Potential energy surfaces of carrageenan models: carrabiose, β-(1→4)-linked d-galactobiose, and their sulfated derivatives Original Research Article

The adiabatic conformational surfaces of several β-linked disaccharides, which correspond to the repeating structures of carrageenans, were calculated using the mm3 force-field. The studies were carried out on the disaccharide β-d-Galp-(1→4)-α-d-Galp and eight sulfated derivatives, as well as on carrabiose (β-d-Galp-(1→4)-3,6-An-α-d-Galp) and five sulfated derivatives. The presence of 3,6-anhydrogalactose does not change the main features of the maps, although it increases the flexibility of the glycosidic linkage. Sulfation neither produces a striking effect on the map shape, nor a shift on the global minimum, which always remains with ψ (θC-1′O-4C-4C-5) in trans orientation, and φ (θO-5′C-1′O-4C-4) with a value close to −80°. This effect differs from that occurring on the α linkage of equivalent disaccharides, for which the sulfation pattern on the β-galactose unit shifts the global minima to different positions. A reduction in the flexibility (originated in a deepening of the global minimum well) is observed by sulfation on position 2 of the β-d-galactose unit, and by sulfation of position 6 of the α-d-galactose unit (when the β-d-galactose unit is 4-sulfated). Within the compounds containing 3,6-anhydrogalactose, the effect of sulfation is even less noticeable. The calculated low-energy regions on carrabiose derivatives agree with X-ray diffraction data on carrageenan fibers and on peracetylated carrabiose dimethyl acetal, and with NOE calculations carried out on κ-carrabiose.