Two-dimensional near-IR correlation spectroscopy study of molten globule-like state of ovalbumin in acidic pH region: Simultaneous changes in hydration and secondary structure

The molten globule‐like states of ovalbumin (OVA) in acid aqueous solutions are investigated by generalized two‐dimensional (2D) Fourier transform near‐IR (FT‐NIR) correlation spectroscopy. This new method allows us to explore the changes in hydration and the secondary structure simultaneously. FT‐NIR spectra are measured for OVA aqueous solutions with concentrations of 1, 2, 3, 4, and 5 wt % over a pH range of 2.4–5.4. Concentration‐perturbed 2D correlation spectra are calculated for the spectra in the 4850–4200 and 7500–5350 cm−1 regions at different pH values. The 2D NIR synchronous spectrum in the 4850–4200 cm−1 region shows a significant change upon going from pH 5.4 to 3.6. An autopeak at 4265 cm−1 that is due to a combination of a symmetric CH2 stretching mode and a CH2 bending mode of side chains seen at pH 5.0 disappears completely in the synchronous spectrum at pH 3.6. This suggests that some amino acid residues of OVA are subjected to microenvironmental changes with decreasing pH. More remarkable changes are observed in the synchronous spectra at pHs below 2.8. A band near 4600 cm−1 arising from a combination of amide B and amide II modes (amide B/II) shifts downward with considerable broadening between pH 3.0 and 2.4, suggesting that the strength of the hydrogen bonds of amide groups of OVA changes significantly. The synchronous and asynchronous spectra in the 4850–4200 cm−1 region show that the intensities of the bands attributable to amide groups and side chains of OVA and that of the band near 4800 cm−1 arising from water change in phase with the increase in the concentration above pH 2.8, but they vary out of phase below pH 2.8. The 2D synchronous map in the 7500–5350 cm−1 region also shows marked changes upon going from pH 2.8 to 2.6. A broad autopeak at around 6950 cm−1 assigned to free water and bound water with weak hydrogen bonds becomes very weak in the synchronous spectrum at pH 2.6, while broad autopeaks around 6450 cm−1 suddenly appear that are due to bound water with several hydrogen bonds and the first overtone of an NH stretching mode of the amide groups of OVA. Therefore, it is very likely that protein hydration and the hydrogen bonds of amide groups change simultaneously in a narrow pH region of 2.8–2.6. It is probably that below pH 2.6 the protein assumes a molten globule‐like state in which the whole molecule is very flexible, and side chains (but not the backbone chain) fluctuate significantly. © 2002 Wiley Periodicals, Inc