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 β-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., α-helical structures at 1650 cm−1, aggregated β-strands at 1620 cm−1, and β-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 β-sheet into α-helix occurs in terms of two parallel pathways, i.e., (1) β-sheets → aggregated β-strands →α-helix, and (2) β-sheets →α-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.

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