Abstract:
Carbohydrates are known to closely modulate their surrounding solvent structure and influence solvation dynamics. Spectroscopic investigations studying the far IR regions (below 1000 cm-1) have observed spectral shifts in the libration band (around 600 cm-1) of water in the presence of monosaccharides and polysaccharides. In this paper we use molecular dynamics simulations to gain atomistic insight into carbohydrate-water interactions and to specifically highlight the differences between additive (non-polarizable) and polarizable simulations. A total of six monosaccharide systems, α and β anomers of glucose, galactose and mannose have been studied using additive and polarizable CHARMM carbohydrate force fields. Solvent was modeled using three additive water models TIP3P, TIP4P and TIP5P in the additive simulations and the polarizable water model SWM4 in the polarizable simulations. The presence of carbohydrate has significant affect on the microscopic water structure with the affects being pronounced for the proximal water molecules. Notably a disruption of the tetrahedral arrangement of proximal water molecules was observed due to the formation of strong carbohydrate-water hydrogen bonds in both the additive and polarizable simulations. However the inclusion of polarization resulted in significant bridge water occupancies, improved ordered water structure (tetrahedral order parameter) and longer carbohydrate-water H-bond correlations as compared to the additive simulations. Additionally the polarizable simulations also allowed the calculation of the power spectra from the dipole-dipole autocorrelation function which corresponds to the IR spectra. From the power spectra we could identify spectral signatures differentiating the proximal and bulk water structures which could not be captured from additive simulations.