Effects of salts on association and conformational equilibria of macromolecules in solution
2008; Linguagem: Inglês
10.1007/bfb0118259
Autores Tópico(s)Surfactants and Colloidal Systems
ResumoThe physical basis and the theoretical modelling of effects of added electrolytes on the association and conformational equilibra of macromolecules in solution are discussed in the light of our recent detailed studies and analyses of such effects on the coil-to-helix transitions of charged and uncharged polysaccharides (Carrageenans and agarose, respectively). Salt effects are divided into 1) generic electrostatic effects (for charged macromolecules), 2) general, ion-specific (but not site-specific) so-called lyotropic effects, which occur also for uncharged macromolecules, and 3) effects of site-specific binding of ions to the macromolecule. Electrostatic salt effects are analyzed within the Poisson-Boltzmann Cell Model (PBCM), and it is found that this model can successfully account for experimentally observed effects on the conformational transitions of carrageenans, which result from changes in the concentrations of the macromolecules and/or of the added salt, and from a variation of the relative proportions of mono- and divalent counterions. Analyses of ion-specific (lyotropic) salt effects on the coil-to-helix transition of agarose suggest that the latter effects result from a change in the interfacial energy contribution to the transition, on addition of salt, due to a non-zero (positive or negative) surface excess of ions at the macromolecular surface which is expected, in general, also for an uncharged macromolecule. The same mechanism als accounts for similar effects of added uncharged cosolutes, such as urea. It is emphasized that watercosolute interactions may be as important as cosolute-macromolecule interactions in the generation of a non-zero surface excess of the cosolute at the macromolecular surface.
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