Binding of alkylsulfonate ligands to bovine β-lactoglobulin: effects on protein denaturation by urea

The effect of protein concentration on the interaction of alkylsulfonate ligands (ALn), characterized by a similar sulfonate head and a progressively longer hydrocarbon tail (AL8, AL10, AL12, AL14 and AL16), with bovine beta-lactoglobulin (β-LG) AB was studied using tryptophan fluorescence enhancement. Our results suggest that the ALn binding site is formed when β-LG monomers associates into dimers and that it is close to the junction region. The higher rigidity of β-LG structure observed on decreasing the pH, could be responsible for the loss of ALn binding at acid pH. The effects caused by ALn binding on β-LG denaturation by urea were also examined. The fraction of protein denatured was determined following λmax of fluorescence emission. The denaturation profiles were analyzed by the dissociation coupled unfolding model. Our results suggest that the protective action toward urea unfolding is exerted by a fraction of ALn bound to β-LG monomer, not related to the dimer existence. Moreover, we conclude that ALn mainly stabilizes the monomer structure.