Macromolecules, protonation equilibria

An association constant (units = M ) for an equilibrium binding reaction of molecules, most frequently the binding interactions of a macromolecule ligand (which can be a proton, substrate, effector, anion, or cation such... 

Both processes (86) and (87) apparently involve formation or destruction of the zwitterion dipole moment Therefore, application of an electric field must di lace the respective chemical equilibrium to some extent depending on the an e 0 between the directions of the zwitterion dipole and the field E. For 0 < 90° an increase of the number of zwitterions is favoured whereas for 0 > 90° this number will be decreased. Therefore, the whole system tends to develop preferential orientation of dipoles parallel to the field. Proton transfer of the kind involved here has generally been proved to be practically difiiision controlled so that the reaction rates could be extremely high. The chemical mechanism of dielectric polarization may thus be fast enough in comparison with the rotational difiusion of the zwitterion, especially if the latter is a macromolecule (e.g . a protein). 

Scheme I. Schematic representation of macromolecules in the water-free globulcur conformation and of those in the highly protonated extended state at equilibrium in the two-phase system.

The equilibrium between globular and extended macromolecules is obviously controlled by electrostatic repulsions and entropic effects involving hydrophobic interactions and solvent structuration. However, the main factor is the ability of hydrophilic protonated tertiary amine to deprotonate easily to hydrophobic tertiary amine. Therefore, the difference between N-protonation and N-methylation in their ability to impart water-solubility to P(TDAE) chains is because the non-methylated chains can expel all their electric charges by deprotonation and thus macroscopically precipitate when hydrophobic interactions become greater than electrostatic repulsive forces. In the case of N-methylated molecules, stable tetralkyl ammonium groups prevent the loss of all the electric charges and thus stabilize the deprotonated macromolecules in their dispersed globular form. 

---