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

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 (D) and penta-lysine (K), 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 CH groups were obtained. Compared with D, the two groups in K change significantly at higher temperature. The result may suggest that K has stronger interaction with water than D. Moreover, three ICs that contain the spectral information of the water species with no (S), one (S), and two (S) hydrogen-bonds were obtained. It was shown that the spectral intensity of S and S increases while that of S 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 S and S decreases and that of S increases for D solution, but the intensity of all the three species decreases for K solution. Furthermore, the concentration effect of K was found to be stronger than D. The result may reveal that D combines with water molecule through forming one hydrogen bond but K interacts with water through a different way.