Reversible effects of medium dielectric constant on structural transformation of β-lactoglobulin and its retinol binding

The secondary structure transformation of beta-lactoglobulin from a predominantly beta-structure into a predominantly alpha-helical one, under the influence of solvent polarity changes is reversible. Independent of the alcohol used--methanol, ethanol, or 2-propanol--the midpoints of the observed structural transformation occur around dielectric constant epsilon approximately 60. The structural change destroying the hydrophobic core formed by the beta-barrel structure leads, at room temperature, to the dissociation of the retinol/beta-lactoglobulin complex in the neighborhood of dielectric constant epsilon approximately 50. However, when the dielectric constant of the medium is raised back to epsilon approximately 70 by the decrease of the temperature, both the refolding of BLG into a beta-structure and the reassociation of the retinol/beta-lactoglobulin complex are observed. The esterification of beta-lactoglobulin carboxyl groups has two effects: on the one hand it accelerates the beta-strand alpha-helix transition induced by alcohols. On the other hand, the esterification of beta-lactoglobulin strengthens its interaction with retinol as it may be deduced from the smaller apparent dissociation constant of retinol/methylated beta-lactoglobulin complex. The binding of retinol to modified or unmodified beta-lactoglobulin has no influence (stabilizing or destabilizing) on the folding changes induced by alcohol.