Stabilization, electrostatic contribution

It follows from previous discussion that the destabilizing electrostatic contribution grows in absolute value with x (with increasing A.). But the influence of the nonuniform electrical force is overwhelmed by the stabilizing bending and stretching contributions. As a result, the traditional smectic model cannot explain how a small transmembrane voltage can lead to membrane breakdown. The obvious solution is to abandon this approach and to develop an alternative, such as the pore formation model. However, as we noticed before, this approach postulates rather than proves the appearance of hydrophobic pores. 

Many investigators of steric stabilization have measured colloidal stability without taking the effort to find out whether the stability actually resulted from electrostatic stabilization. In many published articles it has been concluded that steric stabilization had been attained and further study showed this was not the case. One such example is a recent paper on "steric" stabilization by an additive of the same type used in this work. (12) The published photograph shows the silica particles in oil stabilized at interparticle separations several times the distances provided by the adsorbed films no electrical measurements had been made, but it they had, this particular dispersant would have provided about -200 mV of zeta-potential and given excellent electrostatic repulsion. The reader should be wary of any claims of steric stabilization unless the electrostatic contribution has been measured. 

In such systems the requirement of the electrostatic contribution to colloidal stability is quite different than when no steric barrier is present. In the latter case an energy barrier of about 30 kT is desirable, with a Debye length 1/k of not more than 1000 X. This is attainable in non-aqueous systems (5), but not by most dispersants. However when the steric barrier is present, the only requirement for the electrostatic repulsion is to eliminate the secondary minimum and this is easily achieved with zeta-potentials far below those required to operate entirely by the electrostatic mechanism. 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

The order of elution of peptides (charged compounds) is governed by a combination of electrophoresis and partitioning, with hydrophobic as well as electrostatic contributions. In this study it was demonstrated that sulfonic acid functionalities in the methacrylate monolith provide high stability and maintain a constant EOF over a wide range of pH (2—12). It was also demonstrated that a better separation of a mixture of therapeutic peptides was obtained at high pH values (Figure 16) due to the suppression of electrostatic attraction. 

A related example54 was the decrease in the value of the comproportionation constant with increasing dielectric constant of the medium for the system [Ru(NH3)5(4,4 -bipyridyl)-Ru(NH3)5]6+,5+ 4+, which was attributed to the decrease in the electrostatic contribution to the stability of the mixed valence species as the dielectric constant increased. 

Na+, Mg2 1) and temperature T, using the TBI model (Fig. 22.1), we compute the electrostatic free energies for the duplex GES and the unfolded state GE, respectively. The electrostatic contribution to the folding stability is... 

This is the dependence expected for purely electrostatic stabilization of polyelectrolyte layers. Electro-steric interactions involve steric contributions [Eq. (3)] and electrostatic contributions [Eq. (4)]. For high ionic strength as in the interior of the polyelectrolyte shell the measured shear modulus should exhibit a characteristic G (r) e Kr-dependence which is apparent in a semi-logarithmic presentation of the data as in Fig. 13. The measured shear moduli are plotted as a function of the reduced distance r/2Rm which is... 

F. Dong, M. Vijayakumar, H.-Y. Zhou, Comparison of calculation and experiment implicates significant electrostatic contributions to the binding stability of bamase and barstar, Biophys.J. 2003, 85, 49-60. 

Xiao L, Honig B. Electrostatic contributions to the stability of hyperthermophilic proteins. J. Mol. Biol. 1999 289 1435-1444. 

The contribution of Asp to maintaining His in such an orientation that can accept a proton from Ser is considered to be of minor importance since the electrostatic contribution to the difference in free energy between native and mutant serine proteinases accounts for most of the calculated effects of the mutations (Asp to Asn in trypsin, Asp to Ala in subtilisin). The other stabilizing effect is of the oxyanion hole on the negatively charged TI, which is also electrostatic in nature. 

Empirically determined parameters, Ea and Ca, are assigned to an acid while, Eb and Cb are assigned to a base. When substituted into equation (2), they give the enthalpy of adduct formation for the acid-base pair. Ea and Eb parameters supposedly represent the electrostatic contributions to adduct stability, while Ca and Cb parameters are the susceptibility of the acid and base, respectively, to form covalent bonds. With increasing amount of reliable enthalpy data, the E C model was extended to many different acids and bases. 

The balance of electrostatic and delocalization interactions in an isolated molecule may be perturbed by the influence of the solvent. In calculations based on Eq. 7, the analysis of solvation-energy terms suggested that the electrostatic contribution stabilizing the ap orientation of the acetal s ment is the conformationally dominant term. For example, in 2-methoxyoxane, the difference in energy of the (ap, ap) and (ap, sc) conformers in water, compared to that in the isolated molecule, caused by solute-solvent electrostatic interactions alone, amounts to 4 kJ.mor. Accordingly, the inter-and intra-molecular, electrostatic interactions operate in reverse directions in acetals. Whereas the intramolecular, electrostatic interactions are responsible, together with delocalization interactions, for the aiq)earance of the anomeric, reverse anomeric, and exo-anomeric effects, the solute-solvent electrostatic interactions lessen their im nitude, and may even cancel them. Of course, the solvent may also influence the electron distribution and energy of MO s in a molecule. In this way, the orbital interactions of lone-pairs and delocalization contributions to the anomeric effect may be scaled by the solvent, but this mechanism of the environmental effect is, in most cases, of only minor importance. 

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