Understanding the Interaction Between Oligopeptide and Water in Aqueous Solution Using Temperature-Dependent Near-Infrared Spectroscopy

Abstract

Investigating the interaction between oligopeptide and water is essential for understanding the structure, dynamics and function of proteins. Temperature-dependent near-infrared (NIR) spectroscopy and independent component analysis (ICA) were employed to study the interaction between oligopeptide and water in aqueous solution. The NIR spectra of two homo-oligopeptides, penta-aspartic acid (D₅) and penta-lysine (K₅), in aqueous solution of different concentration were measured at different temperature (30–90 ℃). Independent component analysis was performed to extract the spectral information that changes with temperature. The independent components (ICs) representing the spectral information of NH and CH₂ groups were obtained. Compared with D₅, the two groups in K₅ change significantly at higher temperature. The result may suggest that K₅ has stronger interaction with water than D₅. Moreover, three ICs that contain the spectral information of the water species with no (S₀), one (S₁), and two (S₂) hydrogen-bonds were obtained. It was shown that the spectral intensity of S₀ and S₁ increases while that of S₂ decreases with the temperature, and the changes of oligopeptide solutions are weaker than those of pure water. The results indicate that water structure is sensitive to temperature and the oligopeptide in aqueous solution improves the thermal stability of the water species. When oligopeptide is added, the spectral intensity of S₀ and S₂ decreases and that of S₁ increases for D₅ solution, but the intensity of all the three species decreases for K₅ solution. Furthermore, the concentration effect of K₅ was found to be stronger than D₅. The result may reveal that D₅ combines with water molecule through forming one hydrogen bond but K₅ interacts with water through a different way.