Fourier transform near-infrared (FT-NIR) spectra have been measured for bovine serum albumin (BSA) in an aqueous solution (pH 6.8) with a concentration of 5.0 wt % over a temperature range of 45-85 °C. Not only conventional spectral analysis methods, such as second-derivative spectra and difference spectra, but also chemometrics, such as principal component analysis (PCA) and evolving factor analysis (EFA), have been employed to analyze the temperature-dependent NIR spectra in the 7500-5500 and 4900-4200 cm<sup>-1</sup> regions of the BSA aqueous solution. Intensity changes of bands in the 7200-6600 cm<sup>-1</sup> and 4650-4500 cm<sup>-1</sup> regions in the difference spectra indicate variations of the hydration and secondary structure of BSA in the aqueous solution, respectively. The plot of a band intensity at 7080 cm<sup>-1</sup> in the different spectra shows a clear turning point at 63 °C, revealing that a significant change in the hydration occurs at about 63 °C. The forward and backward eigenvalues (EVs) from EFA suggest that marked changes in the hydration and secondary structure of BSA take place in the temperature ranges of 61-65 °C and 59-63 °C, respectively. In addition, the temperature of 71 °C marked in the EFA plots may correspond to the onset temperature of increase in the intermolecular β-sheet structure.
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