Interaction between proteins and salt solutions. I. Thermodynamic parameters of dilution for gelatin and collagen

Intrinsic viscosity–temperature studies for isoelectric gelatin in KCl and KSCN aqueous solutions and equilibrium degree of swelling, ν −1 , measurements for amorphous crosslinked rat tail tendons in the same solutions were carried out. On increasing salt concentration C s , both [η] and ν −1 increase, go through a maximum, and then decrease at high C s , KCl being more effective than KSCN in bringing about this decrease. The trend observed is similar to the variation of solubility of polypeptides and soluble proteins with C s . By regarding a water–salt solution of a given C s as a single diluent interacting with the protein modified by solvation and binding of ions, usual polymer solution theories were applied to the experimental results. Thus, quantities related to the entropy and enthalpy components of the excess chemical potential of the diluents were obtained. The data indicate that the initial increase of [η] and ν −1 with C s results from the balance of an enthalpy component which, on increasing C s , becomes less favorable to dilution and an entropy component which, conversely, becomes more favorable. The subsequent decrease of [η] and ν −1 with C s is due to the prevailing of the enthalpy component. The maximum is reached at higher C s for KSCN than for KCl, primarily because of the large increase in the entropy component operated by the former salt. The increase of the entropy parameter with C s is associated to a breaking down of the coordinated water structure and to an alteration of the conformation of the macro molecules due to ion–dipole interaction and to ion absorption. The decrease in exothermicity of dilution with C s indicates a reduced thermodynamic affinity of the diluent toward the polymer which probably results from alteration of the nature of both polymer and diluent.