Electrostatic interactions salt effects

Ultrafiltration allows for a partial reduction of salt content in the retentate, approximately 50% of the initial salt content. Salts are partly retained because of electrostatic interactions (Donnan effect) with the biopolymers, which are polyelectrolytes. Nevertheless, this demineralization does not really change the salt concentration in the final powder, which decreases from 15% (treatment without UF) to 10% (UF step). 

The concentration of salt in physiological systems is on the order of 150 mM, which corresponds to approximately 350 water molecules for each cation-anion pair. Eor this reason, investigations of salt effects in biological systems using detailed atomic models and molecular dynamic simulations become rapidly prohibitive, and mean-field treatments based on continuum electrostatics are advantageous. Such approximations, which were pioneered by Debye and Huckel [11], are valid at moderately low ionic concentration when core-core interactions between the mobile ions can be neglected. Briefly, the spatial density throughout the solvent is assumed to depend only on the local electrostatic poten-... 

Cationic samples can be adsorbed on the resin by electrostatic interaction. If the polymer is strongly cationic, a fairly high salt concentration is required to prevent ionic interactions. Figure 4.18 demonstrates the effect of increasing sodium nitrate concentration on peak shapes for a cationic polymer, DEAE-dextran. A mobile phase of 0.5 M acetic acid with 0.3 M Na2S04 can also be used. 

Note that when the concentration of added salt is very low, Debye length needs to be modified by including the charge contribution of the dissociating counterions from the polyelectrolytes. Because the equilibrium interaction is used, their theory predicts that the intrinsic viscosity is independent of ion species at constant ionic strength. At very high ionic strength, the intrachain electrostatic interaction is nearly screened out, and the chains behave as neutral polymers. Aside from the tertiary effect, the intrinsic viscosity will indeed be affected by the ionic cloud distortion and thus cannot be accurately predicted by their theory. 

Fig. 2.16 Effect of electrostatic interactions on 3i4-helix formation in an aqueous environment [1 75 a, 175 b, 176]. y -Peptides 86 and 87 adopt a stable helical conformation mediated by salt bridges near neutral pH. While the propensity of these peptides to adopt a helical conformation is strongly de-...

In fact this "unhydrolyzed" polyacrylamide sample is slightly charged and its low polyectrolyte character is confirmed by a slight difference of red values at pH 7 and 5, for salt free solutions. A really neutral polymer should be necessary to differentiate low effects of electrostatic interactions from non ionic interactions. coordination binding at low pH and hydrogen bonds at pH 7. Nevertheless, at this pH, the adsorption of the chain on Al(0H)3 aggregates can probably be considered as the main origin of the loss of viscosity. 

Electrostatic effects other than ionization are also important. Interactions between reacting ions depend on the local electrical environment of the ions and thus reflect the influence of the dielectric constant of the solvent and the presence of other ions and various solutes that may be present. In dilute solutions the influence of ionic strength on reaction rates is felt in the primary and secondary salt effects (see below). 

The IPA system does not require a co-solvent, but one can be used if this proves advantageous. In the TEAF system a solvent is normally used, though neat TEAF or formic acid can be used if required. The solvent can have a large effect on the reaction rate and optical purity of the product this may in part be because the substrate seems to bind by weak electrostatic interactions with the catalyst, and is also partly due to the pH of the system. Solvents have a dramatic effect on the ionization of formic acid for example, in water the piCa is 3.7, but in DMF it is 11.5. This is because formation of the formate anion becomes less favorable with less polar solvents (see Table 35.2). The piCa of triethy-lamine is far less sensitive. As a consequence, formic acid and triethylamine may remain unreacted and not form a salt. The variation in formic acid piCa can also have a significant impact on the catalyst and substrate, particularly when this is an imine. 

Before we dissolved the salt in water, the ions within the crystal were held together by strong electrostatic interactions, which obeyed Coulomb s law (see p. 313). We call the energetic sum of these interactions the lattice enthalpy (see p. 124). We need to overcome the lattice enthalpy if the salt is to dissolve. Stated another way, salts like magnesium sulphate are effectively insoluble in water because water, as a solvent, is unable to overcome the lattice enthalpy. 

Effect of salt type and concentration The ionic strength of the aqueous solution in eontaet with a reverse micelle phase affects protein partitioning in a number of ways [18,23]. The first is through modification of electrostatic interactions between the protein surface and the surfaetant head groups by modifieation of the eleetrieal double layers adjacent to both the eharged inner mieelle wall and the protein surface. The second effect is to salt out the protein from the mieelle phase because of the inereased propensity of the ionie speeies to migrate to the micelle water pool, reduee the size of the reverse mieelles, and thus displace the protein. 

Based on an X-ray crystal structure analysis of 54, the authors proposed a transition-state where the conformation of the acylammonium salt generated from 54 would be fixed by an attractive electrostatic interaction between the acyl-oxygen and the imidazoyl-2-proton or a dipole minimization effect (Fig. 14) [177]. 

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