Conformational analysis of neocarrabiose and its sulfated and/or pyruvylated derivatives using the MM3 force-field

The adiabatic conformational surfaces of neocarrabiose (3,6,-An-alpha -D-Galp-(1-->3)-beta -D-Galp) and of nine sulfated and/or pyruvylated derivatives were obtained using the MM3 force-field. These maps indicate greater flexibility of the glycosidic linkage than found for similar compounds that are based on alpha -D-galactose instead of 3,6-anhydrogalactose units. Sulfation of the alpha -D-galactose unit on position 2 shifts the global minimum to negative WH (theta (Cl'-O3C3-H3)) angles, whereas sulfation at either position 4 of the same unit or at position 2 of the 3,6-anhydro-alpha -D-galactose unit has less effect. The results are consistent with the X-ray diffraction data on crystalline neocarrabiose and carrageenan fibers. Free energy calculations show that entropy is not uniformly distributed among confomers.