Association of proteins in acidic solutions--a case study with beta-globulin

The investigation of the effect of acid pH on the structure of beta-globulin indicated several transitions as a function of pH. Upon reducing the pH from 7.0, the beta-globulin molecule underwent an expansion due to hydration up to pH 5.0, and a further increase in H+ concentration resulted in unfolding. This is a single step cooperative denaturation as indicated by the viscosity profile. At extreme acid pH values (below pH 2.0) the protein associates or folds to a different conformational motif as shown by blue shift of ultraviolet fluorescence emission maximum and decrease in reduced viscosity values by more than 30% due to an entropically driven hydrophobic interaction. The conformational analysis of beta-globulin showed a decrease up to pH 3.0, followed by an increase in the ordered structure at low pH values indicating that the low pH values stabilized this new conformation. These results are discussed in view of the molten globule structure of proteins.