Other Electrostatic Interactions

Comparison of solution pH with the pKa of a side chain informs about the protonation state. A unique pKa, termed the standard or model pKa, can be experimentally determined for each ionizable side chain in solution when it is incorporated in a model compound, often a blocked amino acid residue [73] (Table 10-1). In a protein environment, however, the pKa value of an ionizable side chain can substantially deviate from the standard value, due to desolvation effects, hydrogen bonding, charge-charge, charge-dipole, and other electrostatic interactions with the... 

The oxidized groups at C-1, C-1 and C-4 are assumed to bind to polar residues of the active sites by hydrogen bonding or by other electrostatic interactions. Indeed, analogs with these moieties at a higher oxidative level show stronger activity. 

In addition to these effects, other electrostatic interactions will affect the redox behavior of redox-active species. A study on flavodoxins for instance, has shown that anionic residues as far away as 13 A have a measurable influence on the FMN cofactor. A different study considering the influence of metal cations on the redox modulation established that Ca binding to a pyrroloquinolinequinone (PQQ) cofactor made the reduction of the oxidized cofactor 570 mV (13kcal/mol) less negative. " ... 

Dispersion forces are attractive forces between atoms at close distances. Even molecules with no permanent dipole moment have, due to the movement of their electrons, local dipole moments which induce dipoles in the opposite molecule, leading to fluctuating electrostatic attractions. At a closer distance repulsive forces develop due to an unfavorable overlap of the van der Waals spheres of both molecules. These relationships are typically described by the Lennard Jones potential, with an r attractive term and an r repulsive term (Figure 2) [59, 116]. Dipole-dipole interactions and dispersion forces are much weaker than other electrostatic interactions. Nevertheless, if there is a close contact between both molecules over a relatively large surface area, they may sum up to large values of overall interaction energies. 

Salt partitioning is largely determined by the electrostatic interactions in gels. In this chapter, we solve the nonlinear Poisson-Boltzmann equation first, and then incoiporate this result with other electrostatic interactions to obtain a model for salt abscRpdon. Then we compare theoretical predictions with experimental results. 

From Figure 1 we can see that linear counterion condensation theory predicts that counterion condensation is only a function of charge density or titrated degree of neutralization, while nonlinear theory predicts that counterion condensation is not only a function of titrated degree of neutralization, but also a function of ionic strength. The lower is the ionic strength, the more obvious is the counterion condensation. In the later discussion, we will use nonlinear counterion condensation as the basis to calculate other electrostatic interactions. 

The hydrogen molecule, H2, provides the simplest possible example of a covalent bond. When two hydrogen atoms are close to each other, electrostatic interactions occur between them. The two positively charged nuclei and the two negatively charged electrons repel each other, whereas the nuclei and electrons attract each other as shown in Figure 8.5(a) . For the H2 molecule to exist as a stable entity, the attractive forces must exceed the repulsive ones. But why is this so ... 

The other important effect in the liquid phase is the direct intermolecular interaction via hydrogen bonding. Van der Waals force and other electrostatic interactions. Figure 3.12a, b illustrates the optimized structures of the dimers of five amides and their convoluted theoretical spectra calculated by TD-DFT/aug-cc-pVTZ. In... 

Of course, it should be remembered that the only type of electrostatic charge being considered is Coulombic. Many nonionic solutes will interact with solution molecules through other types of electrostatic interactions such as polar, induced dipole, hydrogen bonding, etc. These other electrostatic interactions would lead to a nonunity value for, in general. 

The activity coefficient equation (182) for the nonionic solute molecule in solution is unity as it should be for the Coulombic model from which the equation was derived. As discussed above, other electrostatic interactions, other than Coulombic, between nonionic solute and solution molecules in real solutions will make the activity coefficient for nonionic molecules non- unity, especially at high solute concentrations. 

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