Jurij Reščič

Solubility of lysozyme in polyethylene glycol-electrolyte mixtures: the depletion interaction and ion-specific effects.

The solubility of aqueous solutions of lysozyme in the presence of polyethylene glycol and various alkaline salts was studied experimentally. The protein-electrolyte mixture was titrated with polyethylene glycol, and when precipitation of the protein occurred, a strong increase of the absorbance at 340 nm was observed. The solubility data were obtained as a function of experimental variables such as protein and electrolyte concentrations, electrolyte type, degree of polymerization of polyethylene glycol, and pH of the solution; the last defines the net charge of the lysozyme. The results indicate that the solubility of lysozyme decreases with the addition of polyethylene glycol; the solubility is lower for a polyethylene glycol with a higher degree of polymerization. Further, the logarithm of the protein solubility is a linear function of the polyethylene glycol concentration. The process is reversible and the protein remains in its native form. An increase of the electrolyte (NaCl) concentration decreases the solubility of lysozyme in the presence and absence of polyethylene glycol. The effect can be explained by the screening of the charged amino residues of the protein. The solubility experiments were performed at two different pH values (pH = 4.0 and 6.0), where the lysozyme net charge was +11 and +8, respectively. Ion-specific effects were systematically investigated. Anions such as Br(-), Cl(-), F(-), and H(2)PO(4)(-) (all in combination with Na(+)), when acting as counterions to a protein with positive net charge, exhibit a strong effect on the lysozyme solubility. The differences in protein solubility for chloride solutions with different cations Cs(+), K(+), and Na(+) (coions) were much smaller. The results at pH = 4.0 show that anions decrease the lysozyme solubility in the order F(-) < H(2)PO(4)(-) < Cl(-) < Br(-) (the inverse Hofmeister series), whereas cations follow the direct Hofmeister series (Cs(+) < K(+) < Na(+)) in this situation.

Potential of mean force between charged colloids: Effect of dielectric discontinuities

The potential of mean force between two spherical and like-charged macroions in a salt-free aqueous solution has been determined using an extended primitive model and canonical Monte Carlo simulations. The systems considered covered the range from a purely repulsive to a purely attractive potential of mean force as the electrostatic coupling was increased. The macroions were modeled as spherical dielectric cavities, and the polarization surface charge densities occurring at the dielectric discontinuities were expanded in spherical harmonics. The surface polarization gave rise to (i) an attenuation of the counterion accumulation at the macroion surfaces at all cases considered, (ii) an enhanced repulsive potential of mean force in the weak to intermediate electrostatic coupling regime, and (iii) a less attractive at short separation and an enhanced attractive potential of mean force at longer macroion-macroion separation in the strong electrostatic coupling regime.

Attraction between like-charged surfaces induced by orientational ordering of divalent rigid rod-like counterions : theory and simulations

We consider the interaction between two equally charged surfaces in an electrolyte solution composed of long divalent rigid rod-like counterions of arbitrary length. Further, we study the influence of orientational ordering of rigid rod-like counterions on the interaction between charged surfaces. Density functional theory is introduced, where the spatial distribution of charge within the divalent rod-like counterions is represented by two effective charges at a fixed distance. The result of variational procedure gives an integral differential equation for the electrostatic potential which was solved numerically. From the electrostatic potential and the concentration of counterions, the free energy of two charged surfaces interacting in a solution of rod-like counterions is calculated. For large surface charge densities and for long enough divalent rod-like counterions the minimum of the free energy is obtained at a distance between the surfaces which equals the counterion length. This indicates that a bridging mechanism might be responsible for the attraction between like-charged surfaces. The analysis of the orientational distribution function confirms that, at the minimum of the free energy, the rod-like counterions are oriented perpendicularly and thus connect the like-charged surfaces. Finally, canonical Monte-Carlo simulations confirm the theoretical calculations of the osmotic pressure between like-charged surfaces for long enough rod-like counterions.

Thermodynamics of the lysozyme--salt interaction from calorimetric titrations.

It is well-known that the addition of salts influences the properties of proteins in solution. The essential nature of this phenomenon is far from being fully understood, partly due to the absence of the relevant thermodynamic information. To help fill this gap, in this work isothermal titration calorimetry (ITC) was employed to study the ion-lysozyme association in aqueous buffer solutions at pH = 4.0. ITC curves measured for NaCl, NaBr, NaI, NaNO3, NaSCN, KCl, CaCl2, and BaCl2 salts at three different temperatures were described by a model assuming two sets of independent binding sites on the lysozyme. The resulting thermodynamic parameters of binding of anions (counterions) to the first class of sites (N approximately 7) indicate that the binding constant (K approximately 102 M-1) increases in the order Cl- < Br- < I- < NO3- < SCN-. The anion-lysozyme association is entropy driven, accompanied by a small favorable enthalpy contribution and a positive change in heat capacity. It seems that the entropy and heat capacity increase is due to the water released upon binding, while the net exothermic effect originates from the anion-NH3+ pair formation. Moreover, the results reveal that the nature of the cation has little effect on the thermodynamics of the anion-lysozyme association under the given experimental conditions. Taken together, it seems that the observed thermodynamics of association is a result of a combination of both electrostatic and short-range interactions. The anion ordering reflects the strength of water mediated interactions between anions and lysozyme.

Monte Carlo simulation studies of electrolyte in mixture with a neutral component

Monte Carlo simulations are obtained for a multicomponent primitive model mixture consisting of a symmetric 1:1 electrolyte and a neutral particle. The equilibrium structure and thermodynamics are studied as a function of the concentration of the electrolyte and concentration of the neutral species. For the unequal-sized mixture the simulations clearly indicate a transformation in the nature of interaction between two like ions from being repulsive at low concentrations of the neutral component to being attractive at higher concentrations of the neutral component. The computer results are used to assess the accuracy of the corresponding predictions from the symmetric Poisson–Boltzmann, the modified Poisson–Boltzmann, and the hypernetted chain theories. Comparison of the pair distribution functions and thermodynamics shows that the theoretical results are in semiquantitative or better agreement with the simulation results. While the good agreement of the modified Poisson–Boltzmann and the hypernetted chain...

Debye–Hückel theory for mixtures of rigid rodlike ions and salt

Like-charged surfaces are able to attract each other if they are embedded in an electrolyte solution of multivalent rodlike ions, even if the rods are long. To reproduce this ability the Poisson-Boltzmann model has recently been extended so as to account for the rodlike structure of the mobile ions. Our model properly accounts for intraionic correlations but still neglects correlations between different rodlike ions. For sufficiently long rods, the model shows excellent agreement with Monte Carlo simulations and exhibits two minima - a depletion and a bridging minimum - in the interaction free energy. In the present work, we generalize the Poisson-Boltzmann model to systems with polydisperse rod lengths and arbitrary charge distributions along the rods, including the presence of salt. On the level of the linearized Debye-Hückel model we derive a general criterion for whether an electrolyte with given distribution of rodlike ions is able to mediate attraction between like-charged surfaces. We numerically analyze two special cases, namely the influence of salt on symmetric and asymmetric mixtures of monodisperse rodlike ions. The symmetric mixture is characterized by the presence of both negatively and positively charged (but otherwise identical) rodlike ions. For the asymmetric mixture, the system contains rodlike ions of only one type. We demonstrate that the addition of salt retains the depletion minimum but tends to eliminate the bridging minimum.

Monte Carlo simulations of a mixture of an asymmetric electrolyte and a neutral species

The equilibrium properties of a charge and size asymmetric, multicomponent mixture of macroions, counterions, and neutral particles are characterized through Monte Carlo simulations. Particular emphasis is paid to considering the influence on these properties of macroion concentration, neutral component concentration, macroion charge, and counterion charge. A significant result of the simulations is that sufficient increase in the neutral species concentration leads to a gradual change in the qualitative nature of the interaction between two macro-ions from repulsive to attractive. This occurs for all the counterion charges, macroion charges, and macroion concentrations. Overall, the structure and thermodynamics are shaped by the collective and often competing effects of the species. The simulations are supplemented by theoretical calculations using the symmetric Poisson—Boltzmann, the modified Poisson-Boltzmann, and the hypernetted chain approaches. The theories show reasonable consistency, both among th...

Solution of the associative mean spherical approximation for a multicomponent dimerizing hard-sphere multi-Yukawa fluid

The analytical solution of the associative mean spherical approximation (AMSA) for a Yukawa dimerizing multicomponent hard-sphere fluid is derived. The general multi-Yukawa case is discussed. The simpler one-Yukawa case with factorizable coefficients is explicitly solved. As in the previously discussed electrolyte case the solution of the AMSA reduces to the solution of only one nonlinear algebraic equation for the scaling parameter ΓB. The analytical results for the AMSA closure is illustrated by numerical examples and computer simulation for the one-component one-Yukawa dimerizing fluid. Good agreement between theoretical and computer simulation results was found for both the thermodynamic properties and the structure of the system.

Counterion-counterion correlation in the double layer around cylindrical polyions: Counterion size and valency effects

Monte Carlo simulation and Poisson-Boltzmann results on some aspects of structure and thermodynamics of aqueous polyelectrolyte solutions are presented. The polyelectrolyte solution is described by an infinitely long cylindrical polyion surrounded by counterions modeled as rigid ions moving in a continuum dielectric. Ion-ion correlations in the form of volume average of the counterion-counterion distribution function in the double layer surrounding the polyion are reported for mono- and divalent counterions and for a range of polyion concentrations and charge density parameters in each case. These results confirm again strong influence of the charge density parameter of polyions on properties of polyelectrolyte solutions. The structural information is supplemented by the calculated thermodynamic properties such as osmotic coefficients and heats of dilutions; the latter quantity has not been examined yet in detail by computer simulations. The results are discussed in view of the existing experimental data from the literature for these properties.

Recycling of uranyl from contaminated water

Many separation processes are related to the behavior of ions close to charged surfaces. In this work, we examine uranyl ions, which can be considered as rod-like molecular ions with a spatially distributed charge, embedded in a system of like charged surfaces. The analysis of the system is based on an approximate field theory which is accurate from the weak to the strong electrostatic coupling regimes. The numerical results show that close to the charged surface the ions are oriented parallel to the surface, whereas at distances greater than half of the ion length, they are randomly oriented. Due to the restriction of the orientational degrees of freedom, the density of ions at the charged surface decreases to zero. For large surface charge densities, the force between like charged surfaces becomes attractive, as a result of charge correlations. The theoretical results are in good agreement with Monte Carlo simulation results.