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

The adiabatic conformational surfaces of several beta-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 beta-D-Galp-(1 --> 4)-alpha-D-Galp and eight sulfated derivatives, as well as on carrabiose (beta-D-Galp-(1 --> 4)-3.6-An-alpha-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 psi (theta(C-1 O-4 C-4 C-5)) in trans orientation. and phi(theta(0.5 C-1 -O-4 C-4)) with a value close to - 80degrees. This effect differs from that occurring on the alpha linkage of equivalent disaccharides. for which the sulfation pattern on the beta-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 beta-D-galactose unit, and by sulfation of position 6 of the alpha-D-galactose unit (when the beta-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 K-carrabiose. (C) 2002 Elsevier Science Ltd. All rights reserved.