Structural characterization of p-lactoglobulin in solution using two-dimensional FT mid-infrared and FT near-infrared correlation spectroscopy

Two-dimensional (2D) FT-IR correlation analysis was applied to both the mid-IR (MIR) and near-IR (NIR) regions to investigate changes in the secondary structures of beta-lactoglobulin in D2O (or H2O) solvent systems consisting of varying concentrations of bromoethanol, Mid-IR correlation spectra indicate that the amide I bands corresponding to different structures (i.e., alpha-helical structures at 1650 cm(-1), aggregated beta-strands at 1620 cm(-1), and beta-sheet at 1636 cm(-1)) exhibit apparently different spectral response towards varying concentrations of bromoethanol. We propose that the mechanism for the conversion of the beta-sheet into alpha-helix occurs in terms of two parallel pathways, i.e., (1) beta-sheets --> aggregated beta-strands --> alpha-helix, and (2) beta-sheets --> alpha-helix, Although the amide B/amide II combination bands give no spectral features relating to the secondary structure, changes were found in the C-H combination bands that suggest an interaction between the solvent and the protein.