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.
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