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 approximately 2.0-9.0 using vibrational Raman optical activity (ROA). The ROA spectra clearly show that the basic up and down beta-barrel core is preserved over the entire pH range, in agreement with other studies. However, from the shift of a sharp positive ROA band at approximately 1268 to approximately 1294 cm(-1) on going from pH values below that of the Tanford transition, which is centered at pH approximately 7.5, to values above, the Tanford transition appears to be associated with changes in the local conformations of residues in loop sequences possibly corresponding to a migration into the alpha-helical region of the Ramachandran surface from a nearby region. These changes may be related to those detected in X-ray crystal structures which revealed that the Tanford transition is associated with conformational changes in loops which form a doorway to the interior of the protein. The results illustrate how the ability of ROA to detect loop and turn structure separately from secondary structure is useful for studying conformational plasticity in proteins.