Effect of heat treatment on the circular dichroism spectra of bovine beta-lactoglobulin A, B, and C.

Dilute solutions of beta-lactoglobulin (beta-Lg) A, B, and C were heated in phosphate buffer at temperatures between 40 and 94 degrees C for 10 min, cooled, and analyzed using near-UV and far-UV circular dichroism (CD). The decrease in near-UV CD intensity at 293 nm (Deltaepsilon(293)) could be analyzed in terms of a two-state model, and the stability was beta-Lg C > beta-Lg A > beta-Lg B on the basis of the midpoint temperatures for samples heated at pH 6.7 and 7.4. However, the slopes of the curves at the midpoint temperature for variant A were generally less than those for beta-Lg B and beta-Lg C, indicating that the substitution of Val (beta-Lg A) for Ala (beta-Lg B or beta-Lg C) at position 118 had altered the entropic contribution to unfolding of the protein. The changes in CD at 270 nm (Deltaepsilon(270)), an index of significant alteration to disulfide bond dihedral angles, occurred at higher temperatures than those for the Deltaepsilon(293) results. The far-UV CD showed some small changes as a consequence of heat treatment, and the shifts at 205 nm ([theta](205)) fitted a two-state model. Plotting the changes in both Deltaepsilon(293) and [theta](205) against the loss of nativelike and sodium dodecyl sulfate-monomeric protein (assessed by polyacrylamide gel electrophoresis) showed a strong 1:1 relationship between Deltaepsilon(293) or [theta](205) and the loss of nativelike beta-Lg. These results indicated that the initial irreversible stage in the heat-induced aggregation of beta-Lg (nativelike monomer to unfolded monomer) altered the chirality of the environment of Trp(19) and modified the secondary structure of beta-Lg slightly. The differences in the behavior of variants A-C were explicable on the basis of generalized electrostatic and hydrophobicity effects as well as specific amino acid effects.