong, how to repair the present foundations, and discuss current progress toward building a better dialogue.

Biological Science Disciplines↗

PubMed2005

Why forces between proteins follow different Hofmeister series for pH above and below pI.

The relative effectiveness of different anions in crystallizing proteins follows a reversed Hofmeister sequence for pH pI. The phenomenon has been known almost since Hofmeister's original work but it has not been understood. It is here given a theoretical explanation. Classical electrolyte and double layer theory deals only with electrostatic forces acting between ions and proteins. Hydration and hydration interactions are dealt with usually only in terms of assumed hard core models. But there are, at and above biological salt concentrations, other non-electrostatic (NES) ion-specific forces acting that are ignored in such modeling. Such electrodynamic fluctuation forces are also responsible for ion-specific hydration. These missing forces are variously comprehended under familiar but generally unquantified terms, typically, hydration, hydrogen bonding, pi-electron-cation interactions, dipole-dipole, dipole-induced dipole and induced dipole-induced dipole forces and so on. The many important body electrodynamic fluctuation force contributions are accessible from extensions of Lifshitz theory from which, with relevant dielectric susceptibility data on solutions as a function of frequency, the forces can be extracted quantitatively, at least in principle. The classical theories of colloid science that miss such contributions do not account for a whole variety of ion-specific phenomena. Numerical results that include these non-electrostatic forces are given here for model calculations of the force between two model charge-regulated hen-egg-white protein surfaces. The surfaces are chosen to carry the same charge groups and charge density as the protein. What emerges is that for pH pI (where anions are co-ions) the forces increase in the order NaCl<NaI<NaSCN. This is in excellent agreement with both solubility experiments and experiments using SAXS. The results are also consistent with cation effects observed in protein solutions. Our results may provide some insights into a long-standing problem in solution chemistry and biology.

Biomechanical Phenomena↗

PubMed2005

Hofmeister effects in biology: effect of choline addition on the salt-induced super activity of horseradish peroxidase and its implication for salt resistance of plants.

The effect of choline addition on the salt-induced super activity of horseradish peroxidase (HRP) is investigated. HRP is presented in the literature as an efficient H(2)O(2) scavenger, and choline is the precursor of glycine betaine, a strong osmoprotectant molecule. Both the regulations of H(2)O(2) and of osmoprotectant concentrations are implicated in plants in order to counteract salt-induced cell damage. For the oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), sulfate anions were found to play a crucial role in the increase of HRP activity. This induced super activity can be strongly reduced by adding choline chloride. The phenomena provide an example of physicochemical Hofmeister effects playing a central regulatory role in an important biological system.

Choline↗

PubMed2005

Hofmeister effects in surface tension of aqueous electrolyte solution.

The surface tension of electrolyte solutions shows marked specific ion effects. We here show an important role for both ionic solvation energies and ionic dispersion potentials in determining this ion specific surface tension of salt solutions. The ion self-free energy changes when an ion moves from bulk solution into the interfacial region, with its decreasing water density profile. We will show that the solvation energies of different ions correlate very well with the surface tension of salt solutions. Inclusion of this distance-dependent self-free energy contribution brings qualitative agreement with experiments and the right Hofmeister series. This is so not only for surface tension changes but also for measured surface potentials. The inclusion of ionic dispersion interaction potentials further improves the agreement with experiments. We discuss how further progress in the theory of the surface tension of salts can be achieved.

Journal Article↗

PubMed2004

Energy of an ion crossing a low dielectric membrane: the role of dispersion self-free energy.

The Born charging equation predicts that the permeability of a cell membrane to ions by the solubility-diffusion mechanism depends on the ionic radius and on the dielectric constant of the membrane. However, experiments, for example, on red blood cells and on lysosome membranes, show that the permeability depends strongly on the choice of salt anion in a way that cannot be accommodated by differences in ionic size. We demonstrate that one step towards understanding this ion specificity is to take account of the previously ignored dispersion self-free energy of the ion. This is the quantum electrodynamic analogue of the (electrostatic) Born self-energy of an ion. We show that the dispersion self-free energy contribution can be and often is of the same order of magnitude as the Born contribution. To understand the observed specificity, it is essential to take into account of both ionic size and ionic polarizability. In parallel and to reinforce these observations, we also give simple estimates for how self-free energy changes that occur when an ion moves into the air-water interface region (which has a density profile for water molecules) can influence the surface tension of salt solutions. Consistency can be found between the Hofmeister sequences observed in ion permeation and in surface tension of electrolytes when these previously ignored self-free energies are included properly.

Electricity↗

PubMed2004

Dispersion self-free energies and interaction free energies of finite-sized ions in salt solutions.

The role for many-body dipolar (dispersion) potentials in ion-solvent and ion-solvent-interface interactions is explored. Such many-body potentials, accessible in principle from measured dielectric data, are necessary in accounting for Hofmeister specific ion effects. Dispersion self-energy is the quantum electrodynamic analogue of the Born electrostatic self-energy of an ion. We here describe calculations of dispersion self-free energies of four different anions (OH-, Cl-, Br-, and I-) that take finite ion size into account. Three different examples of self-free energy calculations are presented. These are the self-free energy of transfer of an ion to bulk solution, which influences solubility; the dispersion potential acting between one ion and an air-water interface (important for surface tension calculations); and the dispersion potential acting between two ions (relevant to activity coefficient calculations). To illustrate the importance of dispersion self-free energies, we compare the Born and dispersion contributions to the free energy of ion transfer from water to air (oil). We have also calculated the change in interfacial tension with added salt for air (oil)-water interfaces. A new model is used that includes dispersion potentials acting on the ions near the interface, image potentials, and ions of finite size that are allowed to spill over the solution-air interface. It is shown that interfacial free energies require a knowledge of solvent profiles at the interface.

Journal Article↗

PubMed2004

Specific ion effects: Role of salt and buffer in protonation of cytochrome c.

Changes in background salt and buffer are known to influence the properties of proteins. The reasons have remained obscure. The challenge posed by many such problems is this. Can physical chemistry contribute any predictive quantitative insights to what is in effect the simplest macromolecular solution behavior? Or must all remain specific? Our thesis is that it can. For definiteness we consider here as an illustrative example: surface pH and protonation equilibria of cytochrome c. We demonstrate an important role for ionic dispersion forces, missing from previous theoretical treatments. Unlike charge interactions these are different for each ionic species, and act between a protein and both salt and buffer ions. The charge of proteins depends not only on pH, ionic charge, and salt concentration. Taking ionic dispersion forces into account goes some way towards explaining the dependence on ionic species. We demonstrate why the addition of buffer can have profound effects, including reversal of the salt dependence of the protein charge.

Buffers↗

PubMed2003

Hofmeister effects in membrane biology: the role of ionic dispersion potentials.

Membrane biology is notorious for its remarkable, and often strong dependence on the supposedly irrelevant choice of ion pair of background salt solution. While experimentally well known, there has been no progress towards any real theoretical understanding until very recently. We have demonstrated that an important source behind these Hofmeister effects is the ionic excess polarizabilities of ions in solution. Near an interface an ion experiences not only an electrostatic potential, but also a highly specific ionic dispersion potential. At biological concentrations (around 0.1 M and higher) when the electrostatic contribution is highly screened this ionic dispersion potential has a dominating influence. We present the result of model calculations for the interfacial tension and surface potential that demonstrates that inclusion of ionic dispersion potentials is an essential step towards predictive theories. Our results are compared with experimental surface and zeta potential measurements on phospholipid bilayers, zirconia, and cationic micelles.

Cell Membrane↗

PubMed2003

Specific ion effects: why the properties of lysozyme in salt solutions follow a Hofmeister series.

Protein solubility in aqueous solutions depends in a complicated and not well understood way on pH, salt type, and salt concentration. Why for instance does the use of two different monovalent salts, potassium thiocyanate and potassium chloride, produce such different results? One important and previously neglected source of ion specificity is the ionic dispersion potential that acts between each ion and the protein. This attractive potential is found to be much stronger for SCN(-) than it is for Cl(-). We present model calculations, performed within a modified ion-specific double-layer theory, that demonstrate the large effect of including these ionic dispersion potentials. The results are consistent with experiments performed on hen egg-white lysozymes and on neutral black lipid membranes. The calculated surface pH and net lysozyme charge depend strongly on the choice of anion. We demonstrate that the lysozyme net charge is larger, and the corresponding Debye length shorter, in a thiocyanate salt solution than in a chloride salt solution. Recent experiments have suggested that pK(a) values of histidines depend on salt concentration and on ionic species. We finally demonstrate that once ionic dispersion potentials are included in the theory these results can quantitatively be reinterpreted in terms of a highly specific surface pH (and a salt-independent pK(a)).

Chemical Precipitation↗

PubMed2001

Specific ion effects: why DLVO theory fails for biology and colloid systems.

The classical Derjaguin-Landau-Verwey-Overbeek theory that underpins colloid and surface science is shown to be flawed, especially at biological salt concentrations. This is in part because the dispersion forces acting on the ions are ignored. When these are included properly very different results are obtained. These results have substantial implications for biological and for ordinary colloid systems at moderate salt concentrations.

Algorithms↗

PubMed2001

Supra-aggregates.

Mixtures of oil (isooctane), water, and a double-chained ionic surfactant, Cu(AOT)2 [copper(II) bis(2-ethylhexyl)sulfosuccinate] form equilibrium phases consisting of spontaneous emulsions thermodynamically stable systems. The experimental conditions are such that the head polar group of the surfactant is totally hydrated. The emulsion droplets are comprised of supra aggregates, lamellar spherulites in which interior and exterior are bicontinuous microemulsions. The microstructures and component ratios can be predicted from elementary considerations that require only the notion of local and global packing constraints. The phenomenon of supra aggregation, microphases within any topologically closed container, in equilibrium with an external phase, appears quite general. The results throw some light on the meaning of 'phase' in mesostructured fluids.

Journal Article↗

PubMed1999

Mechanism of low-threshold hypersonic cavitation stimulated by broadband laser pump.

A low threshold acoustic cavitation regime was observed for the excitation of hypersonic waves due to a stimulated Brillouin scattering (SBS) mechanism, when the optical pump lies within the uv frequency range. Cavitation occurs if the optical pump bandwidth Delta(+)>>Omega(0), where Omega(0) is the Stokes frequency shift (the hypersonic frequency). In the opposite case (Delta(+)<<Omega(0)), cavitation does not occur despite the fact that the hypersonic wave intensity is much higher. The effect is associated with the stimulation of a broad frequency spectrum of hypersonic pressure in a field provided by the broadband optical pump. In contrast, for a monochromatic optical pump, the hypersonic wave is of single-frequency character. Induction of cavitation at the low intensities of acoustic pressure is attributed to nanobubbles of fixed size that occur in the liquid. The resonant frequency of the nanobubbles coincides with the frequency of some spectral component of hypersound present in the broadband SBS process. That conclusion is reinforced by the further observation that at the same intensity of broadband pumping the cavitation vanishes after degassing the liquid. In parallel experiments on four-photon polarization Rayleigh wing spectroscopy, it was also demonstrated that spectral lines exist in ordinary (not degassed) water, which can be ascribed to resonances of radial vibrations of nanobubbles. Those lines are absent in the degassed water spectrum.

Journal Article↗

PubMed1996

Dressed polyions, counterion condensation, and adsorption excess in polyelectrolyte solutions.

The phenomenon of Manning-Oosawa counterion condensation is given an explicit statistical mechanical and qualitative basis via a dressed polyelectrolyte formalism in connection with the topology of the electrostatic free-energy surface and is derived explicitly in terms of the adsorption excess of ions about the polyion via the nonlinear Poisson-Boltzmann equation. The approach is closely analogous to the theory of ion binding in micelles. Our results not only elucidate a Poisson-Boltzmann analysis, which shows that a fraction of the counterions lie within a finite volume around the polyion even if the volume of the system tends towards infinity, but also provide a direct link between Manning's theta-the number of condensed counterions for each polyion site-and a statistical thermodynamic quantity, namely, the adsorption excess per monomer.

Adsorption↗

PubMed1989

Conformation of surface bound polyelectrolytes: I. Implications for cell electrophoresis.

We present a self-consistent calculation for a layer of short-chain polyelectrolytes grafted to a planar interface. Individual self-avoiding chain configurations are generated on a tetrahedral lattice. By evaluating the complete ensemble of configurations in the presence of a self-consistent field we determine averages for the equilibrium layer height, segment and charge density profiles. These describe the response of the layer to changes in the surrounding medium, in particular to electrolyte concentration. This self-consistent model of a biological cell, which is known to have an extended layer of charge exterior to the membrane bilayer surface, is applied to the problem of cell electrophoretic mobility. We compare the electrophoretic mobility deduced from the application of this model with that for a constant profile of charge and segments and with the case of a constant segment profile with a plane of charge, usually assumed in the literature. We find that, within the region of concentrations studied here and with the chain lengths used, there is general agreement between the different descriptions.

Animals↗