[Beta-lactoglobulin AB fluorescence under different physico-chemical conditions. Denaturation by urea and organic...

Dependences of different fluorescence parameters of bovine beta-lactoglobulin AB on the concentrations of urea (pH 2.8-8.8), ethanol (pH 2.1-10.2), and dioxane (pH 5.3) have been investigated. The denaturation properties (the free energy and the stoichiometry of denaturative interaction) are highly dependent on pH values. The data obtained indicate that the hydrophobic interactions are the determining forces in the stabilization process of the beta-lactoglobulin molecule. The relative contribution of these interactions lowers with pH rise. The denaturation of beta-lactoglobulin AB proceeds through two stages under conditions when the protein octamer exists. Up to 30 vol.% of ethanol and dioxane, the penetration of the organic molecules into the external parts of the protein globule takes place. At the concentration of the solvent exceeding 50 vol.% structural transitions are observed. The comparison of fluorescence and perturbation spectral data enables one to localise tryptophan residues in the protein more precisely. The results of this and former reports lead to hypothesis that beta-lactoglobulin may serve as a transporter of some substances which are unstable to acidic media.