Dehydration-induced Initial Conformational Change of Hydrated Proteins Detected by the Changes in Vibrational Circular Dichroism Activity

Abstract

In the present study, myoglobin (Mb) and casein were investigated as representative proteins with α-helix-rich and random-coil structures, respectively. Conformational changes of hydrated proteins induced by gradual dehydration were monitored by vibrational circular dichroism (VCD) spectroscopy. In myoglobin and casein, representative α-helix-rich and random-coil proteins, respectively, an increase in left-handed optical activity in the amide I band was detected at the initial stage of dehydration, followed by an increase in opposite right-handed activity in both the amide I and II bands with further dehydration. Because the second step was observed with an increase in the turbidity of the proteins, it can be attributed to their aggregation. In contrast, because the increase in left-handed optical activity is induced by the conformational change of the proteins and is followed by the aggregation, it may derive from the increase in the regularity of the local structure in individual myoglobin or casein that triggers the aggregation.

Structural Discrimination between Aβ(1–40) and Aβ(1–42) Peptides in Films with Vibrational Circular Dichroism Spectroscopy

“…We have observed intense and sharp right-handed amide I bands at 1620 -1630 cm -1 based on the white turbidity of myoglobin (α-helical protein) and casein (random-coil protein) in films. 20 Although the optical activity was reverse with the VCD band of the Aβ(1-42) film, the appearance of the intense and sharp band may be due to the increased degree of the crystallinity by the aggregation of Aβ(1-42) up to white turbidity.…”

“…In film preparation, in contrast to the solution state, we have to pay attention to the appearance of the artifact signals due to the anisotropy of the prepared samples. 19,20 We examined the influence of the orientation of the Aβ films on the IR and VCD spectra. The IR and VCD spectra were measured by rotating the film, defined as 0 (normal), 45 and 90 to the direction of the polarized light (Figs.…”

“…20 They were briefly described here. An IR spectrometer (Nicolet 8700, Thermo Fisher Scientific) directly connected with a table-top optical module (TOM) box equipped with a photo-elastic modulator (Hinds Instruments, II/ZS50) was used.…”

“…The wavenumber of the main peak in the amide I band (1632 cm -1 ) is characteristic of the β-sheet conformation in proteins, while the wavenumber of the shoulder (1660 cm -1 ) is the α-helical conformation. 19,20 In the formation of aggregated or fibrous Aβ, it has been considered that water-soluble Aβ monomers with α-helical conformation convert to insoluble Aβ aggregates with a high β-sheet content. 24,25 From the IR absorption spectra, it was revealed that the prepared Aβ films were composed of the main aggregated parts with the β-sheet conformation and of the minor nonaggregated parts with the α-helical conformation.…”

“…VCD spectroscopy, where one measures the circular dichroism activity in the molecular vibration region, is a powerful tool for the discrimination of slight differences between secondary structures of peptides and proteins in aqueous environments. [19][20][21] To form the aggregated states of Aβ(1-40) and Aβ(1-42), we prepared their hydrated films.…”