Molecular dynamics of a tetrasaccharide subunit of chondroitin 4-sulfate in water.

Molecular dynamics (MD) simulations on a tetrasaccharide subunit of chondroitin 4-sulfate (CS4) in aqueous solution were carried out to study its interactions with water. Pair distribution functions and diffusion coefficients were calculated from a 4 ns trajectory and the hydration of different molecular groups was analysed. The average values of the interglycosidic torsion angles found in the simulations are phi 13 = -10 degrees, psi 13 = -85 degrees and phi 13 = 80 degrees, psi 13 = 90 degrees for the beta-(1-->3) linkage, and phi 14 = -10 degrees, psi 14 = -70 degrees for the beta-(1-->4) linkage. Hydrophobic patches formed by sugar ring CH groups were found. The diffusion coefficients of the water molecules vary from 1.4 x 10(-9) to 2.3 x 10(-9) m2 s-1 depending on the distances between the water molecules and the atoms of the CS4 molecule and the type of CS4 atoms, respectively. Reorientation correlation times of the water molecules in the vicinity of different CS4 atoms were estimated to be about 1 ps at a polymer concentration of 4 wt.% CS4. The number of hydrogen bonds between the water molecules and the acceptor atoms of CS4 was determined to be about 20 per disaccharide unit, indicating a higher hydration ability of chondroitin sulfate in comparison with non-sulfated oligosaccharides. Substructures, where water molecules are involved in hydrogen bonds to different sugar rings, were found, which may be important for the stabilisation of the secondary structure of the CS4 molecule.