New insight into the pH-dependent conformational changes in bovine beta-lactoglobulin from Raman optical activity

We have studied the conformation of beta-lactoglobulin in aqueous solution at room temperature over the pH range similar to 2.0-9.0 using vibrational Raman optical activity (ROA). The ROA spectra clearly show that the basic u p and down beta-barrel core is preserved over the entire pH range, in agree ment with other studies. However, from the shift of a sharp positive ROA ba nd at similar to 1268 to similar to 1294 cm(-1) on going from pH values bel ow that of the Tanford transition, which is centered at pH similar to 7.5, to values above, the Tanford transition appears to be associated with chang es in the local conformations of residues in loop sequences possibly corres ponding to a migration into the alpha-helical region of the Ramachandran su rface from a nearby region. These changes may be related to those detected in X-ray crystal structures which revealed that the Tanford transition is a ssociated with conformational changes in loops which form a doorway to the interior of the protein. The results illustrate how the ability of ROA to d etect loop and turn structure separately from secondary structure is useful for studying conformational plasticity in proteins.