Two-Dimensional Fourier Transform Near-Infrared Spectroscopy Study of Heat Denaturation of Ovalbumin in Aqueous Solutions

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Abstract

Heat-induced denaturation of ovalbumin in aqueous solutions has been investigated by generalized two-dimensional (2D) Fourier transform near-infrared (FT-NIR) correlation spectroscopy. New insight has been gained into hydration and the unfolding process of secondary structures of ovalbumin by studying temperature-dependent correlation patterns in 2D synchronous and asynchronous spectra, which are constructed from concentration-perturbed NIR spectra at different temperatures. The correlation patterns have provided information about the correlation and phase relationships between different absorption bands of ovalbumin and water. The hydration of ovalbumin is almost unchanged from 45 to 67 °C, where ovalbumin molecules are in a natively folded state. A sudden change in the hydration is detected in a narrow temperature range of 67−69 °C, where the unfolding of the ordered secondary structures starts. The hydration, again, remains nearly unchanged upon further heating to 80 °C, even though the unfolding process develops progressively until the denatured state. The sudden change in the hydration around 68 °C seems to be caused by the stabilization of slightly unstable hydrogen bonds in the folded state. The change may make the intramolecular hydrophobic cores of ovalbumin less condensed and more accessible to the solvent molecules. On the other hand, the development of unfolding from 69 to 80 °C results in band shifts for combination bands involving free NH stretching−amide II (amide A/II), intramolecular hydrogen-bonded NH stretching−amide II (amide B/II) of ovalbumin. The present experiment demonstrates that the generalized 2D NIR correlation spectroscopy is powerful in detecting subtle but valuable structural information about the protein denaturation in the aqueous solution.

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