The Optical Rotatory Dispersion of the β-Lactoglobulins

Abstract An optical rotatory dispersion study of the three genetic variants of β-lactoglobulin has been carried out in the wave length region of 195 to 300 mµ. The results allow several conclusions. (a) The native secondary structures of all three genetic variants are close to being identical. (b) The structure of the native β-lactoglobulins is predominantly not α-helical. This is shown by the profile of the optical rotatory dispersion curve between 228 and 240 mµ which displays a small maximum instead of a minimum at 233 mµ. (c) The optical rotatory dispersion data is consistent with a native structure composed of 10 to 15% α-helix, with the remainder being essentially equal amounts of random and (probably) β structures. (d) In the native protein, some aromatic residues are present in an ordered, optically active conformation, which is destroyed by denaturation leading either to unfolding or to α-helix formation. (e) Addition of methanol (to more than 30% by volume) causes a sharp transition of the β-structured regions to α-helices; this is followed by a similar slow transformation of the randomly folded regions.